Presenilin-1 and Presenilin-2 Exhibit Distinct yet Overlapping γ-Secretase Activities

Abstract

Presenilin-1 (PS1) and presenilin 2 (PS2) are proposed to be transmembrane aspartyl proteases that cleave amyloid precursor protein and Notch. PS1- and PS2-mediated activities were individually characterized using blastocyst-derived (BD) cells and membranes from PS1+/–-PS2–/– and PS1–/–PS2+/+ mice, respectively. The relative amounts of PS1 and PS2 in the various BD cells were determined from the intensities of the anti-PS1 and anti-PS2 immunoblot signals by comparison with standard curves using radiolabeled PS1 and PS2 standards produced by in vitro transcription and translation. Cellular membranes from wild type, PS1–/–PS2+/+, and PS1+/–-PS2–/– but not PS1–/–PS2–/– BD cells generated the Aβ40 and Aβ42 products from the C100FLAG substrate. PS1-associated γ-secretase displays considerably higher specific activity than PS2-associated γ-secretase. Moreover, the PS1+/–PS2–/– BD cells and corresponding membranes exhibited much higher γ-secretase activity as compared with other BD cells and membranes. The PS1-mediated γ-secretase activity correlated better with the amount of PS1 that is modifiable by a photoactivated active site-directed γ-secretase inhibitor rather than total PS1; hence, only a small portion (<14%) of the PS1 in wild-type membranes appears to be engaged in an active γ-secretase complex. This finding suggests that PS1 may serve other biological functions in addition to that associated with its γ-secretase activity. Furthermore, the PS1 γ-secretase complex and the PS2 γ-secretase complex activities can be discriminated on the basis of their susceptibility to inhibition by a potent γ-secretase inhibitor. The distinct yet overlapping enzymatic properties of the PS1 γ-secretase complex and the PS2 γ-secretase complex imply that these two putative aspartyl class proteases may contribute to different biological processes. Presenilin-1 (PS1) and presenilin 2 (PS2) are proposed to be transmembrane aspartyl proteases that cleave amyloid precursor protein and Notch. PS1- and PS2-mediated activities were individually characterized using blastocyst-derived (BD) cells and membranes from PS1+/–-PS2–/– and PS1–/–PS2+/+ mice, respectively. The relative amounts of PS1 and PS2 in the various BD cells were determined from the intensities of the anti-PS1 and anti-PS2 immunoblot signals by comparison with standard curves using radiolabeled PS1 and PS2 standards produced by in vitro transcription and translation. Cellular membranes from wild type, PS1–/–PS2+/+, and PS1+/–-PS2–/– but not PS1–/–PS2–/– BD cells generated the Aβ40 and Aβ42 products from the C100FLAG substrate. PS1-associated γ-secretase displays considerably higher specific activity than PS2-associated γ-secretase. Moreover, the PS1+/–PS2–/– BD cells and corresponding membranes exhibited much higher γ-secretase activity as compared with other BD cells and membranes. The PS1-mediated γ-secretase activity correlated better with the amount of PS1 that is modifiable by a photoactivated active site-directed γ-secretase inhibitor rather than total PS1; hence, only a small portion (<14%) of the PS1 in wild-type membranes appears to be engaged in an active γ-secretase complex. This finding suggests that PS1 may serve other biological functions in addition to that associated with its γ-secretase activity. Furthermore, the PS1 γ-secretase complex and the PS2 γ-secretase complex activities can be discriminated on the basis of their susceptibility to inhibition by a potent γ-secretase inhibitor. The distinct yet overlapping enzymatic properties of the PS1 γ-secretase complex and the PS2 γ-secretase complex imply that these two putative aspartyl class proteases may contribute to different biological processes. Presenilin-1 (PS1) 1The abbreviations used are: PS1, presenilin-1; PS2, presenilin-2; BD, blastocyst-derived; AD, Alzheimer's disease; APP, amyloid precursor protein; Aβ, β-amyloid; TNT, transcription/translation; PVDF, polyvinylidene difluoride; CHAPSO, 3-[(3-cholamidopropyl)dimethylammonio]-2-hydroxy-1-propanesulfonic acid; NTF, N-terminal fragment; CTF, C-terminal fragment. and presenilin-2 (PS2) are homologous (67% identity) polytopic membrane-spanning proteins that are endoproteolytically processed to form heterodimers. PS1 and PS2 were identified by their association with early onset Alzheimer's disease (AD) in humans. Autosomal dominant inheritance of mutations in the PS1 gene is the most common cause of familial early onset Alzheimer's disease (1Sherrington R. Rogaev E.I. Liang Y. Rogaeva E.A. Levesque G. Ikeda M. Chi H. Lin C. Li G. Holman K. et al.Nature. 1995; 375: 754-760Crossref PubMed Scopus (3585) Google Scholar). Familial early onset Alzheimer's disease-associated mutations are also present in PS2, although they are far less prevalent than in PS1 (2Levy Lahad E. Wasco W. Poorkaj P. Romano D.M. Oshima J. Pettingell W.H. Yu C.E. Jondro P.D. Schmidt S.D. Wang K. et al.Science. 1995; 269: 973-977Crossref PubMed Scopus (2230) Google Scholar). In addition to their purported role in the regulation of amyloid precursor protein (APP) processing, the presenilins have been implicated in Notch signaling, Wnt/β-catenin signaling, apoptosis, capacitative calcium entry (3De Strooper B. Annaert W. Nat. Cell Biol. 2001; 3: E221-E225Crossref PubMed Scopus (48) Google Scholar), and ErbB-4-mediated signaling (4Ni C.Y. Murphy M.P. Golde T.E. Carpenter G. Science. 2001; 294: 2179-2181Crossref PubMed Scopus (756) Google Scholar). Moreover, PS1 and PS2 are closely related to the Caenorhabditis elegans protein, sel-12, which is involved in cell fate decisions (5Levitan D. Doyle T.G. Brousseau D. Lee M.K. Thinakaran G. Slunt H.H. Sisodia S.S. Greenwald I. Proc. Natl. Acad. Sci. U. S. A. 1996; 93: 14940-14944Crossref PubMed Scopus (343) Google Scholar). APP is processed by two proteases, β-secretase and γ-secretase, that generate the N and C termini of the Aβ-peptides, respectively (6Li Y.-M. Mol. Intervent. 2001; 1: 198-207PubMed Google Scholar). The Aβ-peptides are generally 40 or 42 amino acids in length (Aβ40 and Aβ42, respectively). Aβ42 is more prone to aggregation and is believed to be a pivotal player in the etiology of AD. There is considerable evidence showing that the presenilins are closely associated with γ-secretase-mediated cleavage. Firstly, familial early onset Alzheimer's disease mutations in PS1 and PS2 cause increased production of Aβ42 in transfected cells or transgenic mice. Secondly, isolated cultured neurons derived from PS1-deficient mice accumulate APP fragments that fail to be processed by γ-secretase (7De Strooper B. Saftig P. Craessaerts K. Vanderstichele H. Guhde G. Annaert W. Von Figura K. Van Leuven F. Nature. 1998; 391: 387-390Crossref PubMed Scopus (1552) Google Scholar, 8Naruse S. Thinakaran G. Luo J.J. Kusiak J.W. Tomita T. Iwatsubo T. Qian X. Ginty D.D. Price D.L. Borchelt D.R. Wong P.C. Sisodia S.S. Neuron. 1998; 21: 1213-1221Abstract Full Text Full Text PDF PubMed Scopus (333) Google Scholar). Thirdly, γ-secretase activity is abolished in cultured cells derived from mice that are deficient in both PS1 and PS2 (9Herreman A. Serneels L. Annaert W. Collen D. Schoonjans L. De Strooper B. Nat Cell Biol. 2000; 2: 461-462Crossref PubMed Scopus (450) Google Scholar, 10Zhang Z. Nadeau P. Song W. Donoviel D. Yuan M. Bernstein A. Yankner B.A. Nat Cell Biol. 2000; 2: 463-465Crossref PubMed Scopus (359) Google Scholar). Fourthly, mutagenesis of two conserved aspartates in the transmembrane regions of PS1 significantly reduces γ-secretase activity (11Wolfe M.S. Xia W. Ostaszewski B.L. Diehl T.S. Kimberly W.T. Selkoe D.J. Nature. 1999; 398: 513-517Crossref PubMed Scopus (1692) Google Scholar). Thus, it was proposed that PS1 is either a unique diaspartyl cofactor for γ-secretase or is itself γ-secretase, an intramembranous aspartyl protease. Finally, biochemical evidence that PS1 and PS2 contain the active sites of γ-secretase was obtained from studies in which presenilins were covalently modified by active site-directed γ-secretase inhibitors (12Li Y.M. Xu M. Lai M.T. Huang Q. Castro J.L. DiMuzio Mower J. Harrison T. Lellis C. Nadin A. Neduvelil J.G. Register R.B. M.K. M.S. Nature. 2000; PubMed Scopus Google Scholar, Kimberly W.T. Ostaszewski B.L. Diehl T.S. T. Xia W. Selkoe D.J. M.S. Nat. Cell Biol. 2000; 2: PubMed Scopus Google both PS1 PS2 are for the γ-secretase-mediated the different of the PS1-deficient and mice that these proteins The PS1-deficient mice and J. Xia W. Selkoe D.J. S. Full Text Full Text PDF PubMed Scopus Google Scholar, P.C. H. H. D.J. Price D.L. Van Sisodia S.S. Nature. PubMed Scopus Google Scholar), the mice are and and only and with A. D. Annaert W. Saftig P. Craessaerts K. Serneels L. L. F. Vanderstichele H. R. P. D. A. Van Leuven F. De Strooper B. Proc. Natl. Acad. Sci. U. S. A. 1999; PubMed Scopus Google Scholar). PS1 and PS2 are present in a of the of significantly and and M.K. Slunt H.H. Thinakaran G. G. M. E. Price D.L. Sisodia S.S. J. 1996; PubMed Google Scholar). The of the putative in PS1 and PS2 to with different overlapping with PS1 and PS2 to in protein G. F. Levesque G. M. D.M. Levesque L. Rogaeva E. Xu D. Liang Y. M. J. Biol. 1998; Full Text Full Text PDF PubMed Scopus Google Scholar). PS1 and PS2 is from the of either protein to the of C. elegans (5Levitan D. Doyle T.G. Brousseau D. Lee M.K. Thinakaran G. Slunt H.H. Sisodia S.S. Greenwald I. Proc. Natl. Acad. Sci. U. S. A. 1996; 93: 14940-14944Crossref PubMed Scopus (343) Google Scholar). the of the PS1 and PS2-mediated γ-secretase activities have yet to be In to the γ-secretase activities by PS1 and (BD) cells from PS1–/–PS2+/+, and PS1–/–PS2–/– mice Ikeda M. H. Bernstein A. 1999; PubMed Scopus Google were used to membranes that were with an in vitro γ-secretase Y.M. Lai M.T. Xu M. Huang Q. DiMuzio Mower J. M.K. Proc. Natl. Acad. Sci. U. S. A. 2000; PubMed Scopus Google Scholar). In these BD cell were transfected with an the C-terminal of APP and the of were the γ-secretase activities associated with the PS1+/–PS2–/– BD cells and PS1–/–PS2+/+ BD cells are by the PS1 γ-secretase complex and the PS2 γ-secretase respectively. This is by the of γ-secretase activity associated with the PS1 and PS2 deficient BD In and of the of and PS1 and PS2 were the and 2 of the PS1 and PS2 was for and on The was to 40 of 2 of and of membranes The were for and in a for The was with of and in of The products were by and to membranes. The was on the for and with a The amounts of the presenilins from the signals were for the of in the PS1 and PS2 the was the was and by using PS1 and signals were with the for the the amounts of PS1 and PS2 with the intensities of the corresponding was used to the relative of the activities of the PS1 and PS2 immunoblot and Cell cells from PS1–/–PS2+/+, and PS1–/–PS2–/– were obtained as Z. Nadeau P. Song W. Donoviel D. Yuan M. Bernstein A. Yankner B.A. Nat Cell Biol. 2000; 2: 463-465Crossref PubMed Scopus (359) Google Scholar). were cultured in modified and BD cell membranes were as Y.M. Lai M.T. Xu M. Huang Q. DiMuzio Mower J. M.K. Proc. Natl. Acad. Sci. U. S. A. 2000; PubMed Scopus Google with the that the cells were with a was determined using a protein with as a protein of the BD Cell membranes PS1 and PS2 were by and to membranes. The membranes were for the of PS1 and PS2 with and respectively. signals were using the for the as for in vitro γ-secretase and of proteins were as (12Li Y.M. Xu M. Lai M.T. Huang Q. Castro J.L. DiMuzio Mower J. Harrison T. Lellis C. Nadin A. Neduvelil J.G. Register R.B. M.K. M.S. Nature. 2000; PubMed Scopus Google Scholar, Y.M. Lai M.T. Xu M. Huang Q. DiMuzio Mower J. M.K. Proc. Natl. Acad. Sci. U. S. A. 2000; PubMed Scopus Google Scholar). PS2 in the Y.M. Lai M.T. Xu M. Huang Q. DiMuzio Mower J. M.K. Proc. Natl. Acad. Sci. U. S. A. 2000; PubMed Scopus Google that anti-PS1 γ-secretase activity from cell membranes. of the PS1 that PS2 is not present two of with anti-PS1 is PS1 in the the PS2 is not as compared with the that PS2 is not engaged in the complex that displays γ-secretase activity. of and with more PS1- and PS2-mediated γ-secretase to the relative amounts of PS1 and PS2 in the different BD cell PS1 and PS2 standards were produced by in vitro of the corresponding presenilin in the of The radiolabeled products were by and to membranes. the of membranes amounts of the radiolabeled products generated with the PS1 and PS2 PS1 and PS2 contain and respectively. of signals for the different of in PS1 and is of amounts of The the products were also with the or The for the immunoblot was much than the for the was hence, the immunoblot is not by the The intensities of the and immunoblot signals as a of the amounts of PS1 and PS2, are in that the immunoblot for of is more than the corresponding immunoblot for of of PS1 and PS2 in the BD in the various BD cells was characterized by using and PS1 and PS2 were in membranes and respectively). The relative amounts of PS1 and PS2 in the membranes were by with the corresponding PS1 or PS2 standard curves the amount of PS2 in cell membranes appears to be of the amount of PS1 but not PS2 was in PS1+/–PS2–/– membranes 2 and respectively). PS2 but not PS1 was in PS1–/–PS2+/+ cell membranes and respectively). from PS1–/–PS2–/– BD cells not protein that with either the or and respectively). The PS1 in PS1+/–PS2–/– cell membranes was only of that with wild-type cell membranes the higher protein in 2 compared with The of PS1 in the PS1+/–PS2–/– cell membranes as compared with the cell membranes was by with the of PS2 in and cell membranes and a of PS2 in the PS1-deficient In by BD Cell γ-secretase activities by the various BD cell membranes were with the in vitro γ-secretase using the Y.M. Lai M.T. Xu M. Huang Q. DiMuzio Mower J. M.K. Proc. Natl. Acad. Sci. U. S. A. 2000; PubMed Scopus Google Scholar). PS1–/–PS2–/– cell membranes fail to the of the or products not that activity by BD cell membranes using the in vitro γ-secretase is to PS1 or the amount of PS1 present in the PS1+/–PS2–/– cell membranes of the in cell the amount of total generated by these membranes was of that produced by cell membranes membranes from the PS1–/–PS2+/+ BD which serve as a of PS2 γ-secretase the of the Aβ-peptides from total generated by PS1–/–PS2+/+ cell membranes is only of that produced by cell vitro and γ-secretase activity of BD cell of cell membranes cell were transfected with of The cell membranes The cell were transfected with of in a of the in vitro γ-secretase activities of the PS1+/–PS2–/– and PS1–/–PS2+/+ membranes with the of the relative of PS1 and PS2 in these for the enzymatic activities of the PS1 γ-secretase complex and the PS2 γ-secretase complex the that the relative activity of the PS1 γ-secretase complex in the PS1+/–PS2–/– cell is than that of the PS2 γ-secretase complex in the PS1–/–PS2+/+ cell with This on the that PS1 and PS2 are engaged in the putative γ-secretase complex in these membranes. The amount of Aβ42 generated with cell membranes was total PS1+/–PS2–/– and PS1–/–PS2+/+ cell membranes also the of the and from The amount of Aβ42 to total produced by the PS1- and membranes is and respectively. the PS1 γ-secretase complex may be more than the PS2 γ-secretase the Aβ42 The comparison of the activities of PS1 and PS2 suggests that the PS1 γ-secretase complex is for the of the in vitro γ-secretase activity by cell membranes. it is that and cell membranes generate amounts of total more PS1 in the cells compared with the The specific activity of the PS1 γ-secretase complex in the PS1+/–PS2–/– cell membranes appears to the specific activity of the PS1 γ-secretase complex in the cell membranes. the amount of the cell membranes in the in a in the amount of not the of PS1 in the cell membranes not to the of the in vitro to PS1 activity. by BD the C-terminal of various BD cells were transfected with an the APP that to the C-terminal produced by β-secretase cleavage. The BD cells were to the for was on the cells for an and the was and for Aβ40 and The BD cells were with a protein to for with to and cell The amount of the Aβ-peptides that in of the various transfected BD cells for on the protein is in I. The PS1–/–PS2–/– BD cells fail to Aβ40 or Aβ42 not which is an that is with (9Herreman A. Serneels L. Annaert W. Collen D. Schoonjans L. De Strooper B. Nat Cell Biol. 2000; 2: 461-462Crossref PubMed Scopus (450) Google Scholar, 10Zhang Z. Nadeau P. Song W. Donoviel D. Yuan M. Bernstein A. Yankner B.A. Nat Cell Biol. 2000; 2: 463-465Crossref PubMed Scopus (359) Google Scholar). the of PS1 protein in PS1+/–PS2–/– cells is only of that in BD cells the amounts of total by the cells is of that by the The of PS2 protein in PS1–/–PS2+/+ BD cells is higher than that in BD the cells only produced by the The relative activity of the PS1 γ-secretase complex in PS1+/–PS2–/– BD appears to be than that of the PS2 γ-secretase complex in PS1–/–PS2+/+ BD are with the studies with the BD cell that PS1 higher relative activity in the substrate. Furthermore, it suggests that from cells is produced by PS1-mediated γ-secretase activity. The amount of Aβ42 to total by BD cells was The corresponding of Aβ42 by PS1+/–PS2–/– BD cells PS1 and PS1–/–PS2+/+ BD cells PS2 were and respectively. in to studies with the cell it appears that the PS2 γ-secretase complex in the may have a than the PS1 γ-secretase complex to the PS1-mediated with of the PS1 activity of PS1 in the BD cells from either specific activity of the PS1-associated γ-secretase complex or from PS1 not engaged in the active γ-secretase complex. The active site-directed γ-secretase which to the active form of (12Li Y.M. Xu M. Lai M.T. Huang Q. Castro J.L. DiMuzio Mower J. Harrison T. Lellis C. Nadin A. Neduvelil J.G. Register R.B. M.K. M.S. Nature. 2000; PubMed Scopus Google Scholar), was used as a to the of PS1 in the BD cell membranes. displays γ-secretase activity in membranes from and PS1+/–PS2–/– BD cells not amounts of protein from the different BD cell membranes were with and the were to of active γ-secretase. The proteins were isolated with and by using anti-PS1 and anti-PS2 were the PS1–/–PS2–/– cell membranes were not Moreover, the of the PS1 and PS2 were the was in the of not of the signals by more PS1 in cell membranes than in PS1+/–PS2–/– cell membranes. Moreover, is less PS2 in cell membranes than in PS1–/–PS2+/+ cell membranes. that γ-secretase activity better with the amount of PS1 rather than total amount of PS1 in and BD cell membranes. PS1- and PS2-mediated potent γ-secretase M.S. D. Harrison T. P. Nadin A. G. Castro J.L. 2000; PubMed Scopus Google Scholar), of by BD cell membranes The was from the that was with the from cell membranes Y.M. Lai M.T. Xu M. Huang Q. DiMuzio Mower J. M.K. Proc. Natl. Acad. Sci. U. S. A. 2000; PubMed Scopus Google Scholar). The PS1-mediated γ-secretase activity by PS1+/–PS2–/– BD cell membranes and the PS2-mediated γ-secretase activity by PS1–/–PS2+/+ BD cell membranes were both by and respectively). the PS1 γ-secretase complex appears to be more to inhibition by than is the PS2 γ-secretase complex. This of to PS1 and PS2 was also with membranes from derived from and mice not the other γ-secretase more γ-secretase activities by PS1+/–PS2–/– and PS1–/–PS2+/+ BD cell membranes The of C for the PS1- and PS2-mediated γ-secretase activities were and respectively. is that the of the inhibitors determined with the cell membranes are more to the determined with PS1+/–PS2–/– BD membranes than to the determined with PS1–/–PS2+/+ cell membranes evidence that the complex is the of γ-secretase activity in BD cells and their corresponding membranes. The of a active site-directed γ-secretase inhibitor (12Li Y.M. Xu M. Lai M.T. Huang Q. Castro J.L. DiMuzio Mower J. Harrison T. Lellis C. Nadin A. Neduvelil J.G. Register R.B. M.K. M.S. Nature. 2000; PubMed Scopus Google to both PS1 and PS2 suggests that the presenilins have overlapping This is with studies both PS1 and PS2 in APP and Notch (9Herreman A. Serneels L. Annaert W. Collen D. Schoonjans L. De Strooper B. Nat Cell Biol. 2000; 2: 461-462Crossref PubMed Scopus (450) Google Scholar, 10Zhang Z. Nadeau P. Song W. Donoviel D. Yuan M. Bernstein A. Yankner B.A. Nat Cell Biol. 2000; 2: 463-465Crossref PubMed Scopus (359) Google Scholar, A. D. Annaert W. Saftig P. Craessaerts K. Serneels L. L. F. Vanderstichele H. R. P. D. A. Van Leuven F. De Strooper B. Proc. Natl. Acad. Sci. U. S. A. 1999; PubMed Scopus Google Scholar, Ikeda M. H. Bernstein A. 1999; PubMed Scopus Google Scholar). the of the presenilins (67% identity) suggests that PS1 and PS2 may with to their The exhibited by the PS1- and mice may J. Xia W. Selkoe D.J. S. Full Text Full Text PDF PubMed Scopus Google Scholar, P.C. H. H. D.J. Price D.L. Van Sisodia S.S. Nature. PubMed Scopus Google Scholar). the of the PS1 and PS2 in mice and cells from these are not of or but of the these PS1- and PS2-mediated activities were individually using BD cells from and PS1–/–PS2+/+ mice, respectively. a on of immunoblot signals to the relative of PS1 and PS2 proteins in the different BD cell The of PS1 and PS2 proteins in BD cells is The amount of PS1 protein in PS1+/–PS2–/– BD cells is only of that in which is far less than the The less amount of PS1 protein in PS1+/–PS2–/– BD cells more or of the PS1-associated the other the amount of PS2 in PS1–/–PS2+/+ cells is that the of the PS2 in The higher of PS2 in PS1-deficient cells of increased of that are by PS1 in wild-type that the PS1 γ-secretase complex displays activity than the PS2 γ-secretase complex both in the in vitro using the and the using the substrate. that PS1 is more active than PS2 the APP fragments that are generated by of biochemical evidence that PS1 a role in the of APP, a on studies with cells derived from PS1 or (7De Strooper B. Saftig P. Craessaerts K. Vanderstichele H. Guhde G. Annaert W. Von Figura K. Van Leuven F. Nature. 1998; 391: 387-390Crossref PubMed Scopus (1552) Google Scholar, A. D. Annaert W. Saftig P. Craessaerts K. Serneels L. L. F. Vanderstichele H. R. P. D. A. Van Leuven F. De Strooper B. Proc. Natl. Acad. Sci. U. S. A. 1999; PubMed Scopus Google Scholar). The that PS1- and PS2-mediated activities different to γ-secretase inhibitors that PS1 and PS2 have unique active sites that PS1+/–PS2–/– and PS1–/–PS2+/+ BD cells and the corresponding cell membranes γ-secretase activities that generated both Aβ40 and The of Aβ42 total to be and only different for the PS1- and PS2-mediated γ-secretase PS1 generated more than PS2 in the in vitro the was in the The for the of Aβ40 and Aβ42 with the in vitro and is but it may the of or in the studies that Aβ42 is not by only of the two PS1 and PS2 are of The different PS1 in the and cells were not by a corresponding in their γ-secretase The PS1 and γ-secretase activity is not of a in in which the of is in the of a amount of active γ-secretase using a inhibitor to on the of the used in the is much higher than its the of the relative amount of the active in the membranes. The amount of PS1 in cell membranes is only than that in PS1+/–PS2–/– cell PS1 in wild cell membranes is more than that in cell membranes. the amount of the is more with the relative γ-secretase activity by the cell membranes. that PS1 in PS1+/–PS2–/– cell membranes is engaged in the active γ-secretase only total PS1 in the BD cells is engaged in the active γ-secretase complex. The of the γ-secretase complex that are for activity to be that the PS1 may be but not for γ-secretase activity. This is with studies in which of PS1 or of its and fragments not γ-secretase activity D. Lee J. Song L. R. Wong G. E. L. Proc. Natl. Acad. Sci. U. S. A. 2001; PubMed Scopus Google Scholar). G. M. S. D. L. A. Song Rogaeva E. F. T. A. Levesque L. Yu H. E. P. Liang Y. D.M. Xu C. Rogaev E. M. C. Y. R. S. A. P. P. Nature. 2000; PubMed Scopus Google Scholar), C. M. Proc. Natl. Acad. Sci. U. S. A. PubMed Scopus Google Scholar, R. G. J. M. J. M. M. B. Xu W. Li J. M. R. C. D. 3: Full Text Full Text PDF PubMed Scopus Google Scholar), and R. G. J. M. J. M. M. B. Xu W. Li J. M. R. C. D. 3: Full Text Full Text PDF PubMed Scopus Google are proteins that are engaged in the complex and may be for γ-secretase activity. putative proteins may be in the that to γ-secretase activity. The PS1 complex may other functions as and regulation of inhibitors serve as to the functions of the presenilins the other putative functions that only a small portion of the PS1 in membranes is involved in an active γ-secretase complex may to γ-secretase The of γ-secretase activity to the presenilins been the presenilins and the γ-secretase not to be to the P. M. Craessaerts K. I. Vanderstichele H. Saftig P. De Strooper B. Annaert W. J. Cell Biol. 2001; PubMed Scopus Google Scholar). in the early of the the γ-secretase Notch or the M.T. A. D.J. R. Mol. 2000; Full Text Full Text PDF PubMed Scopus Google Scholar). that only a small portion of PS1 is engaged in the active γ-secretase complex in with evidence for a small amount of presenilins on the cell M. J. Saftig P. M. Selkoe D.J. R. J. Biol. 1999; Full Text Full Text PDF PubMed Scopus Google may have are also involved in the of Notch that the Notch which is involved in of transcription G. 2000; PubMed Scopus Google Scholar). studies P. M.S. Proc. Natl. Acad. Sci. U. S. A. 2001; PubMed Scopus Google Scholar, D. M.S. R. Proc. Natl. Acad. Sci. U. S. A. 2001; PubMed Scopus Google that inhibition of γ-secretase activity by inhibitors the Notch activity and cell be to studies PS1 and PS2-mediated γ-secretase activities Notch the of PS1 and PS2 different and inhibitors to γ-secretase inhibitors for Moreover, it is to the of to presenilins by neurons and cells J. PubMed Scopus Google Scholar). γ-secretase activity is believed to in two in the M. X. J. 2000; PubMed Google and γ-secretase activity in neurons and cells may contribute to and in other In studies in the the presenilins and γ-secretase activity. were as the of the PS1 γ-secretase complex and the PS2 γ-secretase complex. other in the PS1 and γ-secretase activity were as the with to the activity of PS1 in wild-type BD The that is exhibited by the presenilins their in a of biological that to be C. and K. for with the and S. for the