K M Ferguson

Pleckstrin homology (PH) domains are found in many signaling molecules and are thought to be involved in specific intermolecular interactions. Their binding to several proteins and to membranes containing 1-alpha-phosphatidylinositol 4,5-bisphosphate [PtdIns(4,5)P2] has been reported. A region that includes the PH domain has also been implicated in binding of phospholipase C-delta 1 (PLC-delta 1) to both PtdIns(4,5)P2 and D-myo-inositol 1,4,5-trisphosphate [Ins(1,4,5)P3] [Cifuentes, M. E., Delaney, T. & Rebecchi, M. J. (1994) J. Biol. Chem. 269, 1945-1948]. We report herein that the isolated PH domain from PLC-delta 1 binds to both PtdIns(4,5)P2 and Ins(1,4,5)P3 with high affinity and shows the same binding specificity seen by others with whole PLC-delta 1. Thus the PH domain is functionally and structurally modular. These results demonstrate stereo-specific high-affinity binding by an isolated PH domain and further support a functional role for PH domains in the regulation of PLC isoforms. Other PH domains did not bind strongly to the compounds tested, suggesting that inositol phosphates and phospholipids are not likely physiological ligands for all PH domains. Nonetheless, since all PH-domain-containing proteins are associated with membrane surfaces, several PH domains bind to specific sites on membranes, and PH domains appear to be electrostatically polarized, a possible general role for PH domains in membrane association is suggested.

Adenylate cyclase can be resolved into at least two proteins, a thermolabile, N-ethylmaleimide-sensitive component and a second protein (or proteins) that is more stable to either of these treatments. Neither component by itself catalyzes the formation of cyclic AMP using MgATP as substrate. However, mixture of the two reconstitutes MgATP-dependent fluoride- and guanyl-5'-yl imidodiphosphate (Gpp(NH)p)-stimulatable adenylate cyclase activity. The more stable component can be resolved from the first in various tissues or cultured cells by treatment of membrnes or detergent extracts with heat or N-ethylmaleimide. The two proteins have also been resolved genetically in two clonal cell lines that are deficient in adenylate cyclase activity. An adenylate cyclase-deficient variant of the S49 lymphoma cell (AC-) contains only the thermolabile activity, while the activity of the more stable protein is found in a complementary hepatoma cell line (HC-1). In addition, AC-S49 cell plasma membranes contain MnATP-dependent adenylate cyclase activity. The protein that catalyzes this reaction appears to be the same as that which can combine with the thermostable component to reconstitute Mg2+-dependent enzyme activity because both activities co-fractionate by gel exclusion chromatography and sucrose density gradient centrifugation, both activities have identical denaturation kinetics at 30 degrees C, and both activities are stabilized at 30 degrees C and labilized at 0 degree C by various nucleotides and divalent cations with similar specificity. It is thus hypothesized that the thermolabile factor is the catalytic subunit of the physiological adenylate cyclase and that the Mn2+-dependent activity is a nonphysiological expression of the catalytic protein. The thermostable moiety of the enzyme, which is proposed to serve a regulatory function, appears to consist of two functional components, based upon differential thermal lability of its ability to reconstitute hormone-, NaF-, or Gpp(NH)p-stimulated adenylate cyclase activity. These components have not, however, been physically separated. The thermolabile and thermostable components can interact in detergent solution or in a suitable membrane. Mixing of the detergent-solubilized regulatory component with AC-membranes that contain only the catalytic protein and beta-adrenergic receptors reconstitutes catecholamine-stimulatable adenylate cyclase activity; however, addition of the catalytic protein to membranes that contain receptor and the regulatory component yields MgATP-dependent enzymatic activity that is unresponsive to hormone.

Mg2+ interacts with the alpha subunits of guanine nucleotide-binding regulatory proteins (G proteins) in the presence of guanosine-5'-[gamma-thio]triphosphate (GTP-gamma S) to form a highly fluorescent complex from which nucleotide dissociates very slowly. The apparent Kd for interaction of G alpha X GTP gamma S with Mg2+ is approximately 5 nM, similar to the Km for G protein GTPase activity X G beta gamma increases the rate of dissociation of GTP gamma S from G alpha X GTP gamma S or G alpha X GTP gamma S X Mg2+ at low concentrations of Mg2+. When the concentration of Mg2+ exceeds 1 mM, G beta gamma dissociates from G beta gamma X G alpha X GTP gamma S X Mg2+. Compared with the dramatic effect of Mg2+ on binding of GTP gamma S to G alpha, the metal has relatively little effect on the binding of GDP. However, G beta gamma increases the affinity of G alpha for GDP by more than 100-fold. High concentrations of Mg2+ promote the dissociation of GDP from G beta gamma X G alpha X GDP, apparently without causing subunit dissociation. The steady-state rate of GTP hydrolysis is strictly correlated with the rate of dissociation of GDP from G alpha under all conditions examined. Thus, there are at least two sites for interaction of Mg2+ with G protein-nucleotide complexes. Furthermore, binding of G beta gamma and GTP gamma S to G alpha is negatively cooperative, while the binding interaction between G beta gamma and GDP is strongly positive.

Purified guanine nucleotide-binding regulatory proteins, as either the oligomers or the isolated nucleotide-binding alpha subunits, display anomalous kinetics of nucleotide binding. This is due to the presence of tightly bound GDP in these preparations. The dissociation of bound GDP is the rate-limiting step for nucleotide binding. GDP can be removed by chromatography in the presence of 1 M (NH4)2SO4 and 20% glycerol, which yields preparations of G proteins that contain less than 0.1 mol of GDP/mol of guanosine 5'-(gamma-thio)triphosphate (GTP gamma S)-binding site. When the GDP is removed, the binding of GTP gamma S displays kinetics consistent with a bimolecular reaction.