Isabelle Callebaut

The regions surrounding the catalytic amino acids previously identified in a few "retaining" O-glycosyl hydrolases (EC 3.2.1) have been analyzed by hydrophobic cluster analysis and have been used to define sequence motifs. These motifs have been found in more than 150 glycosyl hydrolase sequences representing at least eight established protein families that act on a large variety of substrates. This allows the localization and the precise role of the catalytic residues (nucleophile and acid catalyst) to be predicted for each of these enzymes, including several lysosomal glycosidases. An identical arrangement of the catalytic nucleophile was also found for S-glycosyl hydrolases (myrosinases; EC 3.2.3.1) for which the acid catalyst is lacking. A (beta/alpha)8 barrel structure has been reported for two of the eight families of proteins that have been grouped. It is suggested that the six other families also share this fold at their catalytic domain. These enzymes illustrate how evolutionary events led to a wide diversification of substrate specificity with a similar disposition of identical catalytic residues onto the same ancestral (beta/alpha)8 barrel structure.

Innate immunity to viral infection involves induction of the type I IFN response; however, dysfunctional regulation of this pathway leads to inappropriate inflammation. Here, we evaluated a nonconsanguineous family of mixed European descent, with 4 members affected by systemic inflammatory and autoimmune conditions, including lupus, with variable clinical expression. We identified a germline dominant gain-of-function mutation in TMEM173, which encodes stimulator of type I IFN gene (STING), in the affected individuals. STING is a key signaling molecule in cytosolic DNA-sensing pathways, and STING activation normally requires dimerization, which is induced by 2'3' cyclic GMP-AMP (cGAMP) produced by the cGAMP synthase in response to cytosolic DNA. Structural modeling supported constitutive activation of the mutant STING protein based on stabilized dimerization. In agreement with the model predictions, we found that the STING mutant spontaneously localizes in the Golgi of patient fibroblasts and is constitutively active in the absence of exogenous 2'3'-cGAMP in vitro. Accordingly, we observed elevated serum IFN activity and a type I IFN signature in peripheral blood from affected family members. These findings highlight the key role of STING in activating both the innate and adaptive immune responses and implicate aberrant STING activation in features of human lupus.

Inherited mutations in BRCA1 predispose to breast and ovarian cancer, but the biological function of the BRCA1 protein has remained largely elusive. The recent correspondence of Koonin et al. [Koonin, E.V., Altschul, S.F. and Bork, P. (1996) Nature Genet. 13, 266-267] has emphasized the potential importance of the BRCA1 C-terminal region for BRCA1-mediated breast cancer suppression, as this domain shows similarities with the C-terminal regions of a p53-binding protein (53BP1), the yeast RAD9 protein involved in DNA repair, and two uncharacterized, hypothetical proteins (KIAA0170 and SPAC19G10.7). The highlighted domain has been suggested to be the result of an internal duplication, each of the tandem domains being designated as a 'BRCT domain' (for BRCA1 C-terminus). Sequence analysis using hydrophobic cluster analysis reveals here the presence of 50 copies of the BRCT domain in 23 different proteins, including, in addition to BRCA1, 53BP1 and RAD9, XRCC1, RAD4, Ect2, REV1, Crb2, RAP1, terminal deoxynucleotidyltransferases (TdT) and three eukaryotic DNA ligases. Most of these proteins are known to be involved in DNA repair. The BRCT domain is not limited to the C-termini of protein sequences and can be found in multiple copies or in a single copy as in RAP1 and TdT, suggesting that it could well constitute an autonomous folding unit of approx. 90-100 amino acids.