THE ATOMIC MODEL OF THE HUMAN PROTECTIVE PROTEIN CATHEPSIN-A SUGGESTSA STRUCTURAL BASIS FOR GALACTOSIALIDOSIS

Human protective protein/cathepsin A (PPCA), a serine carboxypeptidase , forms a multienzyme complex with beta-galactosidase and neuraminidas e and is required for the intralysosomal activity and stability of the se two glycosidases. Genetic lesions in PPCA lead to a deficiency of b eta-galactosidase and neuraminidase that is manifest as the autosomal recessive lysosomal storage disorder galactosialidosis. Eleven amino a cid substitutions identified in mutant PPCAs from clinically different galactosialidosis patients have now been modeled in the three-dimensi onal structure of the wild-type enzyme. Of these substitutions, 9 are located in positions likely to alter drastically the folding and stabi lity of the variant protein, In contrast, the other 2 mutations that a re associated with a more moderate clinical outcome and are characteri zed by residual mature protein appeared to have a milder effect on pro tein structure. Remarkably, none of the mutations occurred in the acti ve site or at the protein surface, which would have disrupted the cata lytic activity or protective function. Instead, analysis of the 11 mut ations revealed a substantive correlation between the effect of the am ino acid substitution on the integrity of protein structure and the ge neral severity of the clinical phenotype. The high incidence of PPCA f olding mutants in galactosialidosis reflects the fact that a single po int mutation is unlikely to affect both the beta-galactosidase and the neuraminidase binding sites of PPCA at the same time to produce the d ouble glycosidase deficiency, Mutations in PPCA that result in defecti ve folding, however, disrupt every function of PPCA simultaneously.