EXAMINATION OF THE POTENTIAL STRUCTURE OF HUMAN SALIVARY CYSTATINS BASED ON COMPUTER MODELING

The cystatin family of proteins exists in both excreted and intracellu lar forms, and appears to be involved in protective and regulatory rol es, inhibiting a variety of bacterial, viral and intracellular proteas es. The amino acid sequences of several human forms of cystatin are kn own, but currently only the structure of chicken cystatin (approx. 40% homologous to the human forms) has been experimentally determined. Th e objective of this study was to use the X-ray coordinates of chicken cystatin to construct computer models of the structures of three human salivary forms (SN, S and SA). These structures were energy-minimized and subjected to dynamic simulations. The resultant structures were c ompared to determine conformational differences. Global root mean squa re deviations between equivalent atoms ranged from 1.4 Angstrom to 3.9 Angstrom. The closest structural similarity to chicken cystatin invol ved cystatin SN, which also showed the highest (68%) functional sequen ce homology. Local secondary structure was examined in more detail. In comparisons of alpha-carbon position the third beta-strand (77% funct ional sequence conservation) and its preceding loop (60% conserved) sh owed the highest structural conservation in S, while beta-strand 4 sho wed the highest structural conservation in SN and SA. Through out thei r structures, SN and SA were more structurally similar to chicken cyst atin than to salivary cystatin S. There are two regions of conserved, negatively charged residues in the salivary cystatins, which appear to be spaced so that they are capable of interaction with hydroxyapatite . It is concluded that not only does structural modelling by analogy p rovide detailed models of salivary cystatins that can be tested by Fut ure experimentation, but also that examination of the models has revea led potential sites of interaction with hydroxyapatite. (C) 1997 Elsev ier Science Ltd.