The cystatin superfamily is comprised of a large group of ancestrally related proteins, most of which are potent inhibitors of cysteine peptidases (CPs). Human saliva contains relatively abundant proteins that are related ancestrally in sequence to the cystatin superfamily. These proteins are now commonly referred to as 'salivary cystatins', although this is something of a misnomer (see below). It has been several years since the publication of the last comprehensive reviews that focused on the salivary cystatins (Bobek and Levine, 1992;Henskens et al., 1996c). Since then, significant advances have been made regarding the gene organization, protein structure, and properties of human and rat proteins. Therefore, a major purpose of this article is to review this recent work. What is the function of the 'salivary cystatins'? Despite a great deal of research effort, we still do not have a definitive answer to this question. A major impediment is the lack of a disease to associate with a specific defect in a gene: Functionality must be inferred from circumstantial evidence. A central premise of this review is that clues to 'salivary cystatin' function are contained within their phylogenetic history, and that by looking at established or likely functions of their 'siblings' and 'cousins', one can assess the likelihood of this function(s) continuing into the 'salivary cystatins'. A comprehensive review of the large cystatin superfamily is beyond the scope of a single article. This review will aim to outline the main branches of the superfamily and the common elements of their structure and function, but will pay closest attention to the nearest relatives of the 'salivary cystatins'.(1) BACKGROUND Scission of peptide bonds is an essential reaction in living cells, and there is a considerable number of peptidases that catalyze this reaction with various degrees of specificity. CPs use a cysteine residue in the active site as the nucleophile in the reaction (see Dickinson, 2002, and references therein). Release of proteolytic enzymes outside of their normal compartment has the potential to cause serious degradation and pathology-an effect taken advantage of by pathogenic organisms from a wide range of phyla. Therefore, control of proteolytic enzyme activity is mandatory. Building upon a very early report (see Barrett et al., 1986, for a review of this earlier literature) of a bovine trypsin inhibitor in chicken egg white, a search for egg white inhibitors of plant proteinases (the CPs ficin and papain) identified a relatively small (12.7 kDa) protein that was a potent inhibitor (Ki = 10 nM). The term cystatin was coined for this protein, which was shown also to inhibit mammalian CPs of the papain superfamily, including cathepsins B, C, H, and L. Cystatins form 1:1 reversible complexes with CPs, in competition with the substrate, but in some cases the binding is so tight as to be physiologically irreversible.
(2) THE CYSTATIN SUPERFAMILYExamination of mammalian tissues and serum revealed several CP inhibitors ranging in si...
