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PA_M28_1_3: Protease-associated (PA) domain, peptidase family M28, subfamily-1, subgroup 3. A subgroup of PA-domain containing proteins belonging to the peptidase family M28. Family M28 contains aminopeptidases and carboxypeptidases, and has co-catalytic zinc ions. The PA domain is an insert domain in a diverse fraction of proteases. The significance of the PA domain to many of the proteins in which it is inserted is undetermined. It may be a protein-protein interaction domain. At peptidase active sites, the PA domain may participate in substrate binding and/or promoting conformational changes, which influence the stability and accessibility of the site to substrate. Proteins into which the PA domain is inserted include the following members of the peptidase family M28: i) prostate-specific membrane antigen (PSMA), ii) yeast aminopeptidase Y, and ii) human TfR (transferrin receptor)1 and human TfR2. The proteins listed above belong to other subgroups; relatively little is known about proteins in this subgroup.