COMPARATIVE KINETIC-ANALYSIS AND SUBSTRATE-SPECIFICITY OF THE TANDEM CATALYTIC DOMAINS OF THE RECEPTOR-LIKE PROTEIN-TYROSINE-PHOSPHATASE-ALPHA

The catalytic activity and substrate specificity of protein-tyrosine p hosphatase alpha (PTP alpha) is primarily controlled by the membrane p roximal catalytic domain (D1). The membrane distal (D2) domain of PTP alpha by itself is a genuine PTPase, possessing catalytic activity com parable to that of D1 using aryl phosphates as substrates. Surprisingl y, k(cat) and k(cat)/K-m for the D2-catalyzed hydrolysis of phosphotyr osine-containing peptides are several orders of magnitude reduced in c omparison with those of D1. Substitution of the putative general acid/ base Glu-690 in D2 by an Asp, which is invariably found in the WPD mot hs in all cytoplasmic PTPases and all the D1 domains of receptor-like PTPases, only increases the k(cat) for D2 by 4-fold. Thus the much red uced D2 activity toward peptide substrates may be due to structural di fferences in the active sites other than the general acid/base. Altern atively, the D2 domain may have a functional active site with a highly stringent substrate specificity. PTP alpha display modest peptide sub strate selectivity and are sensitive to charges adjacent to phosphotyr osine. In the sequence context of DADEpYLIPQQG (where pY stands for ph osphotyrosine), the minimal sizes recognized by PTP alpha are either A DEpYLI or DADEpY-NH2.