COMPARISON OF THE SPECIFICITY OF BACTERIALLY EXPRESSED CYTOPLASMIC PROTEIN-TYROSINE PHOSPHATASES SHP AND SH-PTP2 TOWARDS SYNTHETIC PHOSPHOPEPTIDE SUBSTRATES

SHP and SH-PTP2 are related cytoplasmic protein-tyrosine phosphatases having two tandem aminoterminal src homology 2 domains linked to a sin gle catalytic domain. There is growing evidence that these two molecul es may exhibit opposing effects within specific signaling pathways. Ho wever, the relative contributions of the src homology 2 domains or the catalytic domains to these opposing effects are not well known. To ev aluate the potential contribution of the catalytic domains, we compare d the substrate specificity of the two phosphatases. As seen previousl y, the catalytic activities of bacterially expressed SHP and SH-PTP2 w ere regulated by the presence of the linked src homology 2 domains. In addition, we characterized a cryptic thrombin cleavage site within th e carboxy-terminus of SHP that led to a striking increase in the activ ity of the catalytic domain. Employing a panel of phosphopeptide subst rates whose sequences were modeled after intracellular phosphorylation sites, both SHP and SH-PTP2 demonstrated a similar specificity patter n. Similar to SH-PTP2, SHP failed to elicit detectable phosphate relea se from several phosphopeptide substrates, while displaying catalytic efficiencies that ranged over approximate to 40-1.6X10(3) M(-1) s(-1) towards other substrates. In contrast, the PTP-1B phosphatase dephosph orylated all of the phosphopeptide substrates tested with approximatel y equal ease. The overall similarity demonstrated by the catalytic dom ains of SHP and SH-PTP2 suggested that differences in the in vivo beha vior of these two molecules might not stem from differences in the sub strate specificity of the catalytic domains, suggesting instead that t he specificity of the src homology 2 domains is more important in this regard.