Porcine liver low M_r phosphotyrosine protein phosphatase: The amino acid sequence

Abstract

Porcine low M_r phosphotyrosine protein phosphatase has been purified and the complete amino acid sequence has been determined. Both enzymic and chemical cleavages are used to obtain protein fragments. FAB mass spectrometry and enzymic subdigestion followed by Edman degradation have been used to determine the structure of the NH₂-terminal acylated tryptic peptide. The enzyme consists of 157 amino acid residues, is acetylated at the NH₂-terminus, and has arginine as COOH-terminal residue. It shows kinetic parameters very similar to other known low Mr PTPases. This PTPase is strongly inhibited by pyridoxal 5′-phosphate (K=21ΜM) like the low Mr PTPases from bovine liver, rat liver (AcP2 isoenzyme), and human erythrocyte (Bslow isoenzyme). The comparison of the 40–73 sequence with the corresponding sequence of other low Mr PTPases from different sources demonstrates that this isoform is highly homologous to the isoforms mentioned above, and shows a lower homology degree with respect to rat AcP1 and human Bfast isoforms. A classification of low M_r PTPase isoforms based on the type-specific sequence and on the sensitivity to pyridoxal 5‡-phosphate inhibition has been proposed.