INTERACTIONS BETWEEN A MINIMAL PROTEIN SERINE THREONINE PHOSPHATASE AND ITS PHOSPHOPEPTIDE SUBSTRATE SEQUENCE/

The protein phosphatase encoded by coliphage lambda (PP lambda) was fo und to be the equivalent of the minimal catalytic core of serine/threo nine protein phosphatases (PP) by biochemical and mutational criteria. Bacterially expressed truncated versions of PP1 and PP5 phosphatases, representing the catalytic cores homologous to PP lambda, exhibited p otent phosphatase activity. Unlike full-length PP1, but like PP lambda , the recombinant cores could use casein, p-nitrophenyl phosphate, and a wide variety of peptides as substrates and were resistant to okadai c acid, microcystin-LR, and trypsin. Mutations of His(173), Asp(208), or Arg(221) had little effect on the activity of the PP1 core protein, indicating its closer identity with PP lambda than with full-length P P1. Terminal deletions of a few amino acids of the cores destroyed the ir activity, supporting their minimal nature, Analysis of PP lambda mu tants suggested an influence of the substrate on metal ion binding. Th e minimal length of a phosphopeptide substrate of PP lambda appeared t o be a phosphorylated serine/threonine flanked by 1 or 2 amino acid re sidues on either side, the N-terminal ones being more effective.