A previous study on a yeast protein tyrosine phosphatase, YPTPI, using synthetic phosphotyrosine-containing peptides with various sequences as substrates revealed that DADEpYDA exhibits high affinity (K_m = 4 μM) toward the enzyme. A modified version of this peptide, GDADEpmFDA, immobilized on a resin, was used in this study as an affinity ligand for the purification of YPTP1. Phosphonomethyl-phenylalanine (pmF) was used as a nonhydrolyzable analog of the phosphotyrosine (pY) residue, with properties similar to pY. A protected form of pmF, Fmoc-pmF(^tBu)₂-OH, was chemically synthesized and introduced during solid-phase peptide sythesis. YPTPI was overexpressed in an E. coli strain carrying a plasmid pT7-7-ptp1. Affinity chromatography of the crude lysate afforded PTP1 (39 kDa) of about 50% purity.