The two tandem homologous catalytic domains of PTPα possess different kinetic properties, with the membrane proximal domain (D1) exhibiting much higher activity than the membrane distal (D2) domain. Sequence alignment of PTPα-D1 and -D2 with the D1 domains of other receptor-like PTPs, and modeling of the PTPα-D1 and -D2 structures, identified two nonconserved amino acids in PTPα-D2 that may account for its low activity. Mutation of each residue (Val-536 or Glu-671) to conform to its invariant counterpart in PTPα-D1 positively affected the catalytic efficiency of PTPα-D2 toward the in vitro substrates para-nitrophenylphosphate and the phosphotyrosyl-peptide RR-src. Together, they synergistically transformed PTPα-D2 into a phosphatase with catalytic efficiency for paranitrophenylphosphate equal to PTPα-D1 but not approaching that of PTPα-D1 for the more complex substrate RR-src. In vivo, no gain in D2 activity toward p59(fyn) was effected by the double mutation. Alteration of the two corresponding invariant residues in PTPα-D1 to those in D2 conferred D2-like kinetics toward all substrates. Thus, these two amino acids are critical for interaction with phosphotyrosine but not sufficient to supply PTPα-D2 with a D1-like substrate specificity for elements of the phosphotyrosine microenvironment present in RR-src and p59(fyn). Whether the structural features of D2 can uniquely accommodate a specific phosphoprotein substrate or whether D2 has an alternate function in PTPα remains an open question.
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