Point mutation in the second phosphatase domain of CD45 abrogates tyrosine phosphatase activity

CD45 is a transmembrane protein tyrosine phosphatase that possesses two phosphatase domains in its cytoplasmic region. Whether both domains function independently as phosphatase enzymes or whether both domains interact to form an active enzyme is unclear. A point mutation of a critical cysteine residue in domain I is known to abolish CD45 activity, implying that the catalytic activity resides in domain I. In this report, mutational analysis of purified, recombinant CD45 cytoplasmic domain protein was performed. It was found that a single amino acid change in domain II (glutamine 1180 to a glycine) resulted in an inactive phosphatase enzyme, whereas two other point mutations in the membrane proximal region of the molecule had no effect on activity. Deletion of the region linking the two phosphatase domains also abolished enzymatic activity. Amino acids crucial for phosphatase activity thus reside in both phosphatase domains of CD45, illustrating that the phosphatase domains of CD45 do not act independently, but are both required for the phosphatase activity of CD45.