THE MBP FUSION PROTEIN RESTORES THE ACTIVITY OF THE FIRST PHOSPHATASEDOMAIN OF CD45

CD45 is a receptor-like protein tyrosine phosphatase critically involv ed in the regulation of initial effector functions in B- and T-cells. The protein comprises two phosphatase (PTP) domains in its cytoplasmic region. However, whether each PTP domain has enzyme activity by itsel f or whether both domains are required to build up a functional enzyme is unclear. We have studied different constructions of human CD45 com prising the two PTP domains, both separately and as a single protein, fused to maltose-binding protein (MBP). In apparent contrast with prev ious studies, we show that the first PTP domain of CD45 (when fused to MBP) may be a viable phosphatase in the absence of the second domain. Phosphatase activity resides in the monomeric form of the protein and is lost after proteolytic cleavage of the fusion partner, indicating that MBP specifically activates the first PTP domain. Furthermore, cha nges in the optimal pH for activity with respect to wild-type CD45 sug gest that protein-protein interactions involving residues in the neigh bourhood of the catalytic site mediate enzyme activation. (C) 1997 Fed eration of European Biochemical Societies.