MAPPING THE FODRIN BINDING DOMAIN IN CD45, A LEUKOCYTE MEMBRANE-ASSOCIATED TYROSINE PHOSPHATASE

CD45 belongs to a family of high molecular mass leukocyte glycoprotein s. It contains both an intrinsic protein tyrosine phosphatase (PTPase) activity and a cytoskeleton binding site in its cytoplasmic domain. C ertain cytoskeletal proteins, such as fodrin (a spectrin like molecule ), are known to play an important role in the regulation of CD45's PTP ase activity. In this study we mapped the fodrin binding domain of CD4 5 by deleting various portions of the cytoplasmic region, followed by the expression of these truncated cDNAs using an in vitro transcriptio n/translation system. The results of these experiments indicate that t he CD45 fodrin binding domain resides between amino acids 825 and 939. Construction of a fusion protein encoding the region between amino ac ids 825 and 939 shows that this particular sequence itself is sufficie nt for fodrin binding. Further analyses indicate that the sequence ((9 30)EENKKKNRN(939)S) in CD45 has good sequence homology with the spectr in binding domain found in the MSP1 glycoprotein of the malarial paras ite. Biochemical studies, using binding competition assays, and a synt hetic peptide containing the sequence (930)EENKKKNRN(939)S, support th e conclusion that the sequence between amino acids 930 and 939 is a cr itical part of CD45's fodrin binding domain. Further analyses indicate that this sequence is also involved in the fodrin-induced up-regulati on of CD45 PTPase activity. Therefore, we suggest that fodrin binding to this domain is required for the onset of CD45-mediated signal trans duction and leukocyte activation.