IDENTIFICATION, CLONING, AND EXPRESSION OF A CYTOSOLIC MEGAKARYOCYTE PROTEIN-TYROSINE-PHOSPHATASE WITH SEQUENCE HOMOLOGY TO CYTOSKELETAL PROTEIN 4.1

We have isolated a cDNA encoding a third type of protein-tyrosine-phosphatase. We screened human megakaryoblastic cell line (MEG-01) and umbilical vein endothelial cell cDNA libraries to obtain a 3.7-kilobase cDNA designated PTPase MEG. Northern blot analysis of MEG-01 RNA detected a 3.7-kilobase transcript, suggesting that a full-length cDNA has been identified. PTPase MEG cDNA contains an open reading frame of 926 amino acids. The cDNA has a G + C-rich 5' untranslated region of 771 nucleotides that has the potential to form stable stem-loop structures and has two upstream ATG codons. The predicted protein (M(r) = 105,910) has no apparent membrane-spanning region and contains a single protein-tyrosine-phosphatase domain (amino acids 659-909) that is 35-40% identical to previously described tyrosine-phosphatase domains. The recombinant phosphatase domain possesses protein-tyrosine-phosphatase activity when expressed in Escherichia coli. The amino-terminal region (amino acids 31-367) is 45% identical to the amino terminus of human erythrocyte protein 4.1, a cytoskeletal protein. The identification of a protein-tyrosine-phosphatase that is related to cytoskeletal proteins implies that cell signaling activities reside not only in transmembrane receptors but in cytoskeletal elements as well.