TYROSINE PHOSPHORYLATION OF A M(R) 38,000 A B-TYPE HNRNP PROTEIN SELECTIVELY MODULATES ITS RNA-BINDING/

The M(r) 38,000 RNA-binding protein (P38) is the major component of tr anslationally repressed messenger ribonucleoproteins in cryptobiotic g astrulae of the brine shrimp Artemia. Partial elucidation of the amino acid sequence of P38 reveals that it is homologous to A/B-type hnRNP proteins. This was confirmed by immunodetection with antibodies specif ic for A/B-type hnRNP proteins from Drosophila melanogaster. P38 can b e phosphorylated in vitro by a src-related protein tyrosine kinase on multiple tyrosine residues located predominantly in the glycine-rich d omain. Tyrosine phosphorylated P38 can be efficiently dephosphorylated by a specific protein tyrosine phosphatase (1B-like) and by protein p hosphatase 2A activated by the phosphotyrosyl phosphatase activator. T yrosine phosphorylation of P38 slightly influences its subsequent phos phorylation by casein kinase II. The latter phosphorylation site is lo cated in the glycine-rich domain of P38. Two dimensional gel electroph oresis resolves P38 into multiple isoforms which shift to more acidic pi values after phosphorylation by protein tyrosine kinase or casein k inase II. From nitrocellulose filter binding and UV crosslinking analy sis, evidence was obtained that tyrosine phosphorylation of P38 impair s its binding to poly(A) but not to poly(U). This demonstrates the inv olvement of tyrosine residues in polynucleotide-specific RNA binding t hat can be regulated by phosphorylation/dephosphorylation.