EVIDENCE FOR REVERSIBLE TYROSINE PROTEIN-PHOSPHORYLATION IN THE OKADAIC ACID-PRODUCING MARINE DINOFLAGELLATE PROROCENTRUM-LIMA

The protist Prorocentrum lima, a primary producer of the tumour promot er okadaic acid, is a member of the dinoflagellate class of marine mic roorganisms. Herein, we have identified and characterized a protein ty rosine kinase (designated PLIK 1A) in P. lima that autophosphorylates almost exclusively on tyrosine residues. PLIK 1A was shown to have an approximate molecular mass of 38 kDa by SDS-PAGE and a native molecula r mass within the range of 47-55 kDa by Superdex-75 gel filtration. Ph osphoamino acid analysis of autophosphorylated PLIK 1A revealed the pr esence of phosphotyrosine and autophosphorylated PLIK 1A reacted with monoclonal anti-phosphotyrosine antibodies in a Western immunoblot. In addition, two protein tyrosine phosphatases were identified in P. lim a that had apparent molecular masses within the ranges of 150-168 kDa and 73-82 kDa as determined by Superdex-200 gel filtration. These P. l ima phosphatases, termed PLPTP-I and PLPTP-II, efficiently dephosphory lated tyrosine phosphorylated myelin basic protein. However, only PLPT P-I was capable of dephosphorylating the tyrosine phosphorylated subst rate angiotensin. Both PLPTP-I and PLPTP-II were able to dephosphoryla te tyrosine autophosphorylated PLIK 1A. These data provide the first e vidence for reversible tyrosine protein phosphorylation in P. lima by protein tyrosine kinases and phosphatases.