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Structural and functional characterization of the Fujinami sarcoma virus transforming protein Weinmaster, Geraldine Ann


The phosphorylation of the Fujinami sarcoma virus transforming protein (FSV P140gag-fps) is complex, reversible and affects its tyrosine specific protein kinase activity and transforming function. The sites of phosphorylation within FSV P140gag-fps have been localized to various regions of the protein using partial proteolysis. The two major phosphotyrosine residues and a major phosphoserine residue are located in the C-terminal portion of the fps region, which contains the kinase active domain. A comparative tryptic phosphopeptide analysis of the gag-fps proteins of three FSV variants shows that the phosphotyrosine containing peptides have similar mobilities. To determine whether tyrosine phosphorylation affects protein function and to evaluate the substrate specificity of the protein kinase intrinsic to FSV P130gag-fps oligonucleotide-directed mutagenesis was used to change tyrosine-1073, the major site of P130gag-fps phosphorylation. Tyrosine-1073 was mutated to a phenylalanine and a glycine, amino acids that cannot be phosphorylated, and to the other commonly phosphorylated hydroxyamino acids, serine and threonine. Neither serine nor threonine were phosphorylated when substituted for tyrosine-1073 indicating a strict specificity for and oncogenic capacities. These data indicate that tyrosine phosphorylation stimulates the biochemical and biological activities of FSV P130gag-fps and suggest that tyrosine phosphorylation modulates protein function. Mutations within the putative ATP-binding site of P130gag-fps at lysine-950 destroy both its kinase and transforming activities, supporting the idea that the tyrosine kinase activity intrinsic to P130gag-fps is essential for its transforming function. The mutant protein was also shown to be phosphorylated at a second tyrosine site, which has been previously identified in wild-type P130gag-fps as a site exclusively phosphorylated in vivo. Phosphorylation of secondary tyrosine residues within a mutant protein devoid of intrinsic tyrosine protein kinase activity suggests that the FSV P130gag-fps may be a target for phosphorylation by cellular tyrosine specific protein kinases.

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