The STP-C488 oncogene of herpesvirus saimiri has transforming activity independent of the rest of the viral genome. Three distinct structural regions can be predicted from the STP-C488 sequence: an acidic amino terminal domain, a collagen domain, and a hydrophobic carboxyl-terminal domain. To study the importance and functional roles of these regions, 25 different mutant forms of STP-C488 were generated. Net negative charge in the 17 amino acid amino-terminal domain was found to be important for protein structure and transformation. Increasing the net negative charge decreased electrophoretic mobility and decreasing net negative charge increased electrophoretic mobility. The three glutamic acid residues and overall acidity in this region were found to be necessary to retain potent transforming activity. Interruption of the 18 collagen-like repeats in the central region also interrupted transforming activity. The hydrophobic region at the carboxyl terminus was found to be important for membrane localization. The acidic amino-terminal domain is likely to be the catalytic or ligand binding site of STP-C488.
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