In vitro mutagenesis of a putative DNA binding domain of SV40 large-T

A large number of deletion and point mutations were introduced into a small region of the SV40 large-T gene that was believed to encode part of a DNA-binding domain. The majority of mutant proteins constructed were unable to stimulate viral DNA replication, but all retained at least some transforming activity. Those replication-defective mutants with lesions affecting amino acid residues between 144 and 156 were postulated also to be defective in the sutoregulation function of large-T to account for their ability to transform Rat-1 cells more avidly than wild-type. Two mutants (Glu 107 → Lys and Ser 189 → Asn) were isolated which exhibited severely reduced transforming activity but which supported normal rates of virus and viral DNA replication. Mutation of individual serine and threonine phosphorylation sites within the amino-terminal half of large-T had little effect on the protein's transforming activity. These and other mutations that affected amino acid residues either side of the region from 127 to 133, previously shown to be essential to the nuclear localisation of large-T [D. Kalderon, W. D. Richardson, A. F. Markham, and A. E. Smith (1984) Nature (London)311, 33–38] did not discernibly impair nuclear accumulation.