Definition of regions in human c-myc that are involved in transformation and nuclear localization

To study the relationship between the primary structure of the c-myc protein and some of its functional properties, we made in-frame insertion and deletion mutants of the normal human c-myc coding domain that was expressed from a retroviral promoter-enhancer. We assessed the effects of these mutations on the ability of c-myc protein to cotransform normal rat embryo cells with a mutant ras gene, induce foci in a Rat-1-derived cell line (Rat-1a), and localize in nuclei. Using the cotransformation assay, we found two regions of the protein (amino acids 105 to 143 and 321 to 439) where integrity was critical: one region (amino acids 1 to 104) that tolerated insertion and small deletion mutations, but not large deletions, and another region (amino acids 144) to 320) that was largely dispensable. Comparison with regions that were important for transformation of Rat-1a cells revealed that some are essential for both activities, but others are important for only one or the other, suggesting that the two assays require different properties of the c-myc protein. Deletion of each of three regions of the c-myc protein (amino acids 106 to 143, 320 to 368, and 370 to 412) resulted in partial cytoplasmic localization, as determined by immunofluorescence or immunoprecipitation following subcellular fractionation. Some abnormally located proteins retained transforming activity; most proteins lacking transforming activity appeared to be normally located.