Mutational analysis of the carboxy-terminal casein kinase II phosphorylation site in human c-myc.

Myc proteins are phosphorylated within two critical regions by casein kinase II (CKII): the central acidic domain and a carboxy-terminal region bordering the basic region-helix-loop-helix segment. In order to test whether the carboxy-terminal phosphorylation site was functionally important we introduced three types of mutations into this region. Two of the mutations would be expected to prevent phosphorylation and minimize negative charge while the third introduced a permanent negative charge. The Myc CKII site mutants were cloned into a retroviral vector and were shown to be efficiently expressed in several different cell types. In one mutant we directly demonstrated loss of the phosphorylation site. When the Myc mutants were used in a cooperative transformation assay of Rat-1 cells with the bcr-abl oncogene we were unable to detect a significant difference in transformation efficiency between wild-type Myc and any of the mutants. While the CKII site is non-functional in this assay, the high levels of Myc produced may have overridden potential CKII regulation.