C-TERMINAL PHOSPHORYLATION OF THE SERUM-RESPONSE FACTOR

The serum-response factor (SRF) is essential for the induction and rep ression of the proto-oncogene c-fos. Phosphorylation of SRF has been i mplicated to be involved in these processes and five phosphorylation s ites have already been mapped within the N-terminal region. Here we sh ow that in vivo additional phosphorylation of SRF does occur. This mod ification is located primarily within amino acids 206-289, which proba bly contain more than one phosphorylation site. Microsequencing allowe d the identification of one phosphorylation site at Ser253, which is a potential target of casein kinase II. Mutational analysis revealed th at, in contrast to N-terminal phosphorylation, Ser253 phosphorylation does not affect DNA-binding properties. In addition, phosphorylation a t Ser253 does not seem to change transactivation activity of SRF but r ather influences its contribution to transcriptional repression. Thus, C-terminal phosphorylation of SRF may modulate c-fos basal repression .