Colony stimulating factor-1 (CSF-1) stimulates temperature dependent phosphorylation and activation of the RAF-1 proto-oncogene product

The serine/threonine kinase RAF-1 is phosphorylated in intact macrophages in response to CSF-1 at 37°C. The augmented phosphorylation of RAF-1 and a concomitant increase in RAF-1 associated serine/threonine kinase activity are kinetically later events than CSF-1 induced protein tyrosine phosphorylation. Furthermore, phospho-amino acid analysis of RAF-1 reveals the presence of phosphoserine, trace amounts of phosphothreonine but no phosphotyrosine and the phosphorylated RAF-1 does not react with anti-phosphotyrosine antibodies. In contrast to CSF-1 induced protein tyrosine phosphorylation, RAF-1 phosphorylation and activation are temperature dependent and do not occur at 4°C. Furthermore, coprecipitation experiments failed to reveal any noncovalent association of RAF-1 with the CSF-1 receptor. Thus, while RAF-1 is not a direct substrate for the CSF-1 receptor tyrosine kinase in vivo, its temperature dependent phosphorylation and activation represent an intriguing aspect of the CSF-1 response.