SYNERGISM IN TERNARY COMPLEX-FORMATION BETWEEN THE DIMERIC GLYCOPROTEIN P67SRF, POLYPEPTIDE P62TCF AND THE C-FOS SERUM RESPONSE ELEMENT

Transcriptional regulation of the c-fos proto-oncogene requires the serum response element (SRE) which is complexed by a multi-protein assembly observed both in vitro and in vivo. Two protein factors, p67(SRF) and p62(TCF) (previously called p62), are required to interact with the SRE for efficient induction of c-fos by serum. By quantitative band shift electrophoresis we measure at least a 50-fold increase in SRE affinity for p67(SRF)/p62(TCF) over p67(SRF) alone. Stoichiometrically we determine that the ternary complex with p62(TCF) involves p67(SRF) in dimeric form. We demonstrate that p67(SRF) is a glycosylated nuclear transcription factor carrying terminal N-acetylglucosamine (GlcNAc) as a post-translational modification. A proteolytic limit digestion product, ~ 13 kd in size, was generated from the p67(SRF) - SRE complex. This p67(SRF)-core domain binds SRE, can dimerize with p67(SRF) and is still able to form a ternary complex with p62(TCF). Therefore, three functional activities can be ascribed to this small p67(SRF)-core domain: specific DNA binding, dimerization and interaction with p62(TCF). We demonstrate that these functions map within the p67(SRF) core fragment containing the region between amino acids 93 and 222.