Characterization of SAP-1, a protein recruited by serum response factor to the c-fos serum response element.

We used a yeast genetic screen to isolate cDNAs that encode a protein, SRF accessory protein-1 (SAP-1), that is recruited to the c-fos serum response element (SRE) as part of a ternary complex that includes serum response factor (SRF). SAP-1 requires DNA-bound SRF for ternary complex formation and makes extensive DNA contacts to the 5' side of SRF, but does not bind DNA autonomously. Ternary complex formation by SAP-1 requires only the DNA-binding domain of SRF, which can be replaced by that of the related yeast protein MCM1. We isolated cDNAs encoding two forms of SAP-1 protein, SAP-1a and SAP-1b, which differ at their C termini. Both SAP-1 proteins contain three regions of striking homology with the elk-1 protein, including an N-terminal ets domain. Ternary complex formation by SAP-1 requires both the ets domain and a second conserved region 50 amino acids to its C-terminal side. SAP-1 has similar DNA binding properties to the previously characterized HeLa cell protein p62/TCF.     

Regulation of Id1 and its association with basic helix-loop-helix proteins during nerve growth factor-induced differentiation of PC12 cells.

Cell differentiation in the nervous system is dictated by specific patterns of gene expression. We have investigated the role of helix-loop-helix (HLH) proteins during differentiation of PC12 pheochromocytoma cells in response to nerve growth factor. Gel mobility shift assays using PC12 cell nuclear extracts demonstrated that active basic HLH complexes exist throughout differentiation. Addition of exogeneous Id1 protein, a negative regulator of basic HLH proteins, disrupted specific complexes formed by PC12 cell nuclear extracts on a CANNTG consensus oligonucleotide. To identify possible novel basic HLH proteins in these complexes, a glutathione S-transferase-Id1 fusion protein was used to screen a PC12 cell cDNA expression library. A single clone representing the rat E2-2 gene was identified. Sequential immunoprecipitations with antibodies to each HLH protein revealed an association between Id1 and E2-2 that could be detected in both untreated and nerve growth factor-treated PC12 cell lysates. These experiments define a new HLH interaction between Id1 and E2-2 in neuronal cells and suggest that neuronal differentiation may be regulated by HLH proteins in a distinctive manner.