FIG. 7.
Inhibition by HBP1 of MyoD family transcriptional activation. (A) HBP1 can inhibit MyoD family activation of a natural differentiation-specific MCK-CAT or a simplified muscle-specific 4R-CAT reporter constructs. MCK-CAT denotes a CAT reporter construct driven by the ∼3 kb of the differentiation-specific muscle creatine kinase promoter (46). 4R-CAT denotes a simplified and muscle-specific reporter in which CAT expression is driven by four reiterated MyoD family binding sites (E-box elements) upstream of a minimal thymidine kinase promoter (51). The effect of HBP1 on transcriptional activation by either MyoD (lanes 1 and 2), Myf5 (lanes 3 and 4), or myogenin (lanes 5 and 6) was quantitated in C2 cells as described in Materials and Methods. In each set, transcriptional activities in the presence or absence of HBP1 were denoted by open or filled columns, respectively. (B) Inhibition of MyoD family transcriptional activation requires the N-terminal region of HBP1. Myogenin was used as a representative member of the MyoD family, and the relative inhibition by wild-type HBP1 (open column) and by ΔHBP403-513 (horizontal stripes) (see Fig. 1 for description) was compared by using assays similar to those described for panel A.