HMG Box Transcription Factor 1 (HBP1)

HBP1 is a sequence-specific HMG transcription factor, of which LEF-1 is the best-characterized family member. First, inspection of the HBP1 protein sequence revealed two consensus RB interaction motifs (LXCXE and IXCXE). Second, HBP1 interaction was selective for RB and p130, but not p107. HBP1, RB, and p130 levels are all up-regulated with differentiation; in contrast, p107 levels decline. Third, HBP1 can function as a transcriptional repressor of the promoter for N-MYC, which is a critical cell cycle and developmental gene. Fourth, because the activation of the N-MYC promoter in cycling cells required the E2F transcription factor,E2F-1 and HBP1 represent opposite transcriptional signals that can be integrated within the N-MYC promoter. Fifth, the expression of HBP1 lead to efficient cell cycle arrest.