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Differential biological activities of mammalian Id proteins in muscle cells.

Id proteins are helix-loop-helix (HLH) transcription factors that lack DNA-binding domains. These proteins form inactive heterodimers with basic HLH (bHLH) factors, inhibiting their DNA-binding and transcriptional activities. Consistent with a proposed role for Id proteins as inhibitors of terminal differentiation, Id1 and Id3 have been shown to negatively regulate myogenesis in cultured muscle cells. Here we have investigated the possibility that Id2 and/or Id4 can act in a similar manner. Surprisingly, while overexpression of Id2 resulted in inhibition of differentiation of Sol 8 myoblast cells, overexpression of Id4 did not. Sol 8 cells stably transfected with Id4 showed no apparent changes in expression of muscle-specific genes upon differentiation. DNA-binding activities present at the muscle creatine kinase (MCK) enhancer E-box and transcription of the MCK enhancer were not altered in Id4-overexpressing cells, compared with vector-transfected cells. Id2 is also a more potent inhibitor of protein/DNA complex formation at the MCK-R enhancer E-box than Identified in vitro. Therefore, our data support the notion that members of the Id family might be involved in the regulation of distinct developmental pathways.