Intracellular localisation and expression of mammalian CDC2 protein during myogenic differentiation.

Myogenic differentiation involves withdrawal of myoblasts from the cell cycle and fusion to form multinucleate myotubes. To examine the role that cell cycle control genes may play in this process, we investigated the steady state levels of CDC2 protein and RNA during myogenesis of L6E9 rat myoblasts. Indirect immunofluorescence using a CDC2 affinity-purified antibody showed that this protein is localised exclusively in the cytoplasm with a higher concentration perinuclearly. Both protein and RNA levels were down-regulated to similar extents early in the differentiation process, as cells became quiescent. There was a further down-regulation of protein after fusion to form myotubes. Autonomous expression of CDC2 protein in L6E9 cells, after stable transfection with a metallothionein: CDC2 gene construct, failed to inhibit the differentiation process. This suggests that, although there is down-regulation in levels of CDC2 RNA and protein during myogenesis, this phenomenon per se does not play a primary role in controlling the differentiation process. If CDC2 is involved in control of differentiation, this must depend on post-translational modification of the protein.