Proliferating myoblasts already express MyoD before the induction of differ
entiation. Overexpression of MyoD in normal and transformed cell lines was
shown to block cells from entering S phase, suggesting that the MyoD growth
suppressive effect must be tightly controlled in growing myoblasts, Here w
e show that during G1 phase, but not in G2, MyoD abundance is down-regulate
d by the ubiquitin-proteasome pathway through phosphorylation of serine 200
. Roscovitine, a specific inhibitor of cyclin-Cdk2 complexes, prevents both
phosphorylation and degradation of MyoD in G1. Inhibition of the ubiquitin
-dependent proteasome pathway by MG132 results in stabilization of MyoD-wt,
with little effect on a MyoD mutant where serine 200 is replaced by an ala
nine. Our results show that MyoD Ser200 is the substrate for phosphorylatio
n by cyclin E-Cdk2 stimulating its degradation by the ubiquitin-proteasome
system which controls MyoD levels in G1. Phosphorylation/degradation of Myo
D at the end of G1 thus represents the regulatory checkpoint in growing myo
blasts allowing progression into S phase in a manner similar to the recentl
y examplified cdk2-phosphorylation/degradation Of p27(Kip1). (C) 2000 Acade
mic Press.