PKCβII modulates translation independently from mTOR and through RACK1
Résumé
RACK1 is an abundant scaffolding protein, which binds active PKCβII increasing its activity in vitro. RACK1 has also been described as a component of the small ribosomal subunit, in proximity of the mRNA exit channel. We tested the hypothesis that PKCβII plays a specific role in translational control and verified whether it may associate with the ribosomal machinery. Here we find that specific inhibition of PKCβI/II reduces translation as well as global PKC inhibition, but without affecting phosphorylation of mTOR targets. These data suggest that PKCβII acts as a specific PKC isoform affecting translation in an mTOR-independent fashion, possibly close to the ribosomal machinery. By far-Western, we found that PKCβII binds ribosomes in vitro. Coimmunoprecipitation studies indicate that a small but reproducible pool of PKCβII is associated with membranes containing ribosomes, suggesting that also in vivo PKCβII may physically interact with the ribosomal machinery. Polysomal profiles show that stimulation of PKC results in increased polysomes/80S ratio, associated with a shift of PKCβII to the heavier part of the gradient. A RACK1-derived peptide that inhibits the binding of active PKCβII to RACK1 reduces polysomes/80S ratio and methionine incorporation, suggesting that binding of PKCβII to RACK1 is important for PKC-mediated translational control. Finally, downregulation of RACK1 by siRNA impairs the PKC-mediated increase of translation. Taken together these data show that PKCβII can act as a specific PKC isoform regulating translation, in an mTOR independent fashion, possibly close to the ribosomal machinery.
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