PKCβII modulates translation independently from mTOR and through RACK1

RACK1 (receptor for activated C kinase 1) is an abundant scaf-folding protein, which binds active PKCβII (protein kinase C βII) increasing its activity in vitro. RACK1 has also been described as a component of the small ribosomal subunit, in proximity to the mRNA exit channel. In the present study we tested the hypothesis that PKCβII plays a specific role in translational control and verified whether it may associate with the ribosomal machinery. We find that specific inhibition of PKCβI/II reduces translation as well as global PKC inhibition, but without affecting phosphorylation of mTOR (mammalian target of rapamycin) targets. These results suggest that PKCβII acts as a specific PKC isoform affecting translation in an mTOR-independent fashion, possibly close to the ribosomal machinery. Using far-Western analysis, we found that PKCβII binds ribosomes in vitro. Co-immunoprecipitation studies indicate that a small but reproducible pool of PKCβII is associated with membranes containing ribosomes, suggesting that in vivo PKCβII may also physically interact with the ribosomal machinery. Polysomal profiles show that stimulation of PKC results in an 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 the polysomes/80S ratio and methionine incorporation, suggesting that binding of PKCβII to RACK1 is important for PKC-mediated translational control. Finally, down-regulation of RACK1 by siRNA (small interfering RNA) impairs the PKC-mediated increase of translation. Taken together the results of the present study show that PKCβII can act as a specific PKC isoform regulating translation, in an mTOR-independent fashion, possibly close to the ribosomal machinery.