Eukaryotic ribosomes host PKC activity

PKC isoform betaII modulates translation and can be recruited on ribosomes via its scaffold RACK1 (receptor for activated protein kinase C 1), which resides on the 40S ribosomal subunit. However, whether a PKC activity exists on the ribosome is not yet demonstrated. We purified native ribosomes by two different techniques, which avoid stripping of initiation factors and other associated proteins. In both cases, purified ribosomes are able to phosphorylate a specific PKC substrate, MARCKS (Myristoylated Alanine-Rich C-Kinase Substrate). MARCKS phosphorylation is switched on by treatment with PKC agonist PMA (Phorbol 12-Myristate 13-Acetate). Consistently, the broad PKC inhibitor BMI (Bisindolyl Maleimide I) abrogates MARCKS phosphorylation. These data show that native ribosomes host active PKC and hence allow the phosphorylation of ribosome-associated substrates like initiation factors and mRNA binding proteins.