Regulation of eukaryotic translation by the RACK1 protein: a platform for signalling molecules on the ribosome.

The receptor for activated C-kinase (RACK1) is a scaffold protein that is able to interact simultaneously with several signalling molecules. It binds to protein kinases and membrane-bound receptors in a regulated fashion. Interestingly, RACK1 is also a constituent of the eukaryotic ribosome, and a recent cryo-electron microscopy study localized it to the head region of the 40S subunit in the vicinity of the messenger RNA (mRNA) exit channel. RACK1 recruits activated protein kinase C to the ribosome, which leads to the stimulation of translation through the phosphorylation of initiation factor 6 and, potentially, of mRNA-associated proteins. RACK1 therefore links signal-transduction pathways directly to the ribosome, which allows translation to be regulated in response to cell stimuli. In addition, the fact that RACK1 associates with membrane-bound receptors indicates that it promotes the docking of ribosomes at sites where local translation is required, such as focal adhesions.