A Receptor for Activated C Kinase Is Part of Messenger Ribonucleoprotein Complexes Associated with PolyA-mRNAs in Neurons

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Abstract
Long-lasting changes in synaptic functions after an appropriate stimulus require altered protein expression at the synapse. To restrict changes in protein composition to activated synapses, proteins may be synthesized locally as a result of transmitter receptor-triggered signaling pathways. Second messenger-controlled mechanisms that affect mRNA translation are essentially unknown. Here we report that a receptor for activated C kinase, RACK1, is a component of messenger ribonucleoprotein (mRNP) complexes. RACK1 is predominantly associated with polysome-bound, polyA-mRNAs that are being actively translated. We find it to be present in a complex with β-tubulin and at least two mRNA-binding proteins, polyA-binding protein 1 and a 130 kDa polyA-mRNA binding protein (KIAA0217). Activation of PKCβ2 in vitro by phosphatidylserine/diacylglycerol or in hippocampal slices by metabotropic glutamate receptor stimulation increased the amount of RACK1/PKCβ2 associated with polysome-bound polyA-mRNAs.In vitro, PKCβ2 can phosphorylate a subset of polyA-mRNA-associated proteins that are also phosphorylated underin vivo conditions. On the basis of these findings plus the somatodendritic localization of RACK1, we hypothesize that metabotropic glutamate receptor-triggered binding of activated PKCβ2 to mRNP complexes bound to polyA-mRNAs is involved in activity-triggered control of protein synthesis.