Download RACK-1, a receptor for activated C kinase, links metabotropic

RACK1, A RECEPTOR FOR ACTIVATED C KINASE, LINKS
METABOTROPIC GLUTAMATE RECEPTOR ACTIVATION WITH
DENDRITIC TRANSLATIONAL CONTROL. F. Angenstein1*, J. E.
Kacharmina2, B. Settlage3, S. T. Moran1, S.-C. Ling1, A. Klintsova1, D. Hunt3, J.
Eberwine2, and W.T.Greenough1. 1Beckman Institute, University of Illinois, UrbanaIL 61801; 2Dept. of Pharmacology, University of Pennsylvania; 3Dept. of Chemistry,
University of Virginia, Charlottesville VA 22901
Local protein synthesis is activated by glutamate in synaptoneurosomes (Weiler,
Greenough PNAS, 90:7168, 1993). To search for transmitter receptor triggered
mechanisms involved in translational control of dendritically localized mRNAs, we
focussed on mRNPs that might be affected by second messenger systems. PolyAmRNAs were purified using oligo(d)T-cellulose. The associated proteins were then
released and identified by mass spectrometry. Among the proteins that form the
mRNP-complexes, we could detect RACK1 (receptor for activated C kinase).
RACK1 appears to be linked via the polyA binding protein (PABP1) to mRNAs.
Furthermore, in hippocampal slices a mGluR dependent binding of PKC 2 to this
mRNA/PABP1/RACK1 complex could be observed. PKC2 is able to phosphorylate
a set of mRNPs in vitro; three of them are phosphorylated under in situ conditions as
well. To confirm that this mechanism – mGluR induced translocation of PKC to
mRNA bound RACK1 – is in fact involved in dendritic translational control, we
monitored the translation of a reporter mRNA construct into c-myc tagged protein in
isolated dendrites. Stimulation of mGluR1,5 by 50 µM DHPG increased the
translation of this construct. However, if this c-myc sequence-tagged mRNA was
administered with a peptide that competed with the binding of PKC  to RACK1, this
receptor triggered enhancement in translation was impaired, an effect that was most
clearly evident if the mRNA possessed a polyA tail. A random rearrangement of the
same amino acids had no effect. Based on these findings we conclude that mGluR
induced binding of activated PKC to mRNAs can trigger postsynaptic protein
synthesis. (supported by FRAXA Research Foundation and NICHD)