Molecular determinants of metabotropic glutamate receptor 1B trafficking.

The metabotropic glutamate receptor mGluR1 undergoes alternative splicing to generate isoforms differing in C-terminal sequence. The mechanism by which these isoforms give different functional responses to agonists in vitro is so far unclear. Using the native mGluR1 and CD2-mGluR1 chimeric molecules, as well as their C-terminal truncations and mutants, we identified an endoplasmic reticulum (ER) retention signal Arg-Arg-Lys-Lys within the C-terminal sequence of mGluR1b. Its presence results in a much reduced cell surface expression of the receptor and chimeric molecules in cell lines and their restricted trafficking in neurones. This motif is also present in the C-terminus of mGluR1a, but its effect is overcome by a region of the mGluR1a-specific C-terminal sequence (amino acids 975-1098). Our results indicate that these splice variants of mGluR1 utilize different targeting pathways and suggest that this may be a general phenomenon in the metabotropic glutamate receptor gene family.