L-glutamate binds a truncated isoform of GRM4 (mGluR4) in type II taste bud cells (inferred from the rat homolog). The truncated isoform of the receptor has approximately 100-fold less affinity for L-glutamate than the full-length isoform located in brain has (inferred from the rat homolog). The binding of L-glutamate to Grm4 activates opening of calcium channels via a pathway that is incompletely characterized but probably involves heterotrimeric G-proteins associated with Grm4. In mice, knockout of Grm4 caused a partial loss of sensitivity to L-glutamate, indicating that Grm4 acts with other receptors in type II taste bud cells to activate signaling in response to glutamate (Yasumatsu et al. 2015).