CORNICHON sorting and regulation of GLR channels underlie pollen tube Ca 2+ homeostasis.

Compared to animals, evolution of plant calcium (Ca2+ ) physiology has led to a loss of proteins for influx and small ligand-operated control of cytosolic Ca2+ , leaving many Ca2+ mechanisms unaccounted for. Here, we show a mechanism for sorting and activation of glutamate receptor-like channels (GLRs) by CORNICHON HOMOLOG (CNIH) proteins. Single mutants of pollen-expressed Arabidopsis thaliana GLRs ( At GLRs) showed growth and Ca2+ flux phenotypes expected for plasma membrane Ca2+ channels. However, higher-order mutants of At GLR3.3 revealed phenotypes contradicting this assumption. These discrepancies could be explained by subcellular At GLR localization, and we explored the implication of At CNIHs in this sorting. We found that At GLRs interact with At CNIH pairs, yielding specific intracellular localizations. At CNIHs further trigger At GLR activity in mammalian cells without any ligand. These results reveal a regulatory mechanism underlying Ca2+ homeostasis by sorting and activation of At GLRs by At CNIHs.