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

Abstract

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 <i>Arabidopsis thaliana</i> GLRs (<i>At</i>GLRs) showed growth and Ca²⁺ flux phenotypes expected for plasma membrane Ca²⁺ channels. However, higher-order mutants of <i>At</i>GLR3.3 revealed phenotypes contradicting this assumption. These discrepancies could be explained by subcellular <i>At</i>GLR localization, and we explored the implication of <i>At</i>CNIHs in this sorting. We found that <i>At</i>GLRs interact with <i>At</i>CNIH pairs, yielding specific intracellular localizations. <i>At</i>CNIHs further trigger <i>At</i>GLR activity in mammalian cells without any ligand. These results reveal a regulatory mechanism underlying Ca²⁺ homeostasis by sorting and activation of <i>At</i>GLRs by <i>At</i>CNIHs.