Regulator of Gene Silencing-Calmodulin associates with mRNA granules and the autophagy protein ATG8

Abstract Regulator-of-gene-silencing calmodulins (rgsCaM) represent a phylogenetic subfamily of calmodulin-like calcium sensors that are targets of viral induced suppression of posttranscriptional gene silencing by secondary siRNAs. The present work shows that a stress (hypoxia) that induces mRNP granule formation also induces the relocalization of rgsCaM to cytosolic granule-like foci that interact with the surface of stress granule and processing body structures. Co-expression of rgsCaM and its binding protein Suppressor of Gene Silencing 3 causes re-localization and integration of rgsCaM into stress granule structures. RgsCaMs contain a conserved topology that consists for four EF hand like domains (three functional and one divergent) that are separated into two calcium binding lobes with an extended amino terminal region. RgsCaM also contains an "ATG8 family interacting motif" (AIM) within its amino-terminal domain that is characteristic of selective autophagy cargo receptors. Co-localization experiments and ratiometric BiFC analyses in Nicotiana benthamiana support the hypothesis that rgsCaM binds directly to ATG8e through this conserved AIM domain, and the two proteins co-localize with mRNP granule markers. Previous reports show that rgsCaM mediates the suppression of gene silencing, at least in part, via turnover of SGS3 via autophagy. A model is proposed for rgsCaM-like proteins as potential mediators of selective autophagy of RNA granules in response to biotic and abiotic stresses.