Calmodulin-like protein 38: a component of ribonucleoprotein particles during hypoxic stress responses in Arabidopsis

Waterlogging stress leads to a crisis in energy metabolism and the accumulation of toxic metabolites due to the hypoxic and/or anoxic environment associated with this condition. To respond and adapt to this situation, higher plants employ an integrated genetic program that leads to the induction of anaerobic response polypeptide genes that encode metabolic and signaling proteins involved in altering metabolic flow and other adaptive responses. The study presented here shows that the Arabidopsis thaliana calmodulin-like protein CML38 is calcium sensor protein that serves as a member of the core anaerobic response gene family and is involved in modulating the survival response to low oxygen stress. CML38 is unique among Arabidopsis genes encoding seven calmodulin and fifty calmodulin-like proteins since it is solely induced over 300-fold in roots within 6 hr of hypoxia/water-logging challenge. Sensitivity towards low oxygen stress challenge results in the reduced survival of CML38 T-DNA knock-out plants, which could be rescued by complementation of the mutant. Based on subcellular localization of CML38, and proteomic studies of CML38 co-immunoprecipitation complexes, it appears that the protein is localized to mRNA-ribonucleoprotein (mRNP) complexes known as stress granules (SGs) and processing bodies (PBs) that serve to sequester cellular mRNA in a non-translatable state (SG) or process it for degradation (PB) during stress conditions. It is postulated that CML38 is a new target for calcium signals that regulate the mRNP complex during the hypoxia/flooding adaptation program in Arabidopsis. Phylogenetic analysis reveals that CML38 is part of a larger family of structurally related calcium sensor proteins, which form the rgs-CaM like family of Arabidopsis. RgsCaM was identified as an endogenous suppressor of posttranscriptional gene silencing in tobacco and as an interaction target of the tobacco etch virus protein helper component proteinase (HC-Pro). The present work also shows that CML38 shares high structural and functional similarities with rgsCaM and also interacts with the helper component proteinase (HC-Pro) in planta. Based on these studies, a potential role of CML38 as a hypoxia-induced calcium sensor that regulates mRNA metabolism and posttranscriptional gene silencing is postulated.