Genome-wide identification and biochemical characterization of calcineurin B-like calcium sensor proteins in Chlamydomonas reinhardtii

Calcium (Ca²⁺) signaling is involved in the regulation of diverse biological functions through association with several proteins that enable them to respond to abiotic and biotic stresses. Though Ca²⁺-dependent signaling has been implicated in the regulation of several physiological processes in Chlamydomonas reinhardtii, Ca²⁺ sensor proteins are not characterized completely. C. reinhardtii has diverged from land plants lineage, but shares many common genes with animals, particularly those encoding proteins of the eukaryotic flagellum (or cilium) along with the basal body. Calcineurin, a Ca²⁺/calmodulin-dependent protein phosphatase, is an important effector of Ca²⁺ signaling in animals, while calcineurin B-like proteins (CBLs) play an important role in Ca²⁺ sensing and signaling in plants. The present study led to the identification of 13 novel CBL-like Ca²⁺ sensors in C. reinhardtii genome. One of the archetypical genes of the newly identified candidate, CrCBL-like1 was characterized. The ability of CrCBL-like1 protein to sense as well as bind Ca²⁺ were validated using two-step Ca²⁺-binding kinetics. The CrCBL-like1 protein localized around the plasma membrane, basal bodies and in flagella, and interacted with voltage-gated Ca²⁺ channel protein present abundantly in the flagella, indicating its involvement in the regulation of the Ca²⁺ concentration for flagellar movement. The CrCBL-like1 transcript and protein expression were also found to respond to abiotic stresses, suggesting its involvement in diverse physiological processes. Thus, the present study identifies novel Ca²⁺ sensors and sheds light on key players involved in Ca²⁺signaling in C. reinhardtii, which could further be extrapolated to understand the evolution of Ca²⁺ mediated signaling in other eukaryotes.