Calsequestrin, a calcium sequestering protein localized at the sarcoplasmic reticulum, is not essential for body-wall muscle function in Caenorhabditis elegans

Calsequestrin is the major calcium-binding protein of cardiac and skeletal muscles whose function is to sequester Ca2+ in the lumen of the sarcoplasmi c reticulum (SR). Here we describe the identification and functional charac terization of a C, elegans calsequestrin gene (csq-1). CSQ-1 shows moderate similarity (50% similarity, 30% identity) to rabbit skeletal calsequestrin , Unlike mammals, which have two different genes encoding cardiac and fast- twitch skeletal muscle isoforms, csq-1 is the only calsequestrin gene in th e C, elegans genome. We show that csq-1 is highly expressed in the body-wal l muscles, beginning in mid-embryogenesis and maintained through the adult stage, In body-wall muscle cells, CSQ-1 is localized to sarcoplasmic membra nes surrounding sarcomeric structures, in the regions where ryanodine recep tors (UNC-68) are located. Mutation in UNC-68 affects CSQ-1 localization, s uggesting that the two possibly interact in vivo. Genetic analyses of chrom osomal deficiency mutants deleting csq-1 show that CSQ-1 is not essential f or initiation of embryonic muscle formation and. contraction, Furthermore, double-stranded RNA injection resulted in animals completely lacking CSQ-1 in body-wall muscles with no observable defects in locomotion. These findin gs suggest that although CSQ-1 is one of the major calcium-binding proteins in the body-wall muscles of C, elegans, it is not essential for body-wall muscle formation and contraction.