Calsequestrin is the major calcium-binding protein of cardiac and skeletal 
muscles whose function is to sequester Ca(2+ )in the lumen of the sarcoplasmic 
reticulum (SR). Here we describe the identification and functional 
characterization 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 the C. elegans genome. We show that csq-1 is highly expressed in the 
body-wall muscles, beginning in mid-embryogenesis and maintained through the 
adult stage. In body-wall muscle cells, CSQ-1 is localized to sarcoplasmic 
membranes surrounding sarcomeric structures, in the regions where ryanodine 
receptors (UNC-68) are located. Mutation in UNC-68 affects CSQ-1 localization, 
suggesting that the two possibly interact in vivo. Genetic analyses of 
chromosomal deficiency mutants deleting csq-1 show that CSQ-1 is not essential 
for 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 findings 
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.