ABSTRACT
In this report we defined the structural and temporal limits within which calreticulin and calsequestrin participate in the muscle cell phenotype, in the L6 model myogenic system. Calreticulin and calsequestrin are two Ca2+ binding proteins thought to participate in intracellular Ca2+ homeostasis. We show that calsequestrin protein and mRNA were expressed when L6 cells were induced to differentiate, during which time the level of expression of calreticulin protein did not change appreciably. Calreticulin mRNA levels, however, were constant throughout L6 cell differentiation except for slight decline in the mRNA levels at the very late stages of L6 differentiation (day 11-12). We also show that the two Ca2+ binding proteins are coexpressed in differentiated L6 cells. Based on its mobility in SDS-PAGE, L6 rat skeletal muscle cells in culture expressed cardiac isoform of calsequestrin. In the mature rat skeletal muscle, calreticulin and calsequestrin were localized to sarcoplasmic reticulum (SR). Calreticulun, but not calsequestrin, staining was also observed in the perinuclear region. These data suggest that expression of calreticulin and calsequestrin may be under different control during myogenesis in rat L6 cells in culture.
Subject(s)
Calcium-Binding Proteins/biosynthesis , Calsequestrin/biosynthesis , Cell Differentiation , Muscle, Skeletal/metabolism , Ribonucleoproteins/biosynthesis , Animals , Calcium/metabolism , Calcium-Binding Proteins/analysis , Calreticulin , Calsequestrin/analysis , Cell Fusion , Cell Line , Creatine Kinase/genetics , Golgi Apparatus/chemistry , Homeostasis , Muscle Fibers, Skeletal/chemistry , Muscle, Skeletal/cytology , Muscle, Skeletal/enzymology , Nuclear Envelope/chemistry , Phosphorylation , RNA, Messenger/biosynthesis , Rats , Retinoblastoma Protein/metabolism , Ribonucleoproteins/analysis , Sarcoplasmic Reticulum/chemistryABSTRACT
Calreticulin was identified in a variety of rabbit tissues by Western blot analysis. Indirect immunofluorescence studies on cultured cells or frozen sections from the corresponding tissues revealed that the protein was distributed to the endoplasmic reticulum or sarcoplasmic reticulum. Calreticulin was found to be an abundant calcium-binding protein in non-muscle and smooth muscle cells and a constituent calcium-binding protein in cardiac and skeletal muscle. From the immunoblot data, calreticulin may exist as an isoform in rabbit neural retina. The present study establishes the ubiquity of calreticulin in intracellular calcium binding.
