Site-directed mutagenesis and deletion of three phosphorylation sites of calsequestrin of skeletal muscle sarcoplasmic reticulum: Effects on intracellular targeting

Calsequestrin (CS) is segregated to the junctional sarcoplasmic reticulum (jSR) of skeletal muscle fibers and is responsible for intraluminal Ca(2+) binding. A chimeric CS-HA1, obtained by adding the nine-amino-acid viral epitope hemagglutinin (HA1) to the carboxy-terminal of CS and shown to be correctly segregated to skeletal muscle jSR in vivo (A. Nori, K. A. Nadalini, A. Martini, R. Rizzuto, A. Villa, and P. Volpe, 1997, Am. J. Physiol. 272, C1420-C1428), is mutagenized in order to identify domains of CS involved in targeting. Since a putative targeting mechanism of CS implies phosphorylation-dependent steps in the endoplasmic reticulum (ER) and/or Golgi complex, five CS-HA1 mutants disrupting the three phosphorylation sites of CS (Thr(189), Thr(229), and Thr(353)) were engineered by either site-directed mutagenesis or deletion: CS-HA1DeltaP1 (Thr(189) --> Ile); CS-HA1DeltaP2 (Thr(229) --> Asn); CS-HA1DeltaP1,2; in which Thr(189) and Thr(229) were changed to Ile and Asn, respectively; and CS-HA1Delta14(COOH) and CS-HA1Delta49 (COOH), in which 14 residues (Glu(354)-Asp(367)) and 49 residues (Asp(319)-Asp(367)), respectively, were deleted at the carboxy-terminal. Mutant cDNAs were transiently transfected in either HeLa cells, cultured myoblasts of rat skeletal muscle, or regenerating soleus muscle fibers of adult rats. Each CS-HA1 mutant was identified by Western blot as a single polypeptide of the predicted molecular weight. The intracellular localization of CS-HA1 mutants was studied by immunofluorescence using specific antibodies against either CS or HA1. CS-HA1 mutants colocalized with ER markers, e.g., calreticulin, and partially overlapped with Golgi complex markers, e.g., alpha-mannosidase II, in HeLa cells and myotubes. CS-HA1 mutants were expressed and retained in ER and ER/SR of HeLa cells and myotubes, respectively, and correctly segregated to jSR of regenerating soleus muscle fibers. Thus, the targeting mechanism of CS in vivo is not affected by phosphorylation(s); i.e., sorting and segregation of CS appear to be independent of posttranslational phosphorylation(s).