Immunolocalization of mitsugumin29 in developing skeletal muscle and effects of the protein expressed in amphibian embryonic cells

The temporal appearance and subcellular distribution of mitsugumin29 (MG29) , a 29-kDa transmembrane protein isolated from the triad junction in skelet al muscle, were examined by immunohistochemistry during the development of rabbit skeletal muscle. MG29 appeared in the sarcoplasmic reticulum (SR) in muscle cells at fetal day 15 before the onset of transverse tubule (T tubu le) formation. In muscle cells at fetal day 27, in which T tubule and triad formation is ongoing, both SR and triad were labeled for MG29, In muscle c ells at newborn 1 day, the labeling of the SR had become weak and the triad s were well developed and clearly labeled for MG29. Specific and clear labe ling for MG29 was restricted to the triads in adult skeletal muscle cells. When MG29 was expressed in amphibian embryonic cells by injection of the cR NA, a large quantity of tubular smooth-surfaced endoplasmic reticulum (sER) was formed in the cytoplasm, The tubular sER was 20-40 nm in diameter and appeared straight or reticular in shape. The tubular sER was formed by the fusion of coated vesicles [budded off from the rough-surfaced endoplasmic r eticulum (rER)] and vacuoles of rER origin. The present results suggest tha t MG29 may play important roles both in the formation of the SR and the con struction of the triads during the early development of skeletal muscle cel ls. Dev Dyn 1999;215:87-95, (C) 1999 Wiley-Liss, Inc.