mRNA transcripts for Homer 1a and Homer 1c have been detected in skeletal muscle [Biochem. Biophys. Res. Commun. 279 (2000) 348]. Here, the subcellular distribution of recombinant HA1-tagged Homer 1c and HA1-tagged Homer 1a was investigated in C(2)C(12) myotubes and in transgenic skeletal muscle fibers of the adult rat by epifluorescent and confocal microscopy. In C(2)C(12) myotubes, Homer 1a was homogeneously localized in the cytosol and also labeled some nuclei whereas Homer 1c displayed a diffuse reticular/punctuate pattern in the cytosol with scattered punctuate labeling around nuclei; no co-localization was observed with the ryanodine receptor/Ca(2+) release channel (RYR1). The subcellular localization of the Homer 1 isoforms was markedly different in transgenic muscle fibers: Homer 1c was diffusely distributed at the I band and enlightened the Z line, whereas Homer 1a labeled both the I band and the A band with distinct reinforcement of the H line; neither Homer 1c nor Homer 1a co-localized with either calsequestrin or RYR1, two sarcoplasmic reticulum markers. Our findings are discussed in relation to reported effects of Homer 1 isoforms on RYR1 function.