S100a0 (αα) Protein, a Calcium‐Binding Protein, Is Localized in the Slow‐Twitch Muscle Fiber

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Abstract
We previously showed that, in contrast to the distribution of S100b (.beta..beta.), S100a0 (.alpha..alpha.) is mainly present in human skeletal and heart muscles at the level of 1-2 .mu.g/mg of soluble protein and is universally distributed at high levels in skeletal and heart muscles of various mammals. To elucidate cellular and ultrastructural localizations of the .alpha. subunit of S100 protein (S100-.alpha.) in skeletal muscle, we used immunohistochemical and enzyme immunoassay methods. The immunohistochemical study revealed that S-100-.alpha. is mainly localized in slow-twitch muscle fibers, whereas the .beta. subunit of S100 protein (S100-.beta.) was not detected in both types of muscle fibers, an observation indicating that the predominant form of S100 protein in the slow-twitch muscle fiber is not S100a or S100b, but S100a0. The quantitative analysis using enzyme immunoassay corroborates the immunohistochemical finding: The S100-.alpha. concentration of mouse soleus muscle (mainly composed of slow-twitch muscle fibers) is about threefold higher than that of mouse rectus femoris muscle (mainly composed of fast-twitch muscle fibers). At the ultrastructural level, S100-.alpha. is associated with polysomes, sarcoplasmic reticulum, the plasma membrane, the pellicle around lipid droplets, the outer membrane of mitochondria, and thin and thick filaments, by immunoelectron microscopy.