Significant roles of microtubules in mature striated muscle deduced from the correlation between tubulin and its molecular chaperone αB-crystallin in rat muscles

To elucidate the significance of cytoskeletal microtubule networks in striated muscles, we analyzed correlation between the content of tubulin (building block of microtubules) and alpha B-crystallin (a molecular chaperone for tubulin) in a variety of striated muscles expressing different myosin heavy-chain (MHC) isoforms. The content of both tubulin and alpha B-crystallin was larger in MHC-I dominant soleus muscle and in MHC-alpha dominant cardiac (atrium and ventricle) muscles; intermediate in MHC-IId dominant masseter, tongue, and diaphragm muscles; and smaller in MHC-IIb dominant plantaris, gastrocnemius, psoas, extensor digitorum longus, and tibialis anterior muscles. Since the muscles of slow-type MHC (MHC-I/alpha) show the most economical features in their function and metabolism, which suit for continuous activity required to sustain posture and blood pumping, the present results afforded additional support to our hypothesis that microtubule networks transduce mechanical environmental demands to morphological and biochemical responses that eventually evolve adaptive transformation in the function and metabolism of the mature muscles. The comparison of tubulin/alpha B-crystalline ratios across the muscles of varied MHC isoforms further suggested that mechanical stress fluctuating at the rhythmic frequency of walking and breathing efficiently activates the hypothesized dynamic function of microtubules.