Insulin and contraction-induced movement of fatty acid transport proteins to skeletal muscle transverse-tubules is distinctly different than to the sarcolemma

Fatty acid (FA) transport proteins are known to exist on the sarcolemma of skeletal muscle. However, it is unknown whether the t-tubules, which comprise ~ 60% of the cell surface, also harbor these proteins. We examined FA transport proteins from both membrane fractions in unstimulated, insulin-stimulated and contracted skeletal muscle. Sarcolemmal and t-tubule membrane fractions were isolated from the same muscle homogenate using a discontinuous sucrose gradient. Our results demonstrate that the relative content of FA transport proteins within the two fractions and the magnitude to which they increase when stimulated were distinctly different. In unstimulated muscle FAT/CD36, FATP4, and FABPpm are abundant on the sarcolemma (3-, 8-, and 10-fold greater than t-tubule, respectively), whereas FATP1 resides primarily within the t-tubule fraction (1- to 2-fold greater than the sarcolemma). With both stimuli, in terms of absolute increase, FAT/CD36 predominantly translocated to the sarcolemma and FATP1 to the t-tubules. There are clear differences in the profile of FA transport proteins and the response to stimuli of the sarcolemma and t-tubules. FATP1, a variable and unresponsive protein on the sarcolemma, appears to reside primarily in the t-tubules where it is responsive to stimuli.