Proteins of the Triad Junction of Skeletal Muscle

Abstract

The basic mechanism of excitation-contraction coupling in skeletal muscle is still not understood despite many years of active research on both the physiology and the biochemistry of the process. Several years ago we embarked on a project of identifying the constituents of the triad junction with the view that this junctional region represents not only the point of physical contact between the external and internal organelle but also the point of dynamic transmission in muscle excitation.^(1,2) In 1986 we described the isolation of a protein of approximate subunit molecular weight 300,000 which spanned the gap between the transverse (T) tubule and the terminal cisternae, and therefore represented a portion of the junctional feet⁽³⁾ Later work by others has demonstrated that this protein is identical to the ryanodine receptor protein and contains Ca²⁺ -channel activity. ^(4–8) This then suggests that the process of Ca²⁺ release from the sarcoplasmic reticulum (SR) takes place at the level of the junctional foot. What is still not clear, however, is the relationship between the T-tubule and the junctional foot. The experiments described in this chapter represent an approach to the understanding of the constituents of the T-tubule and of the junction which forms the full triadic junctional structure.