TRPC3-interacting triadic proteins in skeletal muscle

The expression of TRPC3 (canonical-type transient receptor potential cation channel type 3) is tightly regulated during skeletal muscle cell differentiation, and a functional interaction between TRPC3 and RyR1 [(ryanodine receptor type 1), an SR (sarcoplasmic reticulum) Ca²⁺-release channel] regulates the gain of SR Ca²⁺ release during EC (excitation–contraction) coupling. However, it has not been possible to demonstrate direct protein–protein interactions between TRPC3 and RyR1. To identify possible candidate(s) for a linker protein(s) between TRPC3 and RyR1 in skeletal muscle, in the present study we performed MALDI–TOF (matrix-assisted laser-desorption ionization–time-of-flight) MS analysis of a cross-linked triadic protein complex from rabbit skeletal triad vesicles and co-immunoprecipitation assays using primary mouse skeletal myotubes. From these studies, we found that six triadic proteins, that are known to regulate RyR1 function and/or EC coupling [TRPC1, JP2 (junctophilin 2), homer, mitsugumin 29, calreticulin and calmodulin], interacted directly with TRPC3 in a Ca²⁺-independent manner. However we again found no direct interaction between TRPC3 and RyR1. TRPC1 was identified as a potential physical link between TRPC3 and RyR1, as it interacted with both TRPC3 and RyR1, and JPs showed subtype-specific interactions with both RyR1 and TRPC3 (JP1–RyR1 and JP2–TRPC3). These results support the hypothesis that TRPC3 and RyR1 are functionally engaged via linker proteins in skeletal muscle.