The Microtubule-Associated Protein CLIMP-63 is a New Member of the Calcium Release Complex

Abstract

Muscle contraction is achieved when an efficient excitation signal at the plasma membrane triggers intracellular calcium release. This process called “excitation-contraction (E-C) coupling” relies on a macromolecular protein complex, spanning the plasma membrane and the sarcoplasmic reticulum (SR), containing the calcium channel of the SR, the ryanodine receptor (RyR). This calcium release complex is present exclusively in highly organized membrane structures called triads. A triad is composed of two SR terminal cisternae surrounding a plasma membrane transverse-tubule. This architecture is essential to sustain the activity of the calcium channel RyR1, which is located in the membrane of SR terminal cisternae. However, little is known about the molecular mechanisms allowing the formation and maintenance of SR terminal cisternae. Triadin is a member of this complex, present in the SR membrane and interacting with RyR1. Deletion of the triadin gene leads to partial disorganisation of SR membranes in skeletal muscles, with abnormal orientation of part of the triads. Triadin could play a role in the structure of sarcoplasmic reticulum to allow efficient E-C coupling. We have shown that triadin could indirectly interact with the microtubules, and therefore anchor the sarcoplasmic reticulum to the microtubule network (Fourest-Lieuvin, J.Cell.Science, 2012). Using mass spectrometry analysis of the proteins co-immunoprecipitated with triadin, we have identified a new partner of triadin, CLIMP-63 which could be involved in this function. CLIMP-63 is a shaping protein able to mediate the anchoring of the reticulum to microtubules. CLIMP-63 has been involved in the shaping of endoplasmic reticulum in neurons (Cui-Wang, Cell, 2012). We have dissected the interacting domains between CLIMP-63 and triadin, and studied the consequences of this association for muscle function, and triad formation or maintenance.