Role of triadin in the organization of reticulum membrane at the muscle triad.

The terminal cisternae represent one of the functional domains of the skeletal muscle sarcoplasmic reticulum (SR). They are closely apposed to plasma membrane invaginations, the T-tubules, with which they form structures called triads. In triads, the physical interaction between the T-tubule-anchored voltage-sensing channel DHPR and the SR calcium channel RyR1 is essential because it allows the depolarization-induced calcium release that triggers muscle contraction. This interaction between DHPR and RyR1 is based on the peculiar membrane structures of both T-tubules and SR terminal cisternae. However, little is known about the molecular mechanisms governing the formation of SR terminal cisternae. We have previously shown that ablation of triadins, a family of SR transmembrane proteins that interact with RyR1, induced skeletal muscle weakness in knockout mice as well as a modification of the shape of triads. Here we explore the intrinsic molecular properties of the longest triadin isoform Trisk 95. We show that when ectopically expressed, Trisk 95 can modulate reticulum membrane morphology. The membrane deformations induced by Trisk 95 are accompanied by modifications of the microtubule network organization. We show that multimerization of Trisk 95 by disulfide bridges, together with interaction with microtubules, are responsible for the ability of Trisk 95 to structure reticulum membrane. When domains responsible for these molecular properties are deleted, anchoring of Trisk 95 to the triads in muscle cells is strongly decreased, suggesting that oligomers of Trisk 95 and microtubules contribute to the organization of the SR terminal cisternae in a triad.