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. 2010 Jul;136(1):7–12. doi: 10.1085/jgp.201010461

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© 2010 Beam and Bannister

This article is distributed under the terms of an Attribution–Noncommercial–Share Alike–No Mirror Sites license for the first six months after the publication date (see http://www.rupress.org/terms). After six months it is available under a Creative Commons License (Attribution–Noncommercial–Share Alike 3.0 Unported license, as described at http://creativecommons.org/licenses/by-nc-sa/3.0/).

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Figure 1. — Model for the engagement of skeletal muscle EC coupling based on the current literature. (Top) Sequence alignment is shown for rabbit α_1S, rabbit α_1C, and Musca domestica α_1M. The top panel (α_1S residues 720–765) represents the critical domain (Beam and Horowicz, 2004). The middle panel (α_1S residues 766–799) represents the region of the α_1S II-III loop carboxyl-terminal to the critical domain, ending just before repeat III. Sequence is not shown for the relatively divergent amino-terminal portion of the II-III loop (α_1S residues 662–719). Residues of α_1C or α_1M identical to those of α_1S are shown boxed in black, and residues conserved with those of α_1S are shown boxed in gray. (Bottom) The arc extending from repeat II to repeat III represents the α_1S II-III loop; the yellow portion of the loop represents the critical domain (α_1S residues 720–765). The green entity represents the junctional interaction partner(s) of the α_1S II-III loop. Because the amino-terminal portion of the α_1S II-III loop is accessible to large streptavidin probes, it is depicted as being devoid of junctional interaction partners. Because ultrastructural analysis of α_1S/α_1M chimeras suggests that the carboxyl-terminal portion of the α_1S II-III loop supports junctional contacts that are essential for tetrad formation, the carboxyl-terminal portion of the loop is shown as a surface for interaction with other junctional proteins. In addition, the carboxyl-terminal portion of the loop may also serve as a line of communication (large arrow) between repeat III and the critical domain (yellow) in the center of the α_1S II-III loop; voltage-induced conformational rearrangements (little arrow) in the critical domain engage SR Ca²⁺ release via a transient protein–protein interaction between a portion of the critical domain (red box) and another junctional protein (orange box).