Graphic summary (not to scale) of the mechanism by which MARP1 locks titin – N2A to the thin filament. (A and B) The normal or “free state” of the N2A element in healthy unstressed muscle (A) and the “locked state” with MARP1 (orange) gluing titin’s N2A element to the thin filament (B). (C) Left: Detail of the proposed free and locked states of titin–N2A. Right: immunoglobulin domains in red, N2A-unique sequence in blue, PEVK domains in yellow and fibronectin domains in white.