Fig. 8. Model of the M-band incorporating the spatial constraints on the localization of titin, myomesin and obscurin on the basis of the protein interactions identified here and previously. The M-band lines are marked at the top of the figure. The path of titin is unknown outside the M-band, hence no precise arrangement is shown from the domains immediately N-terminal to the kinase domain. Ig and Fn3 domains are shown as ovals, the titin kinase domain (TK) as a circle and the interdomain-sequence insertions as lines. Titin M-band domains are numbered. Protein interactions are: (1) myomesin My1 and myosin; (2) obscurin/Obsl1 Ig1 with titin M10; (3) myomesin homodimerization via My13; and (4) obscurin/Obsl1 Ig3 with myomesin My4-My5. Note that this model makes no assumptions on whether titin crosses over between myosin filaments or is arranged in a strictly parallel fashion, but mechanical considerations make a crosslinking titin arrangement more plausible.