The non-sliding filaments of the sarcomere

A model is proposed for the 'gap' or 'third' filaments of muscle, here renamed 'T-filaments'. These consist of single titin molecules, spanning the half sarcomere from M-line to Z-line. Of several possibilities for stoichiometry and arrangement, the one most favoured here consists of six T-filaments lying longitudinally on the surface of the A-filament, one against each peripheral subfilament of myosin. C-protein molecules over-lie the T-filaments in transverse and axial rows, with their long axes following a helix. Each C-protein molecule helps to bind a pair of T-filaments to one A-strand (but not to bind A-strands together), and hinders the immune response of titin in the C-zone. The gap filaments arise from the coalescence of T-filaments in the I-band, and their extreme extensibility from an unravelling of a beaded structure in the titin molecule. A layout is presented for the molecule in zones of configuration and function. Possible arrangements of the T-filaments in the M- and N(2)-zones are discussed.