Structural and functional studies of titin's fn3 modules reveal conserved surface patterns and binding to myosin S1 - A possible role in the frank-starling mechanism of the heart

The A-band part of titin, a striated-muscle specific protein spanning from the Z-line to the M-line, mainly consists of a well-ordered super-repeat ar ray of immunoglobulin-like and fibronectin-type III (fn3)-like domains. Sin ce it has been suspected that the fn3 domains might represent titin's bindi ng sites to myosin, we have developed structural models for all of titin's 132 fn3-like domains. A subset of eight experimentally determined used as h omology templates. After grouping the models according to their position wi thin the super-repeat segment of the central A-band titin region, we analyz ed the models with respect to side-chain conservation. This showed that con served residues form an extensive surface pattern predominantly at one side of the domains, whereas domains outside the central C-zone super-repeat re gion show generally less conserved surfaces. Since the conserved surface re sidues may function as protein-binding sites, we experimentally studied the binding properties of expressed multi-domain fn3 fragments. This revealed that fn3 fragments specifically bind to the sub-fragment 1 of myosin. We al so measured the effect of fn3 fragments on the contractile properties of si ngle cardiac myocytes. At sub-maximal Ca2+ concentrations, fn3 fragments si gnificantly enhance active tension. This effect is most pronounced at short sarcomere length, and as a result the length-dependence of Ca2+ activation is reduced. A model of how titin's fn3-like domains may influence actomyos in interaction is proposed. (C) 2001 Academic Press.