Fig. 2. Schematic diagram of rat l-caldesmon constructs used in this study. The N-terminal fragment of l-caldesmon (Cad40; amino acids 1-152) harbours the myosin-binding site (hatched) (Velaz et al., 1990; Bogatcheva et al., 1993; Redwood and Marston, 1993; Huber et al., 1995) and a tropomyosin-binding site (black) (Smith et al., 1987; Redwood and Marston, 1993; Redwood et al., 1993). The C-terminal fragment of l-caldesmon (Cad39; amino acids 236-532) contains two tropomyosin-binding sites, two actin-interacting regions (diamonds) (Wang et al., 1997b; Marston et al., 1998), and the two Ca²⁺/calmodulin binding sites A and B, as indicated (dark-grey boxes) (Wang et al., 1991; Marston et al., 1994; Wang et al., 1996). Both Ca²⁺/calmodulin-binding sites contain a key tryptophan residue (W454 and W487) that is essential for this interaction (Graether et al., 1997); replacement of these tryptophan residues with alanine (W454A and W487A) generates an l-caldesmon mutant that is unable to interact with Ca²⁺/calmodulin (CadCamAB).