The Caldesmon-Actin Interaction

Actin binding sites are confined to the C terminus of CD. Many workers have demonstrated that domains 3 and 4, whether prepared by proteolytic digestion or expressed as recombinant proteins, are indistinguishable from whole CD (Redwood et al., 1990 C.-L.A. Wang et al., 1991a Hayashi et al, 1991 Z. Wang et al., 

Although there is evidence for a weak interaction between domain 3 and actin (Leszyk et al, 1989 Levine et al, 1990 Hayashi et al, 1991), recombinant fragments containing domains 1,2 and 3 did not bind to actin in quantitative assays (Redwood et al, 1990 Wang et al, 1994). This sets a limit of less than 4 x 10 for domain 3 binding to actin. 

Domain 4 contains the high-affinity actin binding sites that are involved in inhibition of the actin filament s activity. CD fragment 606C (Marston and Redwood 1992) (Fig. 4), the equivalent chymotryptic fragments ( 18K and 20K ) (Szpacenko and Dabrowska, 

5 to 1 CD fragment bound per actin—the same as whole CD (Marston and Redwood, 1993 Mezgueldi et al, 1994 Chalovich et al, 1992 Bartegi et al, 1990). 

If these results can be taken at face value, the middle of domain 4 [726-767 (669-710 in chicken)] is involved in the TM enhancement of CD inhibition and binding to actin. Since this sequence is well separated from the putative TM binding sequence in domain 4, the effects of TM are probably indirect. The C-terminal end of this sequence is particularly rich in proline, a motif known to be present in many protein-protein binding sites (Williamson, 1994). Amino acids 752-771 (695-714), 

---