Alignment of caldesmon on the actin-tropomyosin filaments.
We have reported previously that each smooth-muscle caldesmon binds predominantly to a region within residues 142-227 of tropomyosin, but a weaker binding site also exists at the N-terminal region of tropomyosin [Watson, Kuhn, Novy, Lin and Mak (1990) J. Biol. Chem. 265, 18860-18866]. In view of recent evidence for the presence of tropomyosin-binding sites at both the N- and C-terminal domains of caldesmon, we have studied the binding of the N- and C-terminal fragments of human fibroblast caldesmon expressed in Escherichia coli to tropomyosin and its CNBr fragments. The N-terminal fragment, CaD40 (residues 1-152), binds tropomyosin, but the interaction is mostly abolished in the presence of actin. CaD40 binds strongly to Cn1B(142-281) of tropomyosin, but weakly to Cn1A(11-127). The C-terminal fragment, CaD39, which corresponds to residues 443-736 of gizzard caldesmon, binds tropomyosin, and the interaction is enhanced by actin. CaD39 binds to both Cn1A(11-127) and Cn1B(142-281) of tropomyosin. Our results suggest that the N-terminal domain of caldesmon interacts with the C-terminal half of one tropomyosin molecule, whereas the C-terminal domain binds to both N- and C-terminal regions of the adjacent tropomyosin molecule along the actin filament. In addition, the binding of the N-terminal domain of caldesmon to the actin-tropomyosin filament is weak, which may allow this domain to project off the thin filament to interact with myosin.
Tsuruda
T S
TS
Department of Biochemistry, Queen's University, Kingston, Ontario, Canada.
Watson
M H
MH
Foster
D B
DB
Lin
J J
JJ
Mak
A S
AS
D000199:Actins / Q000378:metabolism*; D000818:Animals; D002148:Calmodulin-Binding Proteins / Q000235:genetics / Q000378:metabolism*; D002645:Chickens; D003001:Cloning, Molecular; D004926:Escherichia coli / Q000235:genetics; D006801:Humans; D010446:Peptide Fragments / Q000235:genetics / Q000378:metabolism; D011485:Protein Binding; D011817:Rabbits; D014335:Tropomyosin / Q000378:metabolism*
D000199:Actins; D002148:Calmodulin-Binding Proteins; D010446:Peptide Fragments; D014335:Tropomyosin