Disruption of the actin cytoskeleton in living nonmuscle cells by microinjection of antibodies to domain-3 of caldesmon

Two classes of affinity-purified polyclonal antibodies directed against the four different domains of gizzard caldesmon were prepared and their specificity was verified by immunochemical assays. One set of antibodies recognized exclusively domain-3 spanning residues 483-578 and the other one included IgG reactive toward the remaining domains-1, -2 and -4 corresponding to residues 1-482 and 581-756. Microinjection into cultured fibroblasts of each antibody preparation was employed to probe the functional importance of the interaction between caldesmon and tropomyosin within living nonmuscle cells. Low concentrations of the antibody to domain-3 caused a rapid, severe and reversible disassembly of the microfilament network. In contrast, the antibodies to domains-1, -2 and -4 were ineffective. The effects of the anti-domain-3 IgG were observed not only with the purified antibody but also when present in the whole caldesmon antiserum serving for its isolation. The microfilament disintegrating activity of this antibody was completely abolished upon preincubation with native caldesmon or a proteolytic caldesmon fragment encompassing amino acids 483-578. In enzyme-linked immunosorbent assay competition experiments, tropomyosin, but not F-actin, significantly decreased the binding of the domain-3 antibody to caldesmon. Consistent with recent mutational studies pinpointing to the contribution of domain-3 in the in vitro binding of cellular caldesmon to tropomyosin, the findings suggest that the microinjected domain-3 antibody selectively disrupts the association of tropomyosin with caldesmon domain-3, thereby destabilizing the protein complex and alleviating its known protective action against F-actin severing in the cell. Thus, our data further highlight the in vivo involvement of this particular domain in the attachment of non-muscle caldesmon to tropomyosin as well as the direct participation of the caldesmon-tropomyosin complex in the cytoskeletal organization of the microfilaments.

Characterization of the regulatory domain of gizzard calponin. Interactions of the 145-163 region with F-actin, calcium-binding proteins, and tropomyosin

Earlier, we proposed that the interaction of gizzard calponin with F-actin, promoting the inhibition of the actomyosin ATPase activity, involves the NH2-terminal portion of the calponin segment Ala145-Tyr182 (Mezgueldi, M., Fattoum, A., Derancourt, J., and Kassab, R. (1992) J. Biol. Chem. 267, 15943-15951). In this work, we have directly probed this region for actin binding sites using five peptide analogs covering different stretches of the sequence Thr133-Ile163. Co-sedimentation with F-actin, actomyosin ATPase measurements, and zero-length cross-linking reactions demonstrated that the 19-residue sequence Ala145-Ile163 is essential for actin interaction and ATPase inhibition. Furthermore, each peptide was tested for binding to the Ca(2+)-dependent proteins, caltropin and calmodulin, in both an actomyosin ATPase assay and an affinity chromatographic assay. The results revealed the 11-residue segment Gln153-Ile163, representing the COOH-terminal moiety of the F-actin binding sequence, as a crucial region for the high affinity binding of these regulatory proteins with concomitant removal of the ATPase inhibition. The 153-163 stretch contained also interactive sites for tropomyosin as assessed by affinity chromatography and spectrofluorometry. Collectively, the data support our initial results and highlight the ability of the multifunctional 145-163 region to serve as a potent regulatory domain of the smooth muscle calponin.

Precise identification of the regulatory F-actin- and calmodulin-binding sequences in the 10-kDa carboxyl-terminal domain of caldesmon

The precise location of the regulatory F-actin- and calmodulin-binding sites in the COOH-terminal sequence Trp659-Pro756 of gizzard caldesmon was investigated by subjecting the corresponding 10-kDa CNBr fragment, characterized earlier (Bartegi, A., Fattoum, A., Derancourt, J., and Kassab, R. (1990) J. Biol. Chem. 265, 15231-15238), to limited chymotryptic reactions conducted in the absence and presence of F-actin-tropomyosin. As a result, the F-actin-binding and actomyosin ATPase inhibitory activity was separated from the regulatory Ca(2+)-calmodulin-binding site. Seven chymotryptic peptides accounting for the entire primary structure of the CB10 fragment were isolated, and their complete amino acid sequences were established by combining NH2-terminal sequencing, mass spectrometry, and gel electrophoresis. Reversed-phase high performance liquid chromatography analyses of the binding of F-actin to these peptides revealed the 30-residue sequence Leu693-Trp722 as the unique crucial stretch for actin interaction and ATPase inhibition. This segment was also specifically protected by F-actin against proteolytic degradation. We further determined the functional properties of three synthetic peptides which successively cover the sequences Asn675-Lys695, Leu693-Trp722, and Arg711-Lys729. The first peptide segment specifically bound Ca(2+)-calmodulin as assessed by affinity chromatography and spectrofluorometry and should contain a potent novel calmodulin-binding subsite. The second immediately adjacent peptide inhibited the actomyosin ATPase in a tropomyosin-sensitive manner, as expected. In contrast, the third peptide displayed no detectable function. The results indicate that the overall sequence Asn675-Trp722 represents the essential regulatory unit of the COOH-terminal 10-kDa domain of caldesmon.

Mapping of the functional domains in the amino-terminal region of calponin

Three chymotryptic fragments accounting for almost the entire amino acid sequence of gizzard calponin (Takahashi, K., and Nadal-Ginard, B. (1991) J. Biol. Chem. 266, 13284-13288) were isolated and characterized. They encompass the segments of residues 7-144 (NH2-terminal 13-kDa peptide), 7-182 (NH2-terminal 22-kDa peptide), and 183-292 (COOH-terminal 13-kDa peptide). They arise from the sequential hydrolysis of the peptide bonds at Tyr182-Gly183 and Tyr144-Ala145 which were protected by the binding of F-actin to calponin. Only the NH2-terminal 13- and 22-kDa fragments were retained by immobilized Ca(2+)-calmodulin, but only the larger 22 kDa entity cosedimented with F-actin and inhibited, in the absence of Ca(2+)-calmodulin, the skeletal actomyosin subfragment-1 ATPase activity as the intact calponin. Since the latter peptide differs from the NH2-terminal 13-kDa fragment by a COOH-terminal 38-residue extension, this difference segment appears to contain the actin-binding domain of calponin. Zero-length cross-linked complexes of F-actin and either calponin or its 22-kDa peptide were produced. The total CNBr digest of the F-actin-calponin conjugate was fractionated over immobilized calmodulin. The EGTA-eluted pair of cross-linked actin-calponin peptides was composed of the COOH-terminal actin segment of residues 326-355 joined to the NH2-terminal calponin region of residues 52-168 which seems to contain the major determinants for F-actin and Ca(2+)-calmodulin binding.