Biomedical subjects

M el-Mezgueldi

Publications and source records attributed to M el-Mezgueldi.

4 recordsLinked to original sources

PubMed1998

Structural interactions between actin, tropomyosin, caldesmon and calcium binding protein and the regulation of smooth muscle thin filaments.

The basic structure and functional properties of smooth muscle thin filaments were established about 10 years ago. Since then we and others have been working on the details of how tropomyosin, caldesmon and the Ca(2+)-binding protein regulate actin interaction with myosin. Our work has tended to emphasize the similarities between caldesmon and troponin function whilst others have been more concerned with the differences. The need to resolve the resulting differences has stimulated us to find new and more direct ways of investigating the mechanism of thin filament regulation. In recent years an apparent divergence has opened up between functional measurements, which indicate an allosteric-cooperative regulatory mechanism in which caldesmon and Ca(2+)-binding protein control actin-tropomyosin state in the same way as troponin, and structural measurements which show thin filament structures unlike striated muscle thin filaments. The challenge is to interpret function in terms of structure. We have combined functional studies with expression and mutagenesis of caldesmon and with structural methods including X-ray crystalography of tropomyosin-caldesmon crystals, electron microscopy and helical reconstruction of actin-tropomyosin-caldesmon complexes and high resolution nuclear magnetic resonance spectroscopy of the C-terminus of caldesmon in interaction with actin and calmodulin. We have used this information to propose a structural mechanism for caldesmon regulation of the smooth muscle thin filament.

Actins↗

PubMed1997

3-D image reconstruction of reconstituted smooth muscle thin filaments containing calponin: visualization of interactions between F-actin and calponin.

Calponin is a putative thin filament regulatory protein of smooth muscle that inhibits actomyosin ATPase in vitro. We have used electron microscopy and three-dimensional reconstruction to elucidate the structural organization of calponin on actin and actin-tropomyosin filaments. Calponin density was clearly delineated in the reconstructions and found to occur peripherally along the long-pitch actin-helix. The main calponin mass was located over sub-domain 2 of actin, and connected axially adjacent actin monomers by binding to the "upper" and "lower" edges of sub-domains 1 of each actin. When the reconstructions were fitted to the atomic model of F-actin, calponin appeared to contact actin near the N terminus and at residues 349 to 352 close to the C terminus of sub-domain 1 on one monomer. It also touched residues 92 to 95 of sub-domain 1 on the axially neighboring actin and continued up the side of this monomer as far as residues 43 to 48 of sub-domain 2. These positions are consensus binding sites for a number of actin-associated proteins and are also near to sites of weak myosin interaction. Calponin did not appear to block strong myosin binding sites on actin. In contrast to the calponin mass which appeared monomeric in reconstructions, tropomyosin formed a continuous strand of added density along F-actin. When added to tropomyosin-containing filaments, calponin caused a shift of tropomyosin away from sub-domain 1 towards sub-domain 3 of actin, exposing strong myosin-binding sites that were previously covered by tropomyosin. This structural effect is unlike that of troponin and therefore inhibition of actomyosin ATPase by calponin and troponin cannot be strictly analogous. The location of calponin would allow it to directly compete or interact with a number of actin-binding proteins.

Actins↗

PubMed1996

Expressing functional domains of mouse calponin: involvement of the region around alanine 145 in the actomyosin ATPase inhibitory activity of calponin.

Previously, we attributed the binding of F-actin to the 38-residue stretch of gizzard calponin encompassing the sequence A145-Y182 and postulated the hexapeptide motif VKYAEK, representing residues 142-147, as a putative actin-binding site [Mezgueldi, M., Fattoum, A., Derancourt, J. & Kassab, R. (1992) J. Biol. Chem. 267, 15943-15951]. Herein, the nature of the ATPase inhibitory amino acids of calponin and their relative position within the actin binding domain was investigated by expressing the following fragments of mouse calponin with or without substitution or deletion of the hexapeptide V142-K147: amino acids 1-228 (CaP1-228), 45-228 (CaP45-228), 131-228 (CaP131-228), and CaP1-228 with substitution of A145 with S (CaP1-228A145S) or deletion of V142-K147 (CaP1-228de1142-147). All the recombinant fragments displayed most of the biochemical properties of the smooth muscle purified calponin including (a) expected electrophoretic mobility, (b) heat stability, (c) binding to actin, tropomyosin and calmodulin, and (d) zero-length cross-linking to actin switched by calmodulin in a calcium-dependent fashion. However, while the wild-type recombinant fragments inhibit the acto-S-1 ATPase activity to the same extent as do the parent calponin, modulation of the hexapeptide by either substitution or deletion strongly affect the inhibitory activity with only slightly decreasing actin binding capacity. The data indicate that the stretch VKYAEK is crucial for ATPase inhibition by calponin but represents only part of the actin-binding domain. These results are discussed in terms of multiple contact sites between actin and calponin.

Actins↗

PubMed1996

Calponin.

Calponin is a troponin-T like protein purified from chicken gizzard smooth muscle. It binds to actin, myosin, Ca(2+)-binding proteins and tropomyosin and inhibits the actomyosin ATPase as well as the movement of actin filaments over myosin in vitro. These properties have led to the proposal that calponin may be involved in the Ca(2+)-dependent regulation of actin-myosin interaction and consequently of smooth muscle contraction. Calponin is localized in both the contractile and the cytoskeletal parts of the smooth muscle cell and may have a structural function in smooth muscle cells. It may also regulate the pool of free actin available for cytoskeleton organization. In vitro calponin function is modulated by its interaction with a Ca(2+)-binding protein and/or by its phosphorylation. This suggests that calponin may play an important role in signal transduction from the membrane receptor to the contractile proteins in smooth muscle.

Actins↗