Two identical hydrophobic clusters are present on the same actin monomer: interaction between one myosin subfragment-1 and two actin monomers

Close proximity of myosin loop 3 to troponin determined by triangulation of resonance energy transfer distance measurements

Cooperative activation of the thin filament is known to be influenced by the tight binding of myosin to actin, but the molecular mechanism underlying this contribution of myosin is not well understood. To better understand the structural relationship of myosin with the regulatory troponin complex, resonance energy transfer measurements were used to map the location of troponin relative to a neighboring myosin bound to actin using atomic models. Using a chicken troponin T isoform that contains a single cysteine near the binding interface between troponins T, I, and C, this uniquely labeled cysteine on troponin was found to be remarkably near loop 3 of myosin. This loop has previously been localized near the actin and myosin interface by chemical cross-linking methods, but its functional contributions have not been established. The implications of this close proximity are examined by molecular modeling, which suggests that only restricted conformations of actomyosin can accommodate the presence of troponin at this location near the cross-bridge. This potential for interaction between troponin and myosin heads that bind near it along the thin filament raises the possibility of models in which direct myosin and troponin interactions may play a role in the regulatory mechanism.

Interaction of 75-106 actin peptide with myosin subfragment-1 and its trypsin modified derivative

To explore the role of a hydrophobic domain of actin in the interaction with a myosin chain we have synthesized a peptide corresponding to residues 75-106 of native actin monomer and studied by fluorescence and ELISA the interaction (13+/-2.6x10(-6) M) with both S-1 and (27 kDa-50 kDa-20 kDa) S-1 trypsin derivative of myosin. The loop corresponding to 96-103 actin residues binds to the S-1 only in the absence of Mg-ATP and under similar conditions but not to the trypsin derivative S-1. Biotinylated C74-K95 and I85-K95 peptide fragments were purified after actin proteolysis with trypsin. The C74-K95 peptide interacted with both S-1 and the S-1 trypsin derivative with an apparent Kd(app) of 6+/-1.2x10(-6) M in the presence or absence of nucleotides. Although peptide fragment I85-K95 binds to S-1 with a Kd(app) of 12+/-2.4x10(-6) M, this fragment did not bind to the trypsin S-1 derivative. We concluded that the actin 85-95 sequence should be a potential binding site to S-1 depending of the conformational state of the intact 70 kDa segment of S-1.

Coevolution of actin and associated proteins: an alpha-actinin-like protein in a cyanobacterium (Spirulina platensis)

Actin, together with associated proteins, such as myosin, cross-linking or capping proteins, has been observed in all eukaryotic cells. Presence of actin or actin-like proteins has also been reported in prokaryotic organisms belonging to the cyanobacteria. Our aim was first to extend the characterization of an actin-like protein to another prokaryotic cell, i.e. Spirulina, then to compare the antigenic reactivity of this new protein with that of Synechocystis and skeletal actins. We observed that some of the conserved antigenic epitopes corresponded to actin regions known to interact with cross-linking proteins. We also report for the first time that alpha-actinin and filamin purified from chicken gizzard both interact with a prokaryotic actin-like protein. Finally, we searched for the occurrence of a cross-linking protein in these cyanobacteria and identified a 105-kDa protein as an alpha-actinin-like protein using specific antibodies.

Influence of actin binding on the conformation of the central segment of the heavy chain of skeletal myosin subfragment 1

Chymotryptic subfragment 1 (S1) of fast skeletal muscle myosin was digested with trypsin in a low ionic strength buffer in the presence of actin. Under these conditions, leading to S1-induced polymerization of actin (Cooke, R. and Morales, M.F. (1971) J. Mol. Biol. 60, 249-261), the S1 heavy chain was cleaved between Lys-561 and Ser-562, generating the C-terminal fragment with apparent mass of 31 kDa. In the absence of actin, this peptide bond was inaccessible to trypsin. The yield of the 31 kDa fragment decreased with the increase in the ionic strength of the medium. The cleavage was also partially inhibited by magnesium or calcium chloride at millimolar concentrations. The data suggest that in low salt conditions and at low concentrations of divalent cations, actin induces a conformational change in the C-terminal portion of the 50 kDa central segment of the S1 heavy chain.