Structural and functional variations in skeletal-muscle and scallop muscle actins

Does muscle-type myosin have ADPase activity?

Adenosine diphosphate (ADP) is a nucleotide that is structurally very similar to ATP but lacks one of the two high-energy bonds due to hydrolysis. In muscle studies, ADP is usually considered exclusively as a product formed during myosin cross-bridge cycling and is not otherwise involved in this process. In our study, we question the widely held view of ADP as a final product formed during muscle contraction. Using biophysical and biochemical methods, we managed to show that ADP can act as a substrate for myosins in at least three types of muscles: smooth and striated adductor muscles of bivalves (Mytilidae and Pectinidae), and also vertebrate skeletal muscles. According to our data, the differences in the effect of ATP and ADP on the optical, biochemical, and structural properties of actomyosins are exclusively quantitative. We explain the previous ideas about ADP as a compound capable of inhibiting the ATPase activity of actomyosin by the ability of ATP and ADP to depolymerize the polymeric myosin when the concentration in the medium reaches more than 0.3 mM.

Functional specificity of actin isoforms

Actin, one of the main proteins of muscle and cytoskeleton, exists as a variety of highly conserved isoforms whose distribution in vertebrates is tissue-specific. Synthesis of specific actin isoforms is accompanied by their subcellular compartmentalization, with both processes being regulated by factors of cell proliferation and differentiation. Actin isoforms cannot substitute for each other, and the high-level synthesis of exogenous actins leads to alterations in cell organization and morphology. This indicates that the highly conserved actins are functionally specialized for the tissues in which they predominate. The first goal of this review is to analyze the data on the polymerizability of actin isoforms to show that cytoskeleton isoactins form less stable polymers than skeletal muscle actin. This difference correlates with the dynamics of actin microfilaments versus the stability of myofibrillar systems. The three-dimensional actin structure as well as progress in the analysis of conformational changes in both the actin monomer and the filament allows us to view the data on the structure and polymerization of isoactins in terms of structure-function relationships within the actin molecule. Most of the amino acid substitutions that distinguish actin isoforms are located apart from actin-actin contact sites in the polymer. We suggest that these substitutions can modulate the ability of actin monomers to form more or less stable polymers by long-range (allosteric) regulation of the contact sites.

Correlation between polymerizability and conformation in scallop beta-like actin and rabbit skeletal muscle alpha-actin

In order to investigate the structural basis for functional differences among actin isoforms, we have compared the polymerization properties and conformations of scallop adductor muscle beta-like actin and rabbit skeletal muscle alpha-actin. Polymerization of scallop Ca(2+)-actin was slower than that of skeletal muscle Ca(2+)-actin. Cleavage of the actin polypeptide chain between Gly-42 and Val-43 with Escherichia coli protease ECP 32 impaired the polymerization of scallop Mg(2+)-actin to a greater extent than skeletal muscle Mg(2+)-actin. When monomeric scallop and skeletal muscle Ca(2+)-actins were subjected to limited proteolysis with trypsin, subtilisin, or ECP 32, no differences in the conformation of actin subdomain 2 were detected. At the same time, local differences in the conformations of scallop and skeletal muscle actin subdomains 1 were revealed as intrinsic fluorescence differences. Replacement of tightly bound Ca(2+) with Mg(2+) resulted in more extensive proteolysis of segment 61-69 of scallop actin than in the case of skeletal muscle actin. Furthermore, segment 61-69 was more accessible to proteolysis with subtilisin in polymerized scallop Ca(2+)-actin than in polymerized skeletal muscle Ca(2+)-actin, indicating that, in the polymeric form, the nucleotide-containing cleft is in a more open conformation in beta-like scallop actin than in skeletal muscle alpha-actin. We suggest that this difference between scallop and skeletal muscle actins is due to a less efficient shift of scallop actin subdomain 2 to the position it has in the polymer. The possible consequences of amino acid substitutions in actin subdomain 1 in the allosteric regulation of the actin cleft, and hence in the different stabilities of polymers formed by different actins, are discussed.

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

Two-dimensional hydrophobic clusters analysis (HCA) was used to compare the distribution of hydrophobic clusters along various actin sequence. HCA-deduced patterns were not altered by amino-acid variations throughout the evolution of actin and we observed similar hydrophobic motifs comprising myosin subfragment-1 ATP-independent binding sites. HCA suggested the presence of two groups of identical hydrophobic motifs (A1 and A2) which bound on each side of the S1 (63 kDa-31 kDa) connecting segment in relation with two actin monomers. This connection is important in communications between actin- and nucleotide-binding sites. We postulate that some relation and message between the two motifs A1 and A2 take place through myosin subfragment-1 (63 kDa-31 kDa) connecting segment.

Localization of two myosin-subfragment-1 binding contacts in the 96-132 region of actin subdomain-1

Many direct observations and indirect experimental approaches have pin-pointed two segments (sequences 1-28 and 360-372) in actin subdomain-1 which bind to myosin subfragment-1. In a previous investigation [Labbé, J. P., Méjean, C., Benyamin, Y. & Roustan, C. (1990) Biochem. J. 271, 407-413], we have observed competition between myosin subfragment-1 and anti-actin antibodies specific to epitopes including Thr103. A multisite interface model has also been proposed to take into account myosin-head binding to the N-terminal and C-terminal regions and to more central 40-113 sequence of actin. In the present study, two limited actin segments encompassing residues 96-103 and 112-125 were identified as myosin-head-binding sites. Myosin subfragment-1 competed for monomeric actin with the antibodies directed against sequences 96-105 and 114-120 and its binding to the tryptic 96-113 and synthetic 112-125 actin peptides was prevented by magnesium pyrophosphate but not by calcium pyrophosphate. In the presence of ATP-Mg2+, myosin subfragment-1 was dissociated by filamin from its complex with monomeric actin or with peptide 105-120. Contact points of filamin on actin were previously located in the 105-120 and 360-372 actin sequences [Méjean, C., Lebart, M. C., Boyer, M., Roustan, C. & Benyamin, Y. (1992) Eur. J. Biochem. 209, 555-562]. The in vitro inhibitory effect of filamin on actin-activated Mg2+-ATPase would thus be explained by this competition. Furthermore, the (27-kDa-50-kDa-20-kDa) trypsin-split myosin subfragment-1 which could no longer be activated by actin, did not bind at all to the two sites located in the 96-125 region, but it still interacted with the 360-372 segment. Our results regarding the position of the myosin head on actin monomers in rigor conditions provide evidence on the presence of two topologically independent contact points in the myosin-head/actin interface. One group exposed residues in the 1-7, 21-29, 77-95 and 96-103 actin segment, another, on the opposite side of subdomain-1, included residues from 112-125 and 360-372 sequences.

Molecular genetics of actin function

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The mammalian anti-alpha-smooth muscle actin monoclonal antibody recognizes an alpha-actin-like protein in planaria (Dugesia lugubris s.l.)

The presence of an alpha-smooth muscle (alpha-sm) actin-like protein in planaria (Dugesia lugubris s.l.) is reported. The protein shows a 42 kDa molecular weight determined by sodium dodecyl sulphate polyacrylamide gel electrophoresis and is specifically recognized by the mammalian anti alpha-sm actin monoclonal antibody. When a planarian is induced to regenerate by head amputation, the immunostaining of the alpha-sm actin-like molecule becomes important in the area of growing blastema, reaching a maximum between 70-120 hours after injury. Conventional electron microscopy at the 4-day-regeneration stage shows that blastema-forming cells are a homogeneous population whose morphological features resemble those of migrating mesenchyme-like cells; only the myoblasts show a recognizable phenotype. The immunocytochemical localization of alpha-sm actin-like molecule by immunoperoxidase (light microscopy) and immunogold stains (electron microscopy) was carried out on both intact and injured worms. The antigen was localized mainly at the basal portion of the epidermal cells and in the undifferentiated mesenchyme-like cells. Myoblasts, but not differentiated myofibers, were also labelled by this antibody. The results indicate that in the lower Eumetazoan planarians, as well as in vertebrates, the alpha-sm actin can be considered to be a marker for myoid differentiation. The suggestion that alpha-sm actin can be used as a marker for mesenchyme-like cells in vertebrates and in invertebrates is also discussed.

Characterization of an actin-myosin head interface in the 40-113 region of actin using specific antibodies as probes

Evidence for the participation of the 1-7 and 18-28 N-terminal sequences of actin at different steps of actin-myosin interaction process is well documented in the literature. Cross-linking of the rigor complex between filamentous actin and skeletal-muscle myosin subfragment 1 was accomplished by the carboxy-group-directed zero-length protein cross-linker, 1-ethyl-3-[3-(dimethylamino)propyl]carbodi-imide. After chaotropic depolymerization and thrombin digestion, which cleaves only actin, the covalent complex with Mr 100,000 was characterized by PAGE. The linkage was identified as being between myosin subfragment 1 (S-1) heavy chain and actin-(1-28)-peptide. The purified complex retained in toto its ability to combine reversibly with fresh filamentous actin, but showed a decrease in the Vmax. of actin-dependent Mg2(+)-ATPase. By using e.l.i.s.a., S-1 was observed to bind to coated monomeric actin or its 1-226 N-terminal peptide. This interaction strongly interfered with the binding of antibodies directed against the 95-113 actin sequence. Moreover, S-1 was able to bind with coated purified actin-(40-113)-peptide. Finally, antibodies directed against the 18-28 and 95-113 actin sequence, which strongly interfered with S1 binding, were unable to compete with each other. These results suggest that two topologically independent regions are involved in the actin-myosin interface: one located in the conserved 18-28 sequence and the other near residues 95-113, including the variable residue at position 89. Other experiments support the 'multisite interface model', where the two actin sites could modulate each other during S-1 interaction.

Comparative study of invertebrate actins: antigenic cross-reactivity versus sequence variability

The immunological similarities or differences between various actins from muscles of invertebrates were investigated. We elicited antibodies to actin purified from the adductor muscle of the mollusc, Pecten maximus. The antiserum comprised a major population that was specific for the N-terminal extremity of this isoform. This antibody population was used together with other antibodies specific for various domains on the surface of skeletal muscle actin to compare seven actins from invertebrate muscles. The N-terminal extremity showed close homologies between invertebrates (except for Crustacea) but differences were observed with vertebrate skeletal and cytoplasmic actins. In contrast, sequence 18-28, which constitutes part of the myosin interaction domain, appears to be conserved in all actins studied. Other regions (sequences 40-113, 168-226 and 285-375) presented a variable behaviour depending on the particular invertebrate species.