Studies on the role of actin's aspartic acid 3 and aspartic acid 11 using oligodeoxynucleotide-directed site-specific mutagenesis

Rapid nucleotide exchange renders Asp-11 mutant actins resistant to depolymerizing activity of cofilin, leading to dominant toxicity in vivo

Conserved Asp-11 of actin is a part of the nucleotide binding pocket, and its mutation to Gln is dominant lethal in yeast, whereas the mutation to Asn in human α-actin dominantly causes congenital myopathy. To elucidate the molecular mechanism of those dominant negative effects, we prepared Dictyostelium versions of D11N and D11Q mutant actins and characterized them in vitro. D11N and D11Q actins underwent salt-dependent reversible polymerization, although the resultant polymerization products contained small anomalous structures in addition to filaments of normal appearance. Both monomeric and polymeric D11Q actin released bound nucleotides more rapidly than the wild type, and intriguingly, both monomeric and polymeric D11Q actins hardly bound cofilin. The deficiency in cofilin binding can be explained by rapid exchange of bound nucleotide with ATP in solution, because cofilin does not bind ATP-bound actin. Copolymers of D11Q and wild type actins bound cofilin, but cofilin-induced depolymerization of the copolymers was slower than that of wild type filaments, which may presumably be the primary reason why this mutant actin is dominantly toxic in vivo. Purified D11N actin was unstable, which made its quantitative biochemical characterization difficult. However, monomeric D11N actin released nucleotides even faster than D11Q, and we speculate that D11N actin also exerts its toxic effects in vivo through a defective interaction with cofilin. We have recently found that two other dominant negative actin mutants are also defective in cofilin binding, and we propose that the defective cofilin binder is a major class of dominant negative actin mutants.

Dominant negative mutant actins identified in flightless Drosophila can be classified into three classes

Strongly dominant negative mutant actins, identified by An and Mogami (An, H. S., and Mogami, K. (1996) J. Mol. Biol. 260, 492-505), in the indirect flight muscle of Drosophila impaired its flight, even when three copies of the wild-type gene were present. Understanding how these strongly dominant negative mutant actins disrupt the function of wild-type actin would provide useful information about the molecular mechanism by which actin functions in vivo. Here, we expressed and purified six of these strongly dominant negative mutant actins in Dictyostelium and classified them into three groups based on their biochemical phenotypes. The first group, G156D, G156S, and G268D actins, showed impaired polymerization and a tendency to aggregate under conditions favoring polymerization. G63D actin of the second group was also unable to polymerize but, unlike those in the first group, remained soluble under polymerizing conditions. Kinetic analyses using G63D actin or G63D actin.gelsolin complexes suggested that the pointed end surface is defective, which would alter the polymerization kinetics of wild-type actin when mixed and could affect formation of thin filament structures in indirect flight muscle. The third group, R95C and E226K actins, was normal in terms of polymerization, but their motility on heavy meromyosin surfaces in the presence of tropomyosin-troponin indicated altered sensitivity to Ca(2+). Cofilaments in which R95C or E226K actins were copolymerized with a 3-fold excess of wild-type actin also showed altered Ca(2+) sensitivity in the presence of tropomyosin-troponin.

Actin is a component of the compensation mechanism in pseudorabies virus virions lacking the major tegument protein VP22

Despite being a major component of the pseudorabies virus tegument, VP22 is not required for PRV replication, virulence, or neuroinvasion (T. del Rio, H. C. Werner, and L. W. Enquist, J. Virol. 76:774-782, 2002). In the absence of VP22, tegument assembly compensates in a limited fashion with increased incorporation of cellular actin. Infection of epithelial cell lines expressing fluorescent actin fusion proteins resulted in the incorporation of filamentous and nonfilamentous actin into individual virions that were predominately light, noninfectious particles. We conclude that cellular actin is incorporated in the tegument of wild-type virions and is part of a compensation mechanism for VP22-null virions.

Production of human skeletal alpha-actin proteins by the baculovirus expression system

Mutations within the human skeletal muscle alpha-actin gene cause three different skeletal muscle diseases. Functional studies of the mutant proteins are necessary to better understand the pathogenesis of these diseases, however, no satisfactory system for the expression of mutant muscle actin proteins has been available. We investigated the baculovirus expression vector system (BEVS) for the abundant production of both normal and mutant skeletal muscle alpha-actin. We show that non-mutated actin produced in the BEVS behaves similarly to native actin, as shown by DNase I affinity purification, Western blotting, and consecutive cycles of polymerisation and depolymerisation. Additionally, we demonstrate the production of mutant actin proteins in the BEVS, without detriment to the insect cells in which they are expressed. The BEVS therefore is the method of choice for studying mutant actin proteins causing human diseases.

Acetylation at the N-terminus of actin strengthens weak interaction between actin and myosin

The N-terminus of all actins so far studied is acetylated. Although the pathways of acetylation have been well studied, its functional importance has been unclear. A negative charge cluster in the actin N-terminal region is shown to be important for the function of actomyosin. Acetylation at the N-terminus removes a positive charge and increases the amount of net negative charges in the N-terminal region. This may augment the role of the negative charge cluster. To examine this possibility, actin with a nonacetylated N-terminus (nonacetylated actin) was produced. The nonacetylated actin polymerized and depolymerized normally. In actin-activated heavy meromyosin ATPase assays, the nonacetylated actin showed higher K(app) without significantly changing V(max), compared with those of wild-type actin. This is in contrast to the effect of the N-terminal negative charge cluster, which increases V(max) without changing K(app). These results indicate that the acetylation at the N-terminus of actin strengthens weak actomyosin interaction.

Drosophila ACT88F indirect flight muscle-specific actin is not N-terminally acetylated: a mutation in N-terminal processing affects actin function

Many eukaryotic proteins are co and post-translationally modified at their N termini by removal of one or two amino acid residues and N(alpha)-acetylation. Actins show two different forms of N-terminal processing dependent on their N-terminal sequence. In class II actins, which include muscle actins, the common primary sequence of Met-Cys-Asp-actin is processed to acetyl-Asp-actin. The functional significance of this in vivo is unknown. We have studied the indirect flight muscle-specific actin, ACT88F, of Drosophila melanogaster. Our results show that ACT88F is N-terminally processed in vivo as a class II actin by removal of the first two amino acid residues (Met and Cys), but that uniquely the N terminus is not acetylated. In addition we show that ACT88F is methylated, probably at His73. Flies carrying the mod(-) mutation fail to complete post-translational processing of ACT88F. We propose that the mod gene product is normally responsible for removing N-acetyl-cysteine from actin. The biological significance of this process is demonstrated by observations that retention of the N-acetyl-cysteine in ACT88F affects the flight muscle function of mod(-) flies. This suggests that the extreme N terminus affects actomyosin interactions in vivo, a proposal we have examined by in vitro motility assays of ACT88F F-actin from mod(-) flies. The mod(-) actin only moves in the presence of methylcellulose, a viscosity-enhancing agent, where it moves at velocities slightly, but significantly, reduced compared to wild-type. These data confirm that N-acetyl-cysteine at the N terminus affects actomyosin interactions, probably by reducing formation of the initial actomyosin collision complex, a process known to involve the actin N terminus.

Histidine-tagged wild-type yeast actin: its properties and use in an approach for obtaining yeast actin mutants

Wild-type and an N-terminal 6-histidine-tagged actin have each been expressed by using a yeast strain that contains the actin gene on a plasmid and not on the chromosome. Yeast strains have also been constructed that use two plasmids, one expressing the wild-type protein and the other the 6-histidine-tagged protein. Yeast cells can be grown with either plasmid alone or with both plasmids together and appear to be normal in that the growth rates of all the yeast strains are quite similar, as is the morphology of the yeast cells. The polymerization properties of the 6-histidine-tagged actin appear almost identical to wild-type actin expressed from the chromosome. When the wild-type and 6-histidine-tagged actin are coexpressed, they can be purified by standard techniques and then separated using nickel-nitrilotriacetate chromatography. The method can be used to prepare actin mutants including those that are nonfunctional or might not support yeast growth for other reasons.

Actin and the actomyosin interface: a review

This review deals with the structure of the actin monomer, its assembly into filaments and the loci on F-actin involved in binding myosin. Two distinctly different arrangements of monomers have been suggested for actin filaments. One model proposed by Holmes et al. is well developed. It places the so-called 'large' domain close to the filament axis and the so-called 'small' domain out near the surface of the filament. A second, less-well developed, model proposed by Schutt et al. locates the 'small' domain close to the filament axis and they rotate the monomer so that 'bottom' of the 'large' domain is at the highest radius. We analyze the available evidence for the models of F-actin derived from X-ray diffraction, reconstructions from electron micrographs, fluorescence resonance energy transfer spectroscopy, chemical cross-linking, antibody probes, limited proteolysis, site-directed and natural mutations, nuclear magnetic resonance spectroscopy and other techniques. The result is an actin-centered view of the loci on actin which are probably involved in its interaction with the myosin 'head'. From these multiple contacts we speculate on the sequence of steps between the initial weak-binding state of S-1 to the actin filament through to the stable strong-binding state seen in the absence of free Mg-ATP, i.e., the rigor state.

Deletion of amino acids from the carboxy-terminal end of actin

A series of deletions was made from the C-terminal end of actin by inserting termination codons into a full length cDNA of human alpha-skeletal muscle actin. These included deletions of 2, 3, 10, 20, 30, and 40 amino acids. The cDNA clones were transcribed and the resulting mRNAs were translated in vitro using 35S-labeled methionine. The 35S-labeled actin and actin mutants were then tested for the ability to coassemble with carrier actin, bind DNAse I, bind myosin S-1, bind a 27 kDa proteolytic fragment of alpha-actinin, and incorporate into myofibrils in vitro. Removal of the C-terminal two or three amino acids did not grossly alter the properties of actin tested. Deletion of an additional 7 amino acids (10 amino acids total) significantly decreased coassembly, binding to DNAse I, and incorporation into myofibrils, but did not dramatically reduce binding to myosin S-1 or the 27 kDa fragment of alpha-actinin. Deletion of 20 or more amino acids virtually abolished all normal actin function tested. By examining the structure of actin, we propose that the effect of removing residues 356-365 is due to the important role Trp356 plays in maintaining hydrophobic bonds between three non-contiguous segments of actin. We also suggest that removal of residues 366-372 adversely affected the structure or orientation of the DNAse I binding loop and that this change can account for defects in actin binding to DNAse I, coassembly with wild type actin, and incorporation into myofibrils.

Probing actin incorporation into myofibrils using Asp11 and His73 actin mutants

We used a cell free system Bouché et al.: J. Cell Biol. 107:587-596, 1988] to study the incorporation of actin into myofibrils. We used alpha-skeletal muscle actin and actins with substitutions of either His73 [Solomon and Rubenstein: J. Biol.Chem. 262:11382, 1987], or Asp11 [Solomon et al.: J. Biol. Chem. 263:19662, 1988]. Actins were translated in reticulocyte lysate and incubated with myofibrils. The incorporated wild type actin could be cross-linked into dimers using N,N'-1,4-phenylenebismaleimide (PBM), indicating that the incorporated actin is actually inserted into the thin filaments of the myofibril. The His73 mutants incorporated to the same extent as wild type actin and was also cross-linked with PBM. Although some of the Asp11 mutants co-assembled with carrier actin, only 1-3% of the Asp11 mutant actins incorporated after 2 min and did not increase after 2 hr. Roughly 17% of wild type actin incorporated after 2 min and 31% after 2 hr. ATP increased the release of wild type actin from myofibrils, but did not increase the release of Asp11 mutants. We suggest that (1) the incorporation of wild type and His73 mutant actins was due to a physiological process whereas association of Asp11 mutants with myofibrils was non-specific, (2) the incorporation of wild type actin involved a rapid initial phase, followed by a slower phase, and (3) since some of the Asp11 mutants can co-assemble with wild type actin, the ability to self-assemble was not sufficient for incorporation into myofibrils. Thus, incorporation probably includes interaction between actin and a thin filament associated protein. We also showed that incorporation occurred at actin concentrations which would cause disassembly of F-actin. Since the myofibrils did not show large scale disassembly but incorporated actin, filament stability and monomer incorporation are likely to be mediated by actin associated proteins of the myofibril.

The binding of mutant actins to profilin, ATP and DNase I

Twenty-five mutations were created in the Drosophila melanogaster Act88F actin gene by in vitro mutagenesis and the mutant actins expressed in vitro. The affinity of the mutant actins for ATP, profilin and DNase I was determined. They were also tested for conformational changes by non-denaturing gel electrophoresis. Mutations at positions 364 (highly conserved) and 366 (invariant) caused changes in conformation, reduced ATP binding and increased profilin binding. At position 362 (invariant) only the conservative change from tyrosine to phenylalanine had no effect; other changes at this position affected conformation, ATP and profilin binding. Although only glycine or serine occur naturally at position 368, changes to threonine or glutamine had no effect on the actin. The mutant in which Asp363 was replaced by His and that in which Glu364 was replaced by Lys decreased DNase I binding, yet neither amino acid occurs in the DNase I binding site. Likewise several mutations affect ATP and profilin binding but are distant from the binding sites. We conclude that, although actin has a highly conserved amino acid sequence, individual amino acids can have variable tolerance for substitutions. Also amino acid changes can exert significant effects on the binding of ligands to distant parts of the actin structure.

Characterization of yeast-expressed beta-actins, site-specifically mutated at the tumor-related residue Gly245

The tumorigenic cell line HUT14 expresses a beta-actin carrying a mutation at position 245. In this study, two mutant beta-actins with amino acid changes at position 245 replacing the wild-type glycine by an aspartic acid and a lysine residue, respectively, were produced in the yeast Saccharomyces cerevisiae, purified to homogeneity and characterized with respect to polymerization behaviour and interaction with myosin. The major functional effect of these mutations appears to be an impaired polymerization, while the interaction with myosin seems less influenced. In addition, the results also suggest the presence of a Ca(2+)-binding site in the region of residue 245 in actin.

Functional and ultrastructural effects of a missense mutation in the indirect flight muscle-specific actin gene of Drosophila melanogaster

A single-site mutation of the flight-muscle-specific actin gene of Drosophila melanogaster causes a substitution of glutamic acid 93 by lysine in all the actin encoded in the indirect flight muscle (IFM). In these Act88FE93K mutants, myofibrillar bundles of thick and thin filaments are present but lack Z-discs and all sarcomeric repeats. Dense filament bundles, which are probably aberrant Z-discs, are seen in myofibrils of pupal flies, but early in adult life these move to the periphery of the fibrils and are not seen in skinned adult fibres. Consistent with this observation, alpha-actinin and other high molecular weight proteins, possibly associated with Z-discs, are not detected on SDS/polyacrylamide gels or Western blots of skinned adult IFM. The mutation lies at the beginning of a loop in the small domain of actin, near the myosin binding region. However, that the mutant actin binds myosin heads is shown by (1) rigor crossbridges in electron micrographs, (2) the appropriate rise in stiffness when ATP is withdrawn in mechanical experiments, and (3) equal protection against tryptic digestion provided by rigor binding between actin and myosin in both wild-type and mutant fibres. Reversal of rigor chevron angle along some thin filaments reflects reversal of thin-filament polarity due to lattice disorder. The absence of Z-discs, alpha-actinin and two high molecular weight proteins, and binding studies by others, suggest that the substitution at residue 93 affects the binding of the mutant actin to a protein, possibly alpha-actinin, which is necessary for Z-disc assembly or maintenance.

Interference with myosin subfragment-1 binding by site-directed mutagenesis of actin

Three N-terminal double mutants of beta-actin expressed in the yeast Saccharomyces cerevisiae have been characterized with respect to DNase-I interaction, N-terminal post-translational modification, polymerizability and myosin subfragment-1 binding. The results strongly support earlier suggestions that the acidic residues at the N-terminus of actin are part of the myosin-binding site, while they seem to be of no importance for the other aspects of actin biochemistry tested. The suitability of this expression system for production of recombinant actin in general is discussed.

Post-translational processing of the amino terminus affects actin function

We have studied the importance of N-terminal processing for normal actin function using the Drosophila Act88F actin gene transcribed and translated in vitro. Despite having different charges as determined by two-dimensional (2D) gel electrophoresis, Act88F expressed in vivo and in vitro in rabbit reticulocyte lysate bind to DNase I with equal affinity and are able to copolymerise with bulk rabbit actin equally well. Using peptide mapping and thin-layer electrophoresis we have shown that bestatin [( 3-amino-2-hydroxy-4-phenyl-butanoyl]-L-leucine), an inhibitor of aminopeptidases, can inhibit actin N-terminal processing in rabbit reticulocyte lysate. Although processed and unprocessed actins translated in vitro are able to bind to DNase I equally well, unprocessed actins are less able to copolymerise with bulk actins. This effect is more pronounced when bulk rabbit actin is used but is still seen with bulk Lethocerus actin. Also, the unprocessed actins reduce the polymerisation of the processed actin translated in vitro with the bulk rabbit actin. This suggests that individual actins do interact, even in non-polymerising conditions. The reduced ability of unprocessed actin to polymerise shows that correct post-translational modification of the N terminus is required for normal actin function.

Immunochemical probing of the N-terminal segment on actin: the polymerization reaction

The N-terminal segment of actin contains a cluster of acidic residues which are implicated in macromolecular interactions of this protein. In this work, the interrelationship between the N-terminal segment and the polymerization of actin was studied by using affinity-purified antibodies directed against the first seven N-terminal residues on alpha-skeletal actin (S alpha N). The Fab fragments of these antibodies showed equal affinities for G- and F-actin while the bivalent IgG bound preferentially to the polymerized actin. As monitored by pyrene fluorescence measurements, the binding of Fab to G-actin did not alter the kinetics of the MgCl2-induced polymerization; IgG accelerated this reaction considerably. Consistent with these observations, the binding of Fab to F-actin did not change its morphological appearance in electron micrographs and had no effect on the stability and the rate of dissociation of actin filaments. These results are discussed in terms of their implications to the spatial relationship between the N-terminal segment and the rest of the molecule and the context of the polymerization reaction of actin in vitro and in vivo.