Polylysine Induces an Antiparallel Actin Dimer That Nucleates Filament Assembly

Abstract

An antiparallel actin dimer has been proposed to be an intermediate species during actin filament nucleation. We now show that latrunculin A, a marine natural product that inhibits actin polymerization, arrests polylysine-induced nucleation at the level of an antiparallel dimer, resulting in its accumulation. These dimers, when composed of pyrene-labeled actin subunits, give rise to a fluorescent excimer, permitting detection during polymerization in vitro. We report the crystallographic structure of the polylysine-actin-latrunculin A complex at 3.5-Å resolution. The non-crystallographic contact is consistent with a dimeric structure and confirms the antiparallel orientation of its subunits. The crystallographic contacts reveal that the mobile DNase I binding loop of one subunit of a symmetry-related antiparallel actin dimer is partially stabilized in the interface between the two subunits of a second antiparallel dimer. These results provide a potential explanation for the paradoxical nucleation of actin filaments that have exclusively parallel subunits by a dimer containing antiparallel subunits.Actin filament nucleation occurs very slowly de novo, but it occurs rapidly as a necessary step in actin-based motility (1). The formation of a dimer from monomeric subunits is the most thermodynamically unfavorable nucleation step with an estimated equilibrium dissociation constant of 4.6 M (in contrast to 0.6 mM for conversion of dimer to trimer) in a recent molecular dynamic simulation of nucleation (2). The formation of an effective nucleus may be accelerated in vivo by an actin-binding protein such as gelsolin, which can stabilize dimeric actin, or by a protein complex such as Arp2/3 that is thought to contain two actin-like molecules constrained in an orientation that promotes nucleation (3, 4). Antiparallel actin dimers have been identified as a precursor to actin filament polymerization by covalent cross-linking during polymerization induced with divalent cations (5). A gelsolin-actin complex capable of nucleating filament growth at the slow growing, pointed end of filaments has also been shown by covalent cross-linking to contain two actin subunits in the antiparallel configuration (6). The assumption of an antiparallel configuration of subunits is based on evidence that Cys-374 in the C terminus of actin is the only residue involved in the cross-linking reaction. In contrast, when polymerization is complete, intrafilament cross-linking yields a parallel dimer. More recently, electron microscopy has revealed that newly formed actin filaments show evidence of incorporation of antiparallel dimers. This incorporation results in a branched filament network, implying that the dimers have nucleating activity (7). Interestingly, analysis of a Listeria model of cell motility using high-resolution laser tracking provides evidence that filaments elongate in 5.4 nm steps, consistent with in vivo incorporation of dimeric actin (8).In the current work, we provide evidence that polylysine nucleates actin polymerization by e...

Tropomodulin 3 Binds to Actin Monomers

“…However, unlike these other actin monomer binding proteins, Tmod3 monomer binding affinity was not sensitive to actin nucleotide status (less than 4-fold) or to LatA binding, both of which are known to induce substantial conformational changes in the monomer conformation (45,49). In addition to the potential shared site with these other actin-binding proteins in subdomain 3, there is at least a second putative Tmod3-binding actin peptide (residues 216 -238) in subdomain 4.…”

A Nucleotide State-sensing Region on Actin

“…Because yeast actin polymerization is inhibited in the presence of monovalent cations (Na ϩ and K ϩ (44)) and the F169C mutant has a high Cc (3.2 M in 3 mM MgCl 2 ), we could use high concentrations of inorganic phosphate (up to 54 mM) without causing actin polymerization (as monitored by light scattering; data not shown). Also, pyrene excimer, which can accompany the formation of antiparallel dimers and oligomers at early stages of actin polymerization (30,45,46), has never been detected for Cys-169-pyrene actin under all the conditions tested.…”

Structure of an F-actin Trimer Disrupted by Gelsolin and Implications for the Mechanism of Severing

“…A structure of uncomplexed monomeric actin labeled with the dye tetramethylrhodamine (8) reveals that actin-binding proteins did not alter the structure of the actin monomer significantly. The structure of an antiparallel actin dimer has been determined (9), but how this structure relates to helical F-actin is unclear.…”