Cysteine-374 of actin resides at the gelsolin contact site in the EGTA-resistant actin-gelsolin complex

Biomolecular mimicry in the actin cytoskeleton: mechanisms underlying the cytotoxicity of kabiramide C and related macrolides

This study characterizes the interactions between kabiramide C (KabC) and related macrolides and actin and establishes the mechanisms that underlie their inhibition of actin filament dynamics and cytotoxicity. The G-actin-KabC complex is formed through a two-step binding reaction and is extremely stable and long-lived. Competition-binding studies show that KabC binds to the same site on G-actin as Gelsolin domain 1 and CapG. KabC also binds to protomers within F-actin and results in the severing and capping of the (+) end; these studies suggest that free KabC and related macrolides act as biomimetics of Gelsolin. The G-actin-KabC complex binds to the (+) end of a growing filament, where it functions as a novel, unregulated, (+)-end capper and is largely responsible for the inhibition of motility and cytokinesis in approximately 10 -100 nM KabC-treated cells. KabC and related macrolides are useful probes to study the regulation of the actin filament (+) end and may lead to new therapies to treat diseases of the actin cytoskeleton.

Structural connectivity in actin: effect of C-terminal modifications on the properties of actin

In this study, we use fluorescent probes and proteolytic digestions to demonstrate structural coupling between distant regions of actin. We show that modifications of Cys-374 in the C-terminus of actin slow the rate of nucleotide exchange in the nucleotide cleft. Conformational coupling between the C-terminus and the DNasal loop in subdomain II is observed in proteolytic digestion experiments in which a new C-terminal cleavage site is exposed upon DNasel binding. The functional consequences of C-terminal modification are evident from S-1 ATPase activity and the in vitro motility experiments with modified actins. Pyrene actin, labeled at Cys-374, activates S-1 ATPase activity only half as well as control actin. This reduction is attributed to a lower Vmax value because the affinity of pyrene actin to S-1 is not significantly altered. The in vitro sliding velocity of pyrene actin is also decreased. However, IAEDANS labeling of actin (also at Cys-374) enhances the Vmax of acto-S-1 ATPase activity and the in vitro sliding velocity by approximately 25%. These results are discussed in terms of conformational coupling between distant regions in actin and the functional implications of the interactions of actin-binding proteins with the C-terminus of actin.

Benzophenone photophores in biochemistry

The photoactivatable aryl ketone derivatives have been rediscovered as biochemical probes in the last 5 years. The expanding use of benzophenone (BP) photoprobes can be attributed to three distinct chemical and biochemical advantages. First, BPs are chemically more stable than diazo esters, aryl azides, and diazirines. Second, BPs can be manipulated in ambient light and can be activated at 350-360 nm, avoiding protein-damaging wavelengths. Third, BPs react preferentially with unreactive C-H bonds, even in the presence of solvent water and bulk nucleophiles. These three properties combine to produce highly efficient covalent modifications of macromolecules, frequently with remarkable site specificity. This Perspectives includes a brief review of BP photochemistry and a selection of specific applications of these photoprobes, which address questions in protein, nucleic acid, and lipid biochemistry.

The actin monomers in the ternary gelsolin: 2 actin complex are in an antiparallel orientation

Gelsolin forms ternary complexes with two actin monomers in the presence of Ca2+, which nucleate actin polymerization and cap the barbed ends of filaments. It has therefore been assumed that the two actins are oriented in a similar manner to the terminal subunits in the genetic helix of F-actin. We have tested this using chemical cross-linking with N,N'-1,4-phenylenedimaleimide. For all conditions tested, we identified as the only cross-linked dimeric species an actin dimer indistinguishable from the lower actin dimer of 86 kDa. This lower dimer was previously identified in the initial phase of actin polymerization and also when actin paracrystals are chemically cross-linked [Millonig, R., Salvo, H. & Aebi, U. (1988) J. Cell Biol. 106, 785-796]. It probably defines a contact between adjacent monomers oriented in an antiparallel orientation. In contrast, when F-actin is cross-linked by the same reagent, an upper dimer of apparent molecular mass 115 kDa is formed, which corresponds to adjacent monomers in the genetic helix. The formation of this upper dimer was specifically inhibited by addition of gelsolin to F-actin. Evidence is presented for a Cys374-Cys374 cross-link in the lower dimer. Isolated lower dimer binds to gelsolin in a 1:1 stoichiometry, but it inhibits nucleation of polymerization by gelsolin. Other gelsolin constructs that bind two actin subunits (e.g. the N-terminal half of the molecule, which has severing and capping but no nucleating activity) also form only lower dimer when cross-linked with N,N'-1,4-phenylenedimaleimide. Only segment 2-6 (gelsolin fragment devoid of the N-terminal segment 1) induces an upper dimer orientation of the two actins under nucleating conditions. Our evidence suggests that the two actins associated with gelsolin are not fixed in the orientation of adjacent subunits in F-actin; instead they have a flexible orientation with respect to each other, which permits cross-linking into a stable antiparallel form that does not correspond to the presumed nucleating conformation.

Definition of the EGTA-independent interface involved in the serum gelsolin-actin complex

The gelsolin-actin complex in the presence of Ca2+ revealed at least three interacting sites on the gelsolin molecule located in the S1, S2-3, and S4-6 domains. In the presence of EGTA, the N-terminal domain of gelsolin is known to be involved. However, the corresponding site on the surface of actin is poorly defined. The present result locates the Ca(2+)-independent plasma gelsolin-binding site on the actin surface. Natural and synthetic actin peptides were tested for their possible interaction with gelsolin and monitored by fluorescence anisotropy measurements and e.l.i.s.a. The interface was thus located within the 360-372 actin sequence near the C-terminal extremity. In addition, we used a chymotryptic digest of gelsolin and determined that its N-terminal domain (S1) was implicated in this interface. We conclude that the interaction of the 41-126 region of plasma gelsolin is the counterpart of the 360-372 sequence in subdomain 1 of actin.

The amino-terminal fragment of gelsolin is cross-linked to Cys-374 of actin in the EGTA-resistant actin-gelsolin complex

It has been shown that the EGTA-resistant actin, one of the two actin molecules associated to gelsolin, can be predominantly cross-linked to gelsolin by benzophenone-4-maleimide (BPM), a photoaffinity-labeling reagent, which was conjugated to Cys-374 of actin prior to cross-linking (Doi, Y., Banba, M. and Vertut-Doï, A. (1991) Biochemistry 30, 5769-5777). When a chymotryptic digest of gelsolin containing the amino-terminal 15-kDa fragment was mixed with BPM-actin (42 kDa) and irradiated for cross-linking, a band of 58 kDa appeared on SDS-PAGE which was shown to contain actin molecule by using fluorescently labeled actin. The amino-terminal sequence of the 58-kDa complex was identical to that of gelsolin, confirming that the amino-terminal segment (residues 1-133) of pig plasma gelsolin lies closely to Cys-374 of actin in the EGTA-resistant complex.