Studies on the properties of chemically modified actin. II. Trinitrophenylation

Binding of dystrophin's tandem calponin homology domain to F-actin is modulated by actin's structure

Dystrophin has been shown to be associated in cells with actin bundles. Dys-246, an N-terminal recombinant protein encoding the first 246 residues of dystrophin, includes two calponin-homology (CH) domains, and is similar to a large class of F-actin cross-linking proteins including alpha-actinin, fimbrin, and spectrin. It has been shown that expression or microinjection of amino-terminal fragments of dystrophin or the closely related utrophin resulted in the localization of these protein domains to actin bundles. However, in vitro studies have failed to detect any bundling of actin by either intact dystrophin or Dys-246. We show here that the structure of F-actin can be modulated so that there are two modes of Dys-246 binding, from bundling actin filaments to only binding to single filaments. The changes in F-actin structure that allow Dys-246 to bundle filaments are induced by covalent modification of Cys-374, proteolytic cleavage of F-actin's C-terminus, mutation of yeast actin's N-terminus, and different buffers. The present results suggest that F-actin's structural state can have a large influence on the nature of actin's interaction with other proteins, and these different states need to be considered when conducting in vitro assays.

Anti-actin antibodies. Chemical modification allows the selective production of antibodies to the N-terminal region

The specific properties of sera elicited by various native, unfolded or chemically modified actins were compared to provide a means of obtaining high titres of antibodies directed against the N-terminal (1-39) or the central regions of the actin sequence. The antigenic structure of the N-terminal region of actin was analyzed. It has at least 2 discrete epitopes, one of which appears to be species-specific and is composed of the hydrophilic N-terminal heptapeptide sequence.

Induction by chemically modified actin derivatives of antibody specificity. A relation between modified sites and antibody interactions with monomeric and filamentous actins

A comparison of specific antibodies induced by unfolded actins modified either by oxidation or by arylation of lysine residues was reported. We have focused our work on binding properties with filamentous actin and located its preferential antigenic sites for the anti-arylated-actin antibodies in the C-part of the molecule. An interference of anti-oxidized actin antibodies upon actin polymerisation has also been reported.

Change of reactivity of lysine residues upon actin polymerization

The reactivity of lysine residues of actin was measured by a surface labeling method--limited reductive methylation. After labeling, actin was subjected to CNBr and enzymatic cleavage, and all lysines were obtained either singly in a peptide or as a free residue. The specific activity of each lysine was taken as the measure of its reactivity. In actin denatured in 8 M urea, the reactivity of each lysine residue is approximately equal whereas those in G-actin fall into three categories: Lys-61 and Lys-113 are the most reactive ones; Lys-18, -213, -215, -314, and -358 are hardly reactive; the remainder, including Lys-50, -68, -84, -118, -191, -237, -283, -290, -325, -327, -335, and -372, are moderately reactive. The least reactive ones are probably buried in the native G-actin and all the others are most likely on the surface. Upon actin polymerization the reactivities of Lys-61, -68, -113, and -283 are significantly reduced while that of Lys-335 is strikingly enhanced. The decrease in reactivity could be readily explained if these residues were located in the monomer-monomer contact area although a polymerization-induced conformational change cannot be excluded. Such a conformational change may be invoked to explain the increase in the reactivity of Lys-335. Alternatively, the latter may be interacting with the bound ATP of G-actin, and the increased reactivity might be directly attributable to the loss of gamma-P for ATP accompanying polymerization.

Effect of crosslinking by glutaraldehyde on interaction of F-actin with heavy meromyosin

Crosslinking of F-actin by a bifunctional reagent glutaraldehyde resulted in a marked decrease of viscosity and length of F-actin filaments. The extent and rate of superprecipitation of actomyosin reconstituted from the modified actin were lower than those of unmodified actin-myosin complex, but activation of heavy meromyosin ATPase by the crosslinked actin was higher than by unmodified one. Heavy meromyosin ATPase activated by the crosslinked actin was distinctly less dependent on KCl concentration than that activated by unmodified actin. Turbidity of the modified acto-heavy meromyosin in the presence of ATP exceeded the sum of turbidities of actin and heavy meromyosin, whereas in the case of unmodified acto-heavy meromyosin the turbidity was comparable to that for noninteracting system. The difference in activation of heavy meromyosin. ATPase by the cross-linked and unmodified actin, clearly seen at room temperature, significantly diminished when temperature was lowered to 0 degrees C.