Titin’s Intrinsically Disordered PEVK Domain Modulates Actin Polymerization

TL;DR

This study shows that the disordered PEVK domain of the titin protein can speed up actin polymerization, potentially influencing muscle function.

Contribution

The novel finding is that the PEVK domain modulates actin assembly by enhancing nucleation without affecting critical concentration.

Findings

PEVKII enhances initial and log-phase actin assembly rates in a concentration-dependent manner.

AFM images reveal radially symmetric complexes of short actin filaments in the presence of PEVKII.

PEVK does not alter the critical concentration of actin polymerization.

Abstract

The multi-domain muscle protein titin provides elasticity and mechanosensing functions to the sarcomere. Titin’s PEVK domain is intrinsically disordered due to the presence of a large number of prolines and highly charged residues. Although PEVK does not have canonical actin-binding motifs, it has been shown to bind F-actin. Here, we explored whether the PEVK domain may also affect actin assembly. We cloned the middle, 733-residue-long segment (called PEVKII) of the full-length PEVK domain, expressed in E. coli and purified by using His- and Avi-tags engineered to the N- and C-termini, respectively. Actin assembly was monitored by the pyrene assay in the presence of varying PEVKII concentrations. The structural features of PEVKII-associated F-actin were studied with atomic force microscopy. The added PEVKII enhanced the initial and log-phase rates of actin assembly and the peak F-actin quantity in a concentration-dependent way. However, the critical concentration of actin polymerization was unaltered. Thus, PEVK accelerates actin polymerization by facilitating its nucleation. This effect was highlighted in the AFM images of F-actin–PEVKII adsorbed to the supported lipid bilayer. The sample was dominated by radially symmetric complexes of short actin filaments. PEVK’s actin polymerization-modulating effect may, in principle, have a function in regulating sarcomeric actin length and turnover. Altogether, titin’s PEVK domain is not only a non-canonical actin-binding protein that regulates sarcomeric shortening, but one that may modulate actin polymerization as well.

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