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α-Actinin/titin interaction: A dynamic and mechanically stable cluster of bonds in the muscle Z-disk

  1. Matthias Riefa,d,1
  1. aPhysik Department E22, Technische Universität München, 85748 Garching, Germany;
  2. bDepartment of Structural and Computational Biology, Max F. Perutz Laboratories, University of Vienna, A-1030 Vienna, Austria;
  3. cDepartment of Biochemistry, Faculty of Chemistry and Chemical Technology, University of Ljubljana, SI-1000 Ljubljana, Slovenia;
  4. dMunich Center for Integrated Protein Science, 81377 Munich, Germany

  1. Edited by James A. Spudich, Stanford University School of Medicine, Stanford, CA, and approved December 16, 2016 (received for review August 2, 2016)

Significance

Muscle is the tissue in our body experiencing most extreme mechanical forces. The mechanism of active force generation has been investigated for more than 50 y and is fairly well understood. However, despite its physiological significance, it is still unknown what mechanical linkages hold together the muscle machinery under passive stretching forces. In this paper, we show with direct mechanical single-molecule measurements that an array of titin/α-actinin bonds composes a dynamic network that can provide stable anchoring, maintaining the integrity of the muscle Z-disk even under load. This dynamic network explains how components of the Z-disk are able to rapidly rearrange and, at the same time, form a long-term stable mechanical structure.

Abstract

Stable anchoring of titin within the muscle Z-disk is essential for preserving muscle integrity during passive stretching. One of the main candidates for anchoring titin in the Z-disk is the actin cross-linker α-actinin. The calmodulin-like domain of α-actinin binds to the Z-repeats of titin. However, the mechanical and kinetic properties of this important interaction are still unknown. Here, we use a dual-beam optical tweezers assay to study the mechanics of this interaction at the single-molecule level. A single interaction of α-actinin and titin turns out to be surprisingly weak if force is applied. Depending on the direction of force application, the unbinding forces can more than triple. Our results suggest a model where multiple α-actinin/Z-repeat interactions cooperate to ensure long-term stable titin anchoring while allowing the individual components to exchange dynamically.

Footnotes

  • ↵1To whom correspondence should be addressed. Email: mrief{at}ph.tum.de.

  • Author contributions: M.G., K.D.-C., and M.R. designed research; M.G. performed research; U.M. and J.K. contributed new reagents/analytic tools; M.G. and M.R. analyzed data; and M.G., K.D.-C., and M.R. wrote the paper.

  • The authors declare no conflict of interest.

  • This article is a PNAS Direct Submission.

  • This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10.1073/pnas.1612681114/-/DCSupplemental.

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