The Structure and Regulation of Human Muscle α-Actinin

Abstract

SummaryThe spectrin superfamily of proteins plays key roles in assembling the actin cytoskeleton in various cell types, crosslinks actin filaments, and acts as scaffolds for the assembly of large protein complexes involved in structural integrity and mechanosensation, as well as cell signaling. α-actinins in particular are the major actin crosslinkers in muscle Z-disks, focal adhesions, and actin stress fibers. We report a complete high-resolution structure of the 200 kDa α-actinin-2 dimer from striated muscle and explore its functional implications on the biochemical and cellular level. The structure provides insight into the phosphoinositide-based mechanism controlling its interaction with sarcomeric proteins such as titin, lays a foundation for studying the impact of pathogenic mutations at molecular resolution, and is likely to be broadly relevant for the regulation of spectrin-like proteins.

Dystrophin's central domain forms a complex filament that becomes disorganized by in-frame deletions

“…Similar to the crystal structures of spectrin and α-actinin, the B helices are generally bent in their middle (30,31). However, the largest bends of α-helices are located at the inter-repeat linkers ( Figure S6)(Supplemental Video 1).…”

The sarcomeric cytoskeleton: from molecules to motion

“…All major Z-disk proteins except telethonin show rapid dynamic exchange in cardiac and skeletal myocytes, with half-lives of about 25 s for α-actinin-2 (Ribeiro et al, 2014;Stout et al, 2008;Wang et al, 2005). This suggests that the fast-exchanging components may be in a dynamically regulated equilibrium between a high-affinity Z-disk-bound conformation and a low-affinity cytoplasmic conformation.…”

“…Until recently, the exact localisation of the required flexible links was a matter of speculation, with bending of the α-actinin rod domain a plausible possibility (Luther, 2009;Perz-Edwards and Reedy, 2011). The recent complete structure of α-actinin and separate rod structures (Ribeiro et al, 2014;Ylänne et al, 2001) suggests that this is unlikely: the central dimerisation domain, formed from eight spectrin-like repeats in an antiparallel dimer, is a rigid and inflexible rod. However, flexible segments link the actin-and titin-binding domains at the N-and C-termini.…”

“…Z-repeats bind to EF-hand motifs 3-4 of the C-terminal calmodulin-like domain (CaM) of α-actinin (Atkinson et al, 2001). The interaction of Z-repeats with α-actinin has been studied in detail at the cellular, biochemical, biophysical and structural level, confirming and characterising the interaction of the helical Z-repeats with the α-actinin CaM domain (Atkinson et al, 2000(Atkinson et al, , 2001Ribeiro et al, 2014;Sorimachi et al, 1997;Young et al, 1998). There is a strict correlation of the number of titin Z-repeats with the number of Z-links: chicken pectoralis major muscle, a fast glycolytic muscle with thin Z-disks, expresses titin with two Z-repeats (Peckham et al, 1997), while cardiac and slow skeletal muscles in mammals with thick Z-disks express up to seven Z-repeats Sorimachi et al, 1997).…”

“…The α-actinin-titin interaction is regulated by an intramolecular pseudoligand mechanism, in which a titin Z-repeat-like sequence between the actin-binding domain and the central rod domain interacts with EFhand motifs 3-4 of the juxtaposed CaM domain. This interaction can be released by the binding of acidic phospholipids, especially phosphatidylinositol 4,5-bisphosphate (PIP2), to the actin-binding domain, thus activating titin binding (Ribeiro et al, 2014;Young and Gautel, 2000).…”