Properties of the spectrin‐like structural element of smooth‐muscle α‐actinin

Abstract

The fragment of smooth muscle alpha-actinin, comprising the four spectrin-like structural repeating units, has a high alpha-helix content, similar to that of spectrin, and a hydrodynamic frictional coefficient, indicative of an elongated, probably bent or kinked rod-like structure, as found for spectrin dimer and tetramer. The fragment exists in solution as an extremely stable dimer, which is dissociated only under denaturing conditions and is much more resistant to dissociation by urea than is the spectrin heterodimer. High-resolution proton magnetic resonance spectra reveal that a part of the polypeptide chain gives rise to sharp resonances; this is also true of spectrin and it implies that the individual structural repeating units contain segmentally mobile elements, which may be required to generate the elastic properties of the spectrin family of proteins. Again like spectrin, the alpha-actinin fragment contains multiple binding sites for long-chain fatty acids, as revealed by quenching of tryptophan fluorescence by 2-bromostearate (though not by 9(10)-bromostearate). The results point to extensive structural and functional similarities between the repeating units of all the proteins of the spectrin family.

Analysis of the Phasing of Four Spectrin‐like Repeats in α‐actinin

“…The results presented in this study, together with other features of the structures and physical properties of the proteins Kahana and Gratzer, 1991;Kuroda et al, 1994), imply a close similarity between the a-actinin and spectrin rod domains in almost all known respects.…”

“…We infer that this represents the start of the rod domain, which is therefore approximately 20 residues C-terminal from the position originally predicted from sequence alignments of the four repeats (Baron et al, 1987). This conclusion is supported by our observation that the expressed polypeptide comprising residues 267 -749 is protease-resistant and has an a helicity similar to that of the rod domain released from a-actinin by therniolysin or chymotrypsin (Itnamura et al, 1988;Kahana and Gratzer, 1991). The expressed fragment, moreover, appeared rod-like in the electron microscope after shadowing, and was a dimer, as judged by cross-liking and sedimentation equilibria analysis (data not shown).…”

“…Speicher et al (1992) suggest that the inserts may be implicated in the tight interactions with the partnering repeats in the complementary chain. If so, this would be fully compatible with the remarkable stability of the anti-parallel dimer of the a-actinin rod Kahana and Gratzer, 1991).…”

A 3-D Reconstruction of Smooth Muscle α-Actinin by CryoEm Reveals Two Different Conformations at the Actin-binding Region

“…26 In the discussion that follows, we will refer to a-actinin molecules as the native dimer unless otherwise specified. Interactions between a-actinin molecules will refer to interactions between native molecules (i.e.…”

“…7,11,15 a-Actinin, is dimeric due to the strong association between the R1 -R4 domains, 26 and is not known to form higher order aggregates that might be due to ABD -ABD interactions. This suggests that all of these ABDs may adopt a closed conformation similar to fimbrin while in solution.…”