The Physiological Molecular Shape of Spectrin: A Compact Supercoil Resembling a Chinese Finger Trap

Abstract

The primary, secondary, and tertiary structures of spectrin are reasonably well defined, but the structural basis for the known dramatic molecular shape change, whereby the molecular length can increase three-fold, is not understood. In this study, we combine previously reported biochemical and high-resolution crystallographic data with structural mass spectroscopy and electron microscopic data to derive a detailed, experimentally-supported quaternary structure of the spectrin heterotetramer. In addition to explaining spectrin’s physiological resting length of ~55-65 nm, our model provides a mechanism by which spectrin is able to undergo a seamless three-fold extension while remaining a linear filament, an experimentally observed property. According to the proposed model, spectrin’s quaternary structure and mechanism of extension is similar to a Chinese Finger Trap: at shorter molecular lengths spectrin is a hollow cylinder that extends by increasing the pitch of each spectrin repeat, which decreases the internal diameter. We validated our model with electron microscopy, which demonstrated that, as predicted, spectrin is hollow at its biological resting length of ~55-65 nm. The model is further supported by zero-length chemical crosslink data indicative of an approximately 90 degree bend between adjacent spectrin repeats. The domain-domain interactions in our model are entirely consistent with those present in the prototypical linear antiparallel heterotetramer as well as recently reported inter-strand chemical crosslinks. The model is consistent with all known physical properties of spectrin, and upon full extension our Chinese Finger Trap Model reduces to the ~180-200 nm molecular model currently in common use.

Actin‐spectrin scaffold supports open fenestrae in liver sinusoidal endothelial cells

“…To assess the distribution of spectrin within LSECs, we applied two‐colored dSTORM optical nanoscopy of spectrin and actin. Spectrin always forms heterotetramers (two alpha units and two beta units) . The mammalian alpha subunits are reportedly similar in molecular weight but are structurally distinct and share only limited immunologic cross reactivity .…”

“…Spectrin's functional length is approximately 55 to 65 nm but can extend up to 200 nm, which makes it a perfect candidate to control the fenestrae in their open vs closed states. One could expect that the interaction between spectrin units and actin across the cell membrane is greatly facilitated at the cell periphery because the cell height is reduced and because of the presence of numerous short actin filaments.…”

The functional architecture of axonal actin

“…Consistent with this mechanoprotective role of the actin/spectrin scaffold, axons of C. Elegans worms lacking ß-spectrin progressively break when the animal is moving (Hammarlund et al, 2007). At rest, spectrin tetramers are~50-70 nm long, whereas in axons their~180 nm indicates that they are fully extended, and thus under tension like extended springs (Brown et al, 2015). FRET sensor probes inserted in ß-spectrin show that indeed, spectrins are extended and under tension along axons (Krieg et al, 2014), a finding confirmed by modeling of the axon mechanical properties (Lai and Cao, 2014;Y.…”

“…2A). This spacing value corresponds to the length of individual, extended spectrin tetramers (Bennett et al, 1982;Brown et al, 2015): spectrin appear as periodic stripes along the axon when labeling the axonal ß2-spectrin and α2-spectrin, or the AIS-specific ß4-spectrin (Huang et al, 2017a, 2017b; Leterrier et al, 2015; Xu et al, 2013) ( Fig. 2B & 2C).…”

Composite biopolymer scaffolds shape muscle nucleus: Insights and perspectives fromDrosophila

“…15,16 Moreover, the physiological resting length of spectrin can undergo a 3-fold extension, which can be explained by a compact supercoil model. 17 Excluding the linker regions between SRs, a giant isoform of Nesprin-1 possesses 74 SRs and MSP300 at least 78 SRs 18 (our unpublished data). Thus, their static length should be more than 370 nm and the de facto total length under tension can be much longer.…”