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

Brown JW, Bullitt E, Sriswasdi S, Harper S, Speicher DW, McKnight CJ - PLoS Comput. Biol. (2015)

Bottom Line: 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.

View Article: PubMed Central - PubMed

Affiliation: Department of Physiology and Biophysics, Boston University School of Medicine, Boston, Massachusetts, United States of America; Internal Medicine Residency Program, University of Pittsburgh Medical Center, UPMC Montefiore Hospital, Pittsburgh, Pennsylvania, United States of America.

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.

No MeSH data available.


Related in: MedlinePlus

The “Chinese Finger Trap” model of spectrin.A End View, and B. Side Views of the spectrin repeats that comprise each tetramer are arranged in a similar fashion to the geometry reported by McGough & Josephs, 1990 (See Materials and Methods). C. Schematic representation of all interdomain interactions present within the Chinese Finger Trap model. The green bars indicate all interactions present within both the extended and Chinese Finger Trap model, while the red bars identify the interactions predicted to exist only in the Chinese Finger Trap Model. D. The proposed mechanism by which spectrin can undergo a three fold extension, while remaining a linear filamentous polymer.
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pcbi.1004302.g003: The “Chinese Finger Trap” model of spectrin.A End View, and B. Side Views of the spectrin repeats that comprise each tetramer are arranged in a similar fashion to the geometry reported by McGough & Josephs, 1990 (See Materials and Methods). C. Schematic representation of all interdomain interactions present within the Chinese Finger Trap model. The green bars indicate all interactions present within both the extended and Chinese Finger Trap model, while the red bars identify the interactions predicted to exist only in the Chinese Finger Trap Model. D. The proposed mechanism by which spectrin can undergo a three fold extension, while remaining a linear filamentous polymer.

Mentions: Spectrin’s contour length (defined in S3 Fig) was experimentally determined to be 19.9 nm and, like a spring, was invariant to either contraction or extension [23]. Intriguingly, this value is precisely the length of four spectrin repeats (~5 nm per repeat including one linker region) and, therefore, we, as did McGough & Josephs (but for other reasons), propose that four spectrin repeats comprise each supercoil turn. This quaternary conformation is obtained by a 90° bend between tandem spectrin repeats (Fig 3A), a conformation that we experimentally observe in mini-spectrin tetramers (see below). In the resulting model of the spectrin tetramer (Fig 3), the two, antiparallel strands are phased so that the high affinity interaction between α20,21 and β1–2 is maintained and arranged in accord with inter-strand chemical cross-linking data [29–35]. A striking prediction from this level of modeling is that spectrin is a hollow cylinder at its biological resting length (Fig 3A).


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

Brown JW, Bullitt E, Sriswasdi S, Harper S, Speicher DW, McKnight CJ - PLoS Comput. Biol. (2015)

The “Chinese Finger Trap” model of spectrin.A End View, and B. Side Views of the spectrin repeats that comprise each tetramer are arranged in a similar fashion to the geometry reported by McGough & Josephs, 1990 (See Materials and Methods). C. Schematic representation of all interdomain interactions present within the Chinese Finger Trap model. The green bars indicate all interactions present within both the extended and Chinese Finger Trap model, while the red bars identify the interactions predicted to exist only in the Chinese Finger Trap Model. D. The proposed mechanism by which spectrin can undergo a three fold extension, while remaining a linear filamentous polymer.
© Copyright Policy
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4466138&req=5

pcbi.1004302.g003: The “Chinese Finger Trap” model of spectrin.A End View, and B. Side Views of the spectrin repeats that comprise each tetramer are arranged in a similar fashion to the geometry reported by McGough & Josephs, 1990 (See Materials and Methods). C. Schematic representation of all interdomain interactions present within the Chinese Finger Trap model. The green bars indicate all interactions present within both the extended and Chinese Finger Trap model, while the red bars identify the interactions predicted to exist only in the Chinese Finger Trap Model. D. The proposed mechanism by which spectrin can undergo a three fold extension, while remaining a linear filamentous polymer.
Mentions: Spectrin’s contour length (defined in S3 Fig) was experimentally determined to be 19.9 nm and, like a spring, was invariant to either contraction or extension [23]. Intriguingly, this value is precisely the length of four spectrin repeats (~5 nm per repeat including one linker region) and, therefore, we, as did McGough & Josephs (but for other reasons), propose that four spectrin repeats comprise each supercoil turn. This quaternary conformation is obtained by a 90° bend between tandem spectrin repeats (Fig 3A), a conformation that we experimentally observe in mini-spectrin tetramers (see below). In the resulting model of the spectrin tetramer (Fig 3), the two, antiparallel strands are phased so that the high affinity interaction between α20,21 and β1–2 is maintained and arranged in accord with inter-strand chemical cross-linking data [29–35]. A striking prediction from this level of modeling is that spectrin is a hollow cylinder at its biological resting length (Fig 3A).

Bottom Line: 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.

View Article: PubMed Central - PubMed

Affiliation: Department of Physiology and Biophysics, Boston University School of Medicine, Boston, Massachusetts, United States of America; Internal Medicine Residency Program, University of Pittsburgh Medical Center, UPMC Montefiore Hospital, Pittsburgh, Pennsylvania, United States of America.

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.

No MeSH data available.


Related in: MedlinePlus