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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

Spectrin domain structure.Cartoon representation depicting the domain organization of (A) erythroid and (B) nonerythroid spectrin tetramers (α-spectrin—brown spectrin repeats; β-spectrin—yellow spectrin repeats). The pink pentagons labeled CH are the actin binding domains other known spectrin domains are labeled EF, SH3 and PH. C. Ribbon representation of three consecutive spectrin repeats (brown); the 5-residue linker regions between each repeat are colored red.
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pcbi.1004302.g001: Spectrin domain structure.Cartoon representation depicting the domain organization of (A) erythroid and (B) nonerythroid spectrin tetramers (α-spectrin—brown spectrin repeats; β-spectrin—yellow spectrin repeats). The pink pentagons labeled CH are the actin binding domains other known spectrin domains are labeled EF, SH3 and PH. C. Ribbon representation of three consecutive spectrin repeats (brown); the 5-residue linker regions between each repeat are colored red.

Mentions: Spectrins are 200+ kDa filamentous proteins present in most cell types; although they are best known for their role in the hexagonal arrangement of junctional complexes in erythrocytes and of microvilli in enterocytes. Humans and other mammals express a number of spectrin isoforms produced from multiple genes as well as by alternative gene splicing. The primary sequences of spectrins are mostly comprised of many tandem homologous domains that are ~106 residues in length and are commonly termed “spectrin repeats”. A number of high-resolution structures have been reported for both individual as well as several tandem spectrin repeats. All crystal structures exhibit a similar left-handed, anti-parallel, three-helix coiled-coil topology with short helical connectors between adjacent repeats [1–11]. Spectrin polypeptides are grouped into two categories: (1) α-spectrins, which contain 20 full spectrin repeats plus several other structural motifs and (2) β-spectrins, which contain 16 or more full spectrin repeats, an N-terminal actin-binding domain and a C-terminal nonhomologous domain of varying size. The biologically relevant form of spectrin is typically the heterotetramer formed through the antiparallel lateral association of an α-chain with a β-chain to form the heterodimer, which further dimerizes in a self-limiting, head-to-head fashion with another heterodimer to form the heterotetramer (Fig 1). One common function of the spectrin family of proteins is to act as a versatile actin crosslinker in diverse cell types.


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)

Spectrin domain structure.Cartoon representation depicting the domain organization of (A) erythroid and (B) nonerythroid spectrin tetramers (α-spectrin—brown spectrin repeats; β-spectrin—yellow spectrin repeats). The pink pentagons labeled CH are the actin binding domains other known spectrin domains are labeled EF, SH3 and PH. C. Ribbon representation of three consecutive spectrin repeats (brown); the 5-residue linker regions between each repeat are colored red.
© Copyright Policy
Related In: Results  -  Collection

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

pcbi.1004302.g001: Spectrin domain structure.Cartoon representation depicting the domain organization of (A) erythroid and (B) nonerythroid spectrin tetramers (α-spectrin—brown spectrin repeats; β-spectrin—yellow spectrin repeats). The pink pentagons labeled CH are the actin binding domains other known spectrin domains are labeled EF, SH3 and PH. C. Ribbon representation of three consecutive spectrin repeats (brown); the 5-residue linker regions between each repeat are colored red.
Mentions: Spectrins are 200+ kDa filamentous proteins present in most cell types; although they are best known for their role in the hexagonal arrangement of junctional complexes in erythrocytes and of microvilli in enterocytes. Humans and other mammals express a number of spectrin isoforms produced from multiple genes as well as by alternative gene splicing. The primary sequences of spectrins are mostly comprised of many tandem homologous domains that are ~106 residues in length and are commonly termed “spectrin repeats”. A number of high-resolution structures have been reported for both individual as well as several tandem spectrin repeats. All crystal structures exhibit a similar left-handed, anti-parallel, three-helix coiled-coil topology with short helical connectors between adjacent repeats [1–11]. Spectrin polypeptides are grouped into two categories: (1) α-spectrins, which contain 20 full spectrin repeats plus several other structural motifs and (2) β-spectrins, which contain 16 or more full spectrin repeats, an N-terminal actin-binding domain and a C-terminal nonhomologous domain of varying size. The biologically relevant form of spectrin is typically the heterotetramer formed through the antiparallel lateral association of an α-chain with a β-chain to form the heterodimer, which further dimerizes in a self-limiting, head-to-head fashion with another heterodimer to form the heterotetramer (Fig 1). One common function of the spectrin family of proteins is to act as a versatile actin crosslinker in diverse cell types.

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