Encyclopedia of Signaling Molecules

2018 Edition
| Editors: Sangdun Choi

Spectrin

Reference work entry
DOI: https://doi.org/10.1007/978-3-319-67199-4_101871

Synonyms

Historical Background

Spectrin was first discovered as a membrane-associated component of erythrocytes in 1968 (Marchesi and Steers 1968). The name was suggested due to the fact that the protein is extractable from red blood cell ghosts (specter), membranes isolated by hypotonic lysis of erythrocytes that retain original shape of these cells. A few years later, the spectrin-based membrane skeleton was first visualized in detergent-extracted erythrocytes by electron microscopy. However, it was over 10 years later when electron micrographs of intact erythrocytes revealed hexagonal network of spectrin filaments interconnected by junctional points (Byers and Branton 1985). Meanwhile, it appeared that isoforms of spectrin can be found in other cells (Goodman et al. 1981), which suggested that spectrin-based membrane skeleton might be essential also for nonerythroid cells. Now, after 50 years of spectrin discovery, it is broadly anticipated that membrane skeleton is present in all metazoan cells, although its spatial organization and functions may not necessarily resemble the structure originally described for the erythrocyte membrane (Machnicka et al. 2014). Moreover, various intracellular locations of different isoforms of spectrin, e.g., plasma membrane, nucleus, and Golgi, reflect the variety of physiological roles of these proteins.

Genes

Spectrins are a large family of proteins encoded by numerous genes in metazoan cells. In highly structurally and functionally complex mammalian cells, the diverse isoforms are expressed in a tightly regulated fashion. Two major isoforms of α spectrin in mammals, αI and αII, are encoded by two genes, SPTA1 and SPTAN1, respectively. Remarkably, alternative processing of the mRNA from SPTAN1 may result in up to eight (or at least four) isoforms of αII spectrin expressed in nonerythroid cells. This is in contradiction to the gene encoding αI spectrin characteristic for erythroid cells. Additionally, in mammalian cells five genes encode β spectrins, which include SPTB, SPTBN1, SPTBN2, and SPTBN4 that encode conventional β spectrins (β erythrocytic, βI, βII, and βIV, respectively) and SPTBN5 which encodes large isoform of β spectrin (βV) (Table 1). All genes encoding conventional β spectrins are differentially spliced; for example, in case of βI, βII, and βIV, longer or shorter C-terminal regions may be generated. Smaller repertoire of spectrin genes is characteristic for invertebrates. In Drosophila melanogaster and Caenorhabditis elegans, α spectrin is encoded by a single gene resembling mammalian SPTAN1, while two genes for β-subunits resemble SPTBN2 and SPTBN5 (Dubreuil and Grushko 1998).
Spectrin, Table 1

Genes encoding spectrins in Homo sapiens

Gene

Chromosome

Protein

SPTA1

1

α Spectrin erythrocytic (αI)

SPTAN1

9

α Spectrin non-erythrocytic (αII)

SPTB

14

β Spectrin erythrocytic

SPTBN1

2

β Spectrin non-erythrocytic (βI)

SPTBN2

11

β Spectrin non-erythrocytic 2 (βII)

SPTBN4

19

β Spectrin non-erythrocytic 4 (βIV)

SPTBN5

15

β Spectrin non-erythrocytic 5 (βV)

General Structure

Spectrins are modular proteins of high molecular weight (ranging from 246 to 430 kDa) composed of multiple domains of various architecture and functions. The core structural element is a repeating unit of approximately 106 amino acid residues that fold as triple-helical coiled coils. Such domains are called “spectrin repeats” and are characteristic also for other proteins within the “spectrin” family, e.g., actinin and dystrophin (Le Rumeur et al. 2012). Three sequential helices, of which A and C are parallel and B is antiparallel, are interconnected by short loops and form a common fold of approximately 2 nm in diameter and 5 nm in length (Table 2). The stability of the repeating units is provided mostly by hydrophobic amino acid residues in positions “a” and “d” of the classical heptad pattern (from “a” to “g”) along each of the helices. Helix C and helix A of the next repeat are continuous because of helical structure of the interrepeat linker, although the heptad pattern is not preserved within the latter. α spectrins contain 20 full-length triple-helical repeats along with one incomplete repeat (one helix) at N-terminus. On the other hand, there are 16 (29 in heavy isoforms) helical repeats and one incomplete repeat (two helices) at C-terminus in β spectrins. Despite similarities in structure, the sequence homology between individual spectrin repeats does not exceed 30%.

The functional units of spectrin are tetramers, although the protein also forms higher-order oligomers in native membranes of erythrocytes (Fig. 1). Two repeats closest to N-terminus of β spectrin interact with two most C-terminal repeats of α spectrin to form an antiparallel heterodimer. Two dimers can interact with each other in a head-to-head mode to form a tetramer. The critical role in this process is played by approximately 30-residue long single helix at N-terminus of α-subunit and two C-terminal helices (approximately 70 aa residues) of β-subunit, which recognize each other and recreate full triple-helical spectrin repeat. Remarkably, the tetramerization regions of erythroid and nonerythroid spectrin are not structurally equivalent, which may contribute to higher degree of flexibility of the erythroid membrane skeleton.
Spectrin, Fig. 1

The spectrin-based membrane skeleton. α Spectrin is marked in yellow; β spectrin is marked in purple. In erythrocytes (a), spectrin heterodimers associate to tetramers or higher-order oligomers that link together junctional complexes (black circles); upon membrane expansion, fully extended spectrin tetramers (length of approximately 190 nm) are predominant; highly dynamic spectrin-based skeleton allows the cell to reversibly deform in response to shear forces. In neuronal cells (b), spectrin tetramers are connected to junctional complexes that are periodically spaced along the axons. (c) Schematic representation of spectrin heterotetramer with its major structural and functional features; see the main text to follow the abbreviations

Besides triple-helical repeats, spectrins also comprise several other types of domains (Machnicka et al. 2014) (Fig. 1c and Table 2). C-terminus of α spectrin is folded into EF-hands (calmodulin-like domains), which are able to bind calcium ions in case of αII isoform, oppositely to similar structures within αI. EF-hands most probably play a role in stabilization of spectrin-actin interactions and bind other proteins, such as protein 4.2, in a calcium and calmodulin dependent manner. α spectrins also contain SH3 domain within repeat 9. This domain seems to be involved in various cellular processes related to signaling (see below) and DNA repair (Machnicka et al. 2014). Moreover, αII spectrin contains a short (36 aa residue) insert within repeat 10 called CCC region, where sites for calmodulin binding and calpain/caspase cleavage occur (Czogalla and Sikorski 2005). The protease cleavage is modulated by calcium ions/calmodulin and phosphorylation of this site. However, the physiological significance of the CCC region is still not fully understood.

Each of the β spectrin isoforms bears a tandem of two CH (calponin homology) domains at their N-termini. These domains are responsible for binding actin filaments located in junctional points. CH domains can also bind to other proteins of membrane skeleton, such as 4.1, and the interaction with the latter is strongly enhanced by phosphatidylinositides (viz., PI(4,5)P2) (Boguslawska et al. 2014). Another structural motif that recognizes phosphatidylinositides is PH (pleckstrin homology) domain, which is located at C-terminus of the “long” isoforms of β spectrin. However, in contrast to PH domains of many other proteins, the one found in spectrin shows rather weak specificity and affinity toward its target lipids. To date, little is known about the role of PH domain of spectrin, although it seems to be indispensable for PI(4,5)P2-independent polarized membrane skeleton assembly in midgut copper cells of fruit flies.
Spectrin, Table 2

Structural motifs of spectrins deposited in Protein Data Bank

PDB ID

Protein

Domain

2SPC

α Spectrin (D. melanogaster)

Repeat 14

1U5P

α Spectrin (G. gallus)

Repeats 15, 16

1U4Q

α Spectrin (G. gallus)

Repeats 15, 16, 17

1AJ3

α Spectrin (G. gallus)

Repeat 16

1CUN

α Spectrin (G. gallus)

Repeats 16, 17

1SHG, 1U06

α Spectrin (G. gallus)

SH3 domain

3F31

α Spectrin (H. sapiens)

Tetramerization site

1OWA

α Spectrin (H. sapiens)

Tetramerization site

3LBX

α Spectrin + β Spectrin (H. sapiens)

Tetramerization sites

1S35

β Spectrin (H. sapiens)

Repeats 8, 9

3KBT

β Spectrin (H. sapiens)

Repeats 13, 14, 15

3F57, 3EDU

β Spectrin (H. sapiens)

Repeats 14, 15

3EDV

β Spectrin (H. sapiens)

Repeats 14, 15, 16

1AA2, 1BKR

β Spectrin (H. sapiens)

CH2 domain

1MPH

β Spectrin (M. musculus)

PH domain

1WJM

β Spectrin (H. sapiens)

PH domain

Spectrin as a Structural Molecule

It is widely accepted that the major role of spectrin-based membrane skeleton of erythrocytes is stabilization of the plasma membrane and cell shape maintenance. Erythrocytes experience particularly strong mechanical stress due to the high-speed flow of the bloodstream and the passage through capillary vessels of diameter much smaller than the size of the cells. The remarkable flexibility of spectrins is hidden in the structure of spectrin repeats and their sequential arrangement within the protein. Fully extended spectrin tetramer is up to 200 nm long, but at physiological conditions, its length is in the range of approximately 50 nm (Nans et al. 2011). Remarkably, spectrin molecule may be arranged in a cylinder-like shape, where the pitch of each spectrin repeat increases while the molecule is extended (Brown et al. 2015). On the other hand, the high degree of elasticity and mechanical stability of the erythrocyte membrane skeleton is also reflected by a broad range of oligomeric states of spectrin.

There are two major macromolecular complexes that anchor spectrin-based skeleton to the erythrocyte membrane (Lux 2016). The core of the junctional complex consist of a short actin protofilament and a number of transmembrane proteins, including glycophorin C (GPC), glucose transporter 1 (GLUT1), anion exchanger 1 (AE1), Rh protein complex, Duffy antigen/chemokine receptor (DARC), and Kell/XK transporter complex. In addition, the complex is supplemented with peripheral proteins, such as protein 4.2, membrane palmitoylated protein 1 (MPP1/p55), adducin, tropomyosin, tropomodulin, dematin, and protein 4.1. The latter plays a crucial role in the organization of the junctional complex, mostly due to the formation of ternary complex with MPP1 and GPC. Ankyrin is the key element of the second membrane – spectrin skeleton linkage. The protein interacts with repeats 14 and 15 of β spectrin via its ZU5 domain with high affinity. On the other hand, ankyrin repeats 7–12 bind one dimer of AE1, while repeats 18–20 bind the second AE1 dimer. The spectrin-ankyrin-AE1 complex is further enriched in other proteins, such as protein 4.2, Rh, and RhAG. AE1 also interacts with other integral membrane proteins, e.g., glycophorin A and glycophorin B (GPA and GPB, respectively).

The three-dimensional topology of membrane skeleton in intact, native erythrocytes is a complex network of predominantly higher-order oligomers of spectrin linked together at junctional points. However, upon expansion of the membrane skeleton, the hexagonal lattice of straight and extended spectrin filaments was observed, where the equilibrium is shifted toward heterotetramer formation (Fig. 1a). It is still an open question whether the arrangement of membrane skeleton known from erythrocytes is common for other, nonerythroid cells. For example, high degree of similarity was found in case of D. melanogaster motoneuron axons in the vicinity of neuromuscular junctions. On the other hand, in neuronal axons, but not in dendrites, junctional complexes are spaced along the axon with periodicity that correspond to the length of extended spectrin heterotetramers which link these complexes (Xu et al. 2013) (Fig. 1b). Remarkably, sodium channels are distributed in a pattern that correlates with the spectrin-based membrane skeleton. However, it seems that the role of spectrin is not restricted to targeting and stabilization but includes also phosphorylation-dependent regulatory mechanism of sodium channels.

Examples of Other Functions of Spectrin

The structure of spectrin repeats and their repetitive arrangement within spectrins and other spectrin-related proteins correlate with their function as structural and mechanical modules. However, more and more evidence suggest that such protein domains also serve as docking sites for cytoskeletal proteins, components of signaling cascades, and membrane lipids. The issue of how such a regular and repetitive structural motif shows sufficient ligand specificity has been thoroughly explored in the case of ankyrin-binding domain (ABD) of β spectrin (Czogalla and Sikorski 2010). Although the domain comprises of repeats 14 and 15, both of which show common structural features of tandem triple-helical repeats, some conservative amino acid residues form docking site for ankyrin and recognition of ankyrin is based rather on shape complementarity than induced fit (Ipsaro et al. 2009). Ankyrin-mediated interactions of various transmembrane proteins, e.g., ion channels, transporters, cell adhesion molecules, and membrane receptors with spectrin skeleton, are critical for multiple physiological processes and proper lateral distribution of these molecules within cellular membranes (Machnicka et al. 2014).

Remarkably, the ankyrin-binding domain of β spectrin overlaps with a site within the repeat 14 which specifically recognizes membranes enriched in phosphatidylethanolamine (PE) and the interactions of that site with lipids can be inhibited by ankyrin (Wolny et al. 2011). Upon lipid binding, the common triple-helical repeat structure undergoes major conformational changes, which result in exposition of some of the hydrophobic amino acid residues that become capable to penetrate the core of a lipid bilayer (Czogalla et al. 2008). Other high-affinity lipid-binding sites that were identified within different spectrin repeats were recognizing phosphatidylserine-containing membranes (Boguslawska et al. 2014).

Triple-helical spectrin repeats serve also as anchors to other molecules, such as neural cell adhesion molecule (NCAM), a synaptic element involved in mechanical stabilization of neuronal contacts (Machnicka et al. 2014). Phosphorylation-dependent interaction of spectrin (repeat 12 of α-subunit) with protein 14-3-3 is a molecular switch of neurite outgrowth stimulated by NCAM. Another example of adhesion molecule, which activity depends on its interactions with spectrin, is Lu/BCAM. Disruption of these interactions causes enhanced adhesion of erythrocytes to laminin. Such interactions in epithelial and endothelial cells play a role in actin reorganization during cell adhesion and spreading. Spectrins associate also with NMDA receptors, although the site of interaction within the former protein is not defined. This may suggest that spectrin is a key element linking signal cascades dependent on Ca2+ and tyrosine kinases/phosphatases. Brain spectrin is also involved in the docking of synaptic vesicles to the presynaptic membrane via approximately 25 amino acid residue sites near N-terminus of β-subunit that interacts with synapsin. Deletion of spectrin in D. melanogaster causes disorganization of the spatial arrangement of synaptic components and loss of proper synaptic transmission.

Spectrin is involved in signaling pathways (Machnicka et al. 2012), mostly via its SH3 domain located in α subunits. The range of processes this domain is involved in includes signal transmission that leads to Rac activation, cell adhesion, lamellipodia extension, and cell spreading mediated by molecules regulating organization and dynamics of actin skeleton. The SH3 domain of αI isoform binds tyrosine kinase-binding protein (hssh3bp1/e3B1/Abi1), while nonerythroid spectrin interacts with tyrosine kinases/phosphatases (e.g., c-Src), sodium/proton exchangers, sodium channels, vasodilator-stimulated phosphoprotein (VASP), and actin-binding proteins (e.g., Tes and Ena/VASP-like). On the other hand, the loss of β spectrin is associated with distorted signaling of transforming growth factor β (TGFβ), which is caused by mislocation of Smad 3/4 proteins. Thus, downregulation and expression of an isoform of βII spectrin in mice often lead to development of tumor associated with dysregulation of cell cycle control and impaired TGFβ signaling. It is also well established that spectrin-based membrane skeleton is involved in lymphocyte activation due to the interactions with CD45. The direct binding of spectrin to CD45 stimulates the protein tyrosine phosphatase activity of the latter and facilitates targeting of CD45 and CD3 to cellular surface.

Spectrin-Related Diseases

Due to the critical role of spectrin-based skeleton in maintaining mechanical properties of membranes, defects related to this structure are of particular significance in erythrocytes and are usually reflected by enhanced fragility, fragmentation, and premature elimination of the cells from the bloodstream. These kinds of disorders lead to hemolytic anemias, such as hereditary spherocytosis (HS), elliptocytosis (HE), pyropoikilocytosis (HPP), and stomatocytosis (HSt) (Da Costa et al. 2013). HS is the most widely spread inherited disease related to the membrane of erythrocytes. Loss of membrane surface area with regard to intracellular volume leads to decreased deformability and spheroidal shape of the cells. Up to 30% of HS show minor to moderate form of the disease, which is typical for defects in SPTB gene. More than 25 various HS-related mutations in this gene are known, including missense mutations (e.g., spectrin “Atlanta” and spectrin “Kissimmee”) in the region of interactions with protein 4.1. Mutations in SPTA1 gene (<5% of HS) cause more severe form of the disease. Elliptic shape of erythrocytes is a hallmark of HE, which in 90% of cases is related to mutations in spectrin genes. Interestingly, individuals with HE exhibit a resistance to P. falciparum, as the parasite is not able to efficiently bind to the altered cells. The majority of HE is linked to missense mutations either within the N-terminal helix and first triple-helical repeat of α-subunit or within the two C-terminal helices of β-subunit, which is related to the reduced ability of spectrin heterodimers to form tetramers. More than 25 mutations in SPTA1 gene are identified as HE-related, while SPTB gene is less affected.

Generally, more and more reports provide data on the fundamental importance of spectrin in neuronal cells (Goellner and Aberle 2012), which is also supported by the fact that the protein constitutes up to 3% of total protein in brain. Mutations in αII spectrin are related to the West syndrome, a rare epileptic disease that is linked to distortions in neurotransmitter function. A deletion within the linker between triple-helical repeats 19 and 20 and duplication within the latter repeat affect the region responsible for dimerization of spectrin. In this context spectrin-mediated stabilization of membrane proteins and axonal transport may be disturbed. Mutations in SPTBN2 gene are connected with spinocerebellar ataxia (SCA), characterized by degeneration of the cerebellum, brainstem, and spinal cord. Deletions in this gene refer to the triple-helical repeat 3 which plays a role in the dimerization process of spectrin. Another mutation related to spinocerebellar ataxias results in a substitution (L253P) in a highly conserved region of CH domain of βII spectrin. The latter is highly expressed in Purkinje neurons, where it stabilizes membrane integral proteins, such as glutamate transporter EAAT4. Proper plasma membrane localization of EAAT4 and glutamate receptors (i.e., GluR gamma2) were reported to be impaired due to the mutations in spectrin, which may lead to abnormalities in glutamate-mediated signaling and cell death that lead to ataxia (Perkins et al. 2010). Moreover, βII spectrin is also associated with Golgi and intracellular vesicles and interacts with dynactin-dynein complex (via Arp1 subunit). The L253P mutation may disrupt interactions with the actin skeleton and, as a consequence, the normal trafficking and stabilization of membrane proteins.

Summary

It is generally accepted that spectrin forms a mechanical scaffold which supports lipid membranes and is able to control the mobility and activity of integral proteins such as membrane channels, transporters, adhesion molecules, and receptors. The scaffold itself is flexible and dynamic, although its spatial arrangement is still not fully resolved in case of majority of nonerythroid cells. Spectrins also contain functional domains, including SH3, PH, phosphorylation, and proteolytic sites which directly link membrane skeleton with signal transduction pathways. Taking together, spectrin emerges as a multifaceted protein. However, the mechanisms regulating multiple functions of this protein and the role of spectrin in some physiological processes are still unclear.

See Also

References

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Copyright information

© Springer International Publishing AG 2018

Authors and Affiliations

  1. 1.Department of Cytobiochemistry, Faculty of BiotechnologyUniversity of WrocławWrocławPoland