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Structure of the integrin alphaIIb transmembrane segment.

ABSTRACT: Integrin cell-adhesion receptors transduce signals bidirectionally across the plasma membrane via the single-pass transmembrane segments of each alpha and beta subunit. While the beta3 transmembrane segment consists of a linear 29-residue alpha-helix, the structure of the alphaIIb transmembrane segment reveals a linear 24-residue alpha-helix (Ile-966 -Lys-989) followed by a backbone reversal that packs Phe-992-Phe-993 against the transmembrane helix. The length of the alphaIIb transmembrane helix implies the absence of a significant transmembrane helix tilt in contrast to its partnering beta3 subunit. Sequence alignment shows Gly-991-Phe-993 to be fully conserved among all 18 human integrin alpha subunits, suggesting that their unusual structural motif is prototypical for integrin alpha subunits. The alphaIIb transmembrane structure demonstrates a level of complexity within the membrane that is beyond simple transmembrane helices and forms the structural basis for assessing the extent of structural and topological rearrangements upon alphaIIb-beta3 association, i.e. integrin transmembrane signaling.

SUBMITTER: Lau TL

PROVIDER: S-EPMC3259656 | biostudies-literature | 2008 Jun

REPOSITORIES: biostudies-literature

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Structure of the integrin alphaIIb transmembrane segment.

Lau Tong-Lay TL Dua Varun V Ulmer Tobias S TS

The Journal of biological chemistry 20080416 23

Integrin cell-adhesion receptors transduce signals bidirectionally across the plasma membrane via the single-pass transmembrane segments of each alpha and beta subunit. While the beta3 transmembrane segment consists of a linear 29-residue alpha-helix, the structure of the alphaIIb transmembrane segment reveals a linear 24-residue alpha-helix (Ile-966 -Lys-989) followed by a backbone reversal that packs Phe-992-Phe-993 against the transmembrane helix. The length of the alphaIIb transmembrane heli ...[more]

PMID: 18417472

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Project description:Integrins link the cytoskeleton to the extracellular matrix and regulate key signaling events that coordinate cellular processes such as secretion, migration, and proliferation. A single integrin molecule can exist in a resting state that does not bind extracellular ligands or in an active state that can engage ligands and form large signaling complexes. Activation signals are transduced between the cytosolic region and the extracellular region by a binary on/off switch in the integrin's transmembrane (TM) domain; the integrin's alpha and beta subunits each have a single TM helix that forms an alpha/beta heterodimer in the resting state, and the TM heterodimer separates to transduce an activation signal across the membrane. In this article, two methods used to generate models of the TM heterodimer, both converging on the same structure, are described. The first model was generated by a Monte Carlo algorithm that selected conformations based on their agreement with published experimental mutagenesis results. The second model was generated by threading the integrin's sequence onto TM helix dimers parsed from the Protein Data Bank and by selecting conformations based on their agreement with published experimental cysteine crosslinking results. The two models have similar structures; however, they differ markedly from some previously published models. To distinguish conformations that reflect the native integrin, we compared the Monte Carlo model, the threaded model, and four published models with experimental mutagenesis and cysteine crosslinking results. The models presented here had high correlation coefficients when compared with experimental findings, and they are in excellent agreement, both in terms of accuracy and in terms of precision, with a recent NMR structure. These results demonstrate that multiple approaches converged on the same structure of the resting integrin's TM heterodimer, and this conformation likely reflects the integrin's native structure.

Project description:Using green fluorescent protein (GFP) as an autofluorescent tag, we report the first successful visualization of a beta3 integrin in a living cell. GFP fused in frame to the cytoplasmic tail of either alphaIIb or beta3 allowed normal expression, heterodimerization, processing and surface exposure of alphaIIbGFPbeta3 and alphaIIb(beta3)GFP receptors in Chinese hamster ovary (CHO) cells. Direct microscopic observation of the autofluorescent cells in suspension following antibody-induced alphaIIb(beta3) capping revealed an intense autofluorescent cap corresponding to unlabelled immunoclustered GFP-tagged alphaIIb(beta3). GFP-tagged alphaIIbbeta3 receptors mediated fibrinogen-dependent cell adhesion, were readily detectable in focal adhesions of unstained living cells and triggered p125(FAK) tyrosine phosphorylation similar to wild-type alphaIIb(beta3) (where FAK corresponds to focal adhesion kinase). However, GFP tagged to beta3, but not to alphaIIb, induced spontaneous CHO cell aggregation in the presence of soluble fibrinogen, as well as binding of the fibrinogen mimetic monoclonal antibody PAC1 in the absence of alphaIIb(beta3) receptor activation. Time-lapse imaging of living transfectants revealed a characteristic redistribution of GFP-tagged alphaIIb(beta3) during the early stages of cell attachment and spreading, starting with alphaIIb(beta3) clustering at the rim of the cell contact area, that gradually overlapped with the boundary of the attached cell, and, with the onset of cell spreading, to a reorganization of alphaIIb(beta3) in focal adhesions. Taken together, our results demonstrate that (1) fusion of GFP to the cytoplasmic tail of either alphaIIb or beta3 integrin subunits allows normal cell surface expression of a functional receptor, and (2) structural modification of the beta3 integrin cytoplasmic tail, rather than the alphaIIb subunit, plays a major role in alphaIIb(beta3) affinity modulation. With the successful direct visualization of functional alphaIIb(beta3) receptors in living cells, the generation of autofluorescent integrins in transgenic animals will become possible, allowing new approaches to study the dynamics of integrin functions.

Dataset Information

Structure of the integrin alphaIIb transmembrane segment.

Publications

Structure of the integrin alphaIIb transmembrane segment.

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