A Secondary Structure Model of the Integrin a Subunit N-Terminal Domain Based on Analysis of Multiple Alignments

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Abstract
The integrins are α/β heterodimeric proteins which mediate cell-matrix and cell-cell inter-actions. Current data indicate that the N-terminal moiety of the a subunit is involved in ligand binding. This region of the receptor is made up of a seven-fold repeated sequence of unknown structure which contains EF-hand-like putative divalent cation-binding sites. Recent studies have shown that multiple sequence alignments can be analysed to yield secondary structure predictions. Therefore, to obtain a model structure for the integrin a subunit N-terminal domain repeat, a large alignment of the seven repeats from sixteen integrin sequences was generated. Two methods of analysis were used: First, Chou and Fasman and Garnier, Osguthorpe and Robson predictions were carried out for individual sequences and the consensus predictions derived. Consensus hydrophobicity and chain flexibility data were also used to provide additional data. Second, sites of conservation and variation were analysed by a computer program STAMA (STructure After Multiple Alignment) to yield a secondary structure prediction. The two analyses gave essentially the same predicted structure: undefined region, loop, α-helix, β-strand, divalent cation-binding loop, β-strand, putative turn, loop, β-strand. This is the first model structure to be presented for an integrin domain. Its implications for integrin function are discussed.