Integrins and evolution:
Integrins are large heterodimeric cell-surface receptors, which detect and transmit changes in mechanical forces resulting from interactions between a cell and the extracellular matrix (Takada et al., 2007, review). Cell-cell and cell-matrix adhesion, mediated by integrins, is divalent-cation dependent, and plays "key" role in inflammation, antigen-specific immunity, cell development, proliferation and differentiation (Hynes 1992; Arnaout et al., 2007, review). Integrins are known to contribute to different stages of a variety of common diseases, such as tumor metastasis, immune dysfunction, viral infections, osteoporosis and others (Hynes 1992; Arnaout et al., 2007). Integrins are probably one of the most complex cell adhesion molecules both structurally and functionally.
Integrins basically consist of two functional subunits. These are α and β subunits. The α subunits can be further classified based on I-domain containing or non I-domain containing. In humans 18 α and 8 β subunits are expressed forming 24 α/β heterodimeric receptors.
The evolutionary study of integrins in bony vertebrates has already shown that, integrins are quite conserved (Huhtala et al., 2005, Johnson et al., 2009). However a number of genomes are missing in between the prokaryotes and the eukaryotes and this is the reason that there is a need for further investigation on the early origin of integrins especially bacteria.
Importance of divalent cation:
The integrins α subunit is composed of several domains of which 7-bladed β-propeller is one of the key functional domains. The β-propeller is located towards the N-terminal of the integrin α and is very important in ligand binding. Integrins are complex receptors, which work through interactions with their ligands. Binding of ligands to integrins as well as the integrin function in general are tightly regulated through binding of several divalent metal cations, predominantly Mg2+ and Ca2+, to different sites on the integrin structure. Binding of divalent cations to integrins governs both their function through coordination of ligand binding and separately interactions between different domains within α and β subunits. For example, in all known RGD-recognizing integrins, the carboxylic side-chain of the aspartate residue of the RGD peptide binds to the Mg2+-coordinated MIDAS (the metal ion dependent adhesion site) site of the I-like domain of the β-subunit, while the arginine side-chain directly binds to the N-terminal domain of the α subunit (Xiong et al., 2001). In this case of the RGD-recognizing integrins, the divalent cation tightly coordinates binding of the RGD ligand, and thus, protein function. At the same time, proper orientation of the 7-bladed β-propeller domain within the integrin αsubunit is governed by another divalent cation, Ca2+, which is present in several β-blades of the 7-bladed β-propeller fold. It was earlier shown that the calcium-binding motif found in the β-blades of the N-terminal β-propeller domain of the α subunit is present in a large number of other unrelated Ca2+-binding proteins as well (Rigden and Galperin, 2004). The first part of my project (Chouhan et al., 2009) focused largely on further substantiating the findings of the previous work (Johnson et al., 2009) and from a structural and biological point of view in the current and upcoming studies will focus on:
(a) Identification and description of various structural attributes responsible for interactions between integrin monomers which will help to answer the question about the mechanism of integrin dimerisation. Additionally, the determination of the structural divalent-cation recognition motifs will allow gaining an insight in receptor/ligand interactions, and thus, the integrin function.
(b) Comparative analysis of sequences from various genomes which will allow tracing evolutionary origin of different elements of integrin structure and thus allowing filling missing details on the origin of human type functional integrins.
The project will require the implementation of the following techniques: