PROSITE documentation PDOC00216

Integrins beta chain EGF (I-EGF) domain signature and profile

Integrins [1,2] are a large family of cell surface receptors that mediate cell to cell as well as cell to matrix adhesion. Some integrins recognize the R-G-D sequence (see <PDOC00016>) in their extracellular matrix protein ligand. Structurally, integrins consist of a dimer of an α and a β chain. Each subunit has a large N-terminal extracellular domain followed by a transmembrane domain and a short C-terminal cytoplasmic region. Some receptors share a common β chain while having different α chains. The sequence of a number of different β chains has been determined and are listed below:

• Integrin β-1, which associates with α-1 to form a laminin receptor, with α-2 to form a collagen receptor, with α-4 to interact with VCAM-1, with α-5 to form a fibronectin receptor, and with α-8.
• Integrin β-2, which associates with α-L (LFA-1) to interact with ICAM-1, and with α-M (MAC-1) or α-X (p150,95) to form the receptor for the iC3b fragment of the third complement component.
• Integrin β-3, which associates with α-IIB to form a receptor for fibrinogen, fibronectin, vitronectin and VWF, and with α-V to form a vitronectin receptor.
• Integrin β-4, which associates with α-6.
• Integrin β-5, which associates with α-V.
• Integrin β-6 [3].
• Integrin β-7 [4].
• Integrin β-8, which associates with α-V [5].
• The Drosophila myospheroid protein, a probable integrin β chain.

The C-terminus of the extracellular region of all the integrin β chains has four cysteine-rich tandem repeats of forty amino acids, which are variants of the EGF-like domain (see <PDOC00021>), termed integrin- or I-EGF domains. The I-EGF domain is a small globular domain mainly composed by loops with a small anti-parallel β-sheet constituted of two β-strands (see <PDB:1L3Y>. The structure is stabilized by four disulfide bonds between the first and fifth, second and fourth, third and sixth, and seventh and eighth Cys residues. Three disulfide bonds are shared with classical EGF-like domains. The disulfide unique to I-EGF domains links the N-terminus to the turn between the two β-strands. Compared to classical EGF-like modules with three disulfide bonds, the I-EGF module is less elongated, with a nosecone-like shape. The anti-parallel sheet between the two β-strands is shortened because four highly conserved residues among classical EGF-like modules are deleted in I-EGF modules. I-EGF repeats in the integrin β subunit stalk region relay activation signals to the ligand-binding headpiece [5,6].

We have developed a pattern from a section of the I-EGF domain that includes five of the conserved cysteines. We also developed a profile which covers the entire I-EGF domain.