Humans have at least 18 α subtypes and 8 β subtypes which together generate 24 known binding pairs for the integrins heterodimer (reviewed in ). The α subunit of the integrin heterodimer especially the αI domain determines the ligand specificity for cell-ECM adhesion (reviewed in ). The characteristic of integrin subunits is their ability to bind diverse matrix molecules imparted by the heterogeneity of the monomers; this plasticity is instrumental for cell-ECM binding and subsequent mechanotransduction events.
The amino acid sequence: arginine-glycine-aspartic acid, or RGD motif, is commonly accepted as a general integrin-binding motif on target ligands, however, individual integrins can also recognize other protein-specific motifs (reviewed in ).
Common ECM components that are bound by integrins (with respective recognition sequence) are
- Fibronectin (RGD, LDV)
- Collagen(triple helical GFOGER)
- Glycoproteins (e.g. tenascin C , osteopontin, nefronectin)
Immunologically important integrin ligands are the inter-cellular adhesion molecules (ICAMs), immunoglobulin superfamily members present on inflamed endothelium and antigen-presenting cells.
Integrins are broadly grouped into four categories based on their ligand-specificity (reviewed in ):
- RGD receptors (α5β1, αVβ3, αVβ1,αVβ5, αVβ6, αVβ8, and αIIbβ3)
- Laminin receptors (α1β1, α2β1, α3β1, α6β1, α7β1, and α6β4)
- Leukocyte-specific receptors (αLβ2, αMβ2, αXβ2, and αDβ2)
- Collagen receptors (α1β1, α2β1, α3β1, α10β1, and α11β1)
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