The Role of the Specificity-Determining Loop of the Integrin β Subunit I-like Domain in Autonomous Expression, Association with the α Subunit, and Ligand Binding

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Abstract
Integrin β subunits contain a highly conserved I-like domain that is known to be important for ligand binding. Unlike integrin I domains, the I-like domain requires integrin α and β subunit association for optimal folding. Pactolus is a novel gene product that is highly homologous to integrin β subunits but lacks associating α subunits [Chen, Y., Garrison, S., Weis, J. J., and Weis, J. H. (1998) J. Biol. Chem.273, 8711−8718] and a ∼30 amino acid segment corresponding to the specificity-determining loop (SDL) in the I-like domain. We find that the SDL is responsible for the defects in integrin β subunit expression and folding in the absence of α subunits. When transfected in the absence of α subunits into cells, extracellular domains of mutant β subunits lacking SDL, but not wild-type β subunits, were well secreted and contained immunoreactive I-like domains. The purified recombinant soluble β1 subunit with the SDL deletion showed an elongated shape in electron microscopy, consistent with its structure in αβ complexes. The SDL segment is not required for formation of α5β1, α4β1, αVβ3, and α6β4 heterodimers, but is essential for fomation of α6β1, αVβ1, and αLβ2 heterodimers, suggesting that usage of subunit interface residues is variable among integrins. The β1 SDL is required for ligand binding and for the formation of the epitope for the α5 monoclonal antibody 16 that maps to loop segments connecting blades 2 and 3 of β-propeller domain of α5, but is not essential for nearby β-propeller epitopes.