The N-terminal ≈440 aa of integrin α subunits contain seven sequence repeats. These are predicted here to fold into a β-propeller domain. A homologous domain from the enzyme phosphatidylinositol phospholipase D is predicted to have the same fold. The domains contain seven four-stranded β-sheets arranged in a torus around a pseudosymmetry axis. The trimeric G-protein β subunit (G beta) appears to be the most closely related β-propeller. Integrin ligands and a putative Mg²⁺ ion are predicted to bind to the upper face of the β-propeller. This face binds substrates in β-propeller enzymes and is used by the G protein β subunit to bind the G protein α subunit. The integrin α subunit I domain, which is structurally homologous to the G protein α subunit, is tethered to the top of the β-propeller domain by a hinge that may allow movement of the domains relative to one another. The Ca²⁺-binding motifs in integrin α subunits are on the lower face of the β-propeller.