Mutations in the Drosophila αPS2 integrin subunit uncover new features of adhesion site assembly

The Drosophila αPS2βPS integrin is required for diverse development events, including muscle attachment. We characterized six unusual mutations in the αPS2 gene that cause a subset of the null phenotype. One mutation changes a residue in αPS2 that is equivalent to the residue in αV that contacts the arginine of RGD. This change severely reduced αPS2βPS affinity for soluble ligand, abolished the ability of the integrin to recruit laminin to muscle attachment sites in the embryo and caused detachment of integrins and talin from the ECM. Three mutations that alter different parts of the αPS2 β-propeller, plus a fourth that eliminated a late phase of αPS2 expression, all led to a strong decrease in αPS2βPS at muscle ends, but, surprisingly, normal levels of talin were recruited. Thus, although talin recruitment requires αPS2βPS, talin levels are not simply specified by the amount of integrin at the adhesive junction. These mutations caused detachment of talin and actin from integrins, suggesting that the integrin-talin link is weaker than the ECM-integrin link.