Identification of Integrin β Subunit Mutations that Alter Heterodimer Function In Situ

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Abstract
We conducted a genetic screen for mutations in myospheroid, the gene encoding the Drosophila βPS integrin subunit, and identified point mutants in all of the structural domains of the protein. Surprisingly, we find that mutations in very strongly conserved residues will often allow sufficient integrin function to support the development of adult animals, including mutations in the ADMIDAS site and in a cytoplasmic NPXY motif. Many mutations in the I-like domain reduce integrin expression specifically when βPS is combined with activating αPS2 cytoplasmic mutations, indicating that integrins in the extended conformation are unstable relative to the inactive, bent heterodimers. Interestingly, the screen has identified alleles that show gain-of-function characteristics in cell culture, but have negative effects on animal development or viability. This is illustrated by the allele mysb58; available structural models suggest that the molecular lesion of mysb58, V409>D, should promote the “open” conformation of the β subunit I-like domain. This expectation is supported by the finding that αPS2βPS (V409>D) promotes adhesion and spreading of S2 cells more effectively than does wild-type αPS2βPS, even when βPS is paired with αPS2 containing activating cytoplasmic mutations. Finally, comparisons with the sequence of human β8 suggest that evolution has targeted the “mysb58” residue as a means of affecting integrin activity.