Differences in regulation of Drosophila and vertebrate integrin affinity by talin

Integrin-mediated cell adhesion is essential for development of multicellular organisms. In worms, flies, and vertebrates, talin forms a physical link between integrin cytoplasmic domains and the actin cytoskeleton. Loss of either integrins or talin leads to similar phenotypes. In vertebrates, talin is also a key regulator of integrin affinity. We used a ligand-mimetic Fab fragment, TWOW-1, to assess talin's role in regulating Drosophila alpha PS2 beta PS affinity. Depletion of cellular metabolic energy reduced TWOW-1 binding, suggesting alpha PS2 beta PS affinity is an active process as it is for vertebrate integrins. In contrast to vertebrate integrins, neither talin knockdown by RNA interference nor talin head overexpression had a significant effect on TWOW-1 binding. Furthermore, replacement of the transmembrane or talin-binding cytoplasmic domains of alpha PS2 beta PS with those of human alpha IIb beta 3 failed to enable talin regulation of TWOW-1 binding. However, substitution of the extracellular and transmembrane domains of alpha PS2 beta PS with those of alpha IIb beta 3 resulted in a constitutively active integrin whose affinity was reduced by talin knockdown. Furthermore, wild-type alpha IIb beta 3 was activated by overexpression of Drosophila talin head domain. Thus, despite evolutionary conservation of talin's integrin/cytoskeleton linkage function, talin is not sufficient to regulate Drosophila alpha PS2 beta PS affinity because of structural features inherent in the alpha PS2 beta PS extracellular and/or transmembrane domains.