Systematic survey of conformational states in β1 and β4 integrins by negative-stain electron microscopy

Structural analyses of β2 and β3 integrins have revealed that they generally assume a compact bent conformation in the resting state and undergo a global conformational transition involving extension during upregulation of ligand affinity, collectively called the “switchblade model”. This hypothesis, however, had not been extensively tested for other classes of integrins. We have prepared a set of recombinant integrin ectodomain fragments including αvβ3, α2β1, α3β1, α5β1, α6β1, and α6β4, and used negative-stain electron microscopy to examine their structures under various conditions. In contrast to αvβ3 integrin that exhibited severely bent conformation in the low affinity, 5 mM Ca2+ condition, all β1 integrin heterodimers displayed a mixed population of half-bent to fully extended conformations, but no severely bent conformation was observed. Moreover, they did not undergo significant conformational change upon activation by Mn2+. α6β4 was even more resistant to conformational regulation, showing a completely extended structure regardless of the buffer conditions. These results suggest that the mechanism of conformational regulation of integrins are more diverse and complex than previously thought, requiring more experimental scrutiny for each integrin subfamily member.