Integrin intra-heterodimer affinity inversely correlates with integrin activatability

Integrins are heterodimeric cell surface receptors composed of an alpha and beta subunit that mediate cell adhesion to extracellular matrix proteins such as fibronectin. We previously studied integrin alpha 5 beta 1 activation during zebrafish somitogenesis, and in the present study, we characterize the integrin alpha V fibronectin receptors. Integrins are activated via a conformational change, and we perform single-molecule biophysical measurements of both integrin activation via fluorescence resonance energy transfer (FRET)-fluorescence lifetime imaging microscopy (FLIM) and integrin intra-heterodimer stability via fluorescence cross-correlation spectroscopy (FCCS) in living embryos. We find that integrin heterodimers that exhibit robust cell surface expression, including alpha V beta 3, alpha V beta 5, and alpha V beta 6, are never activated in this in vivo context, even in the presence of fibronectin matrix. In contrast, activatable integrins, such as integrin alpha V beta 1, and alleles of alpha V beta 3, alpha V beta 5, alpha V beta 6 that are biased to the active conformation exhibit poor cell surface expression and have a higher intra-heterodimer dissociation constant (K-D). These observations suggest that a weak integrin intra-heterodimer affinity decreases integrin cell surface stability and increases integrin activatability.