Transmembrane domain helix packing stabilizes integrin aIIbβ3 in the low affinity state

Regulated changes in the affinity of integrin adhesion receptors ("activation") play an important role in numerous biological functions including hemostasis, the immune response, and cell migration. Physiological integrin activation is the result of conformational changes in the extracellular domain initiated by the binding of cytoplasmic proteins to integrin cytoplasmic domains. The conformational changes in the extracellular domain are likely caused by disruption of intersubunit interactions between the alpha and beta transmembrane (TM) and cytoplasmic domains. Here, we reasoned that mutation of residues contributing to alpha/beta interactions that stabilize the low affinity state should lead to integrin activation. Thus, we subjected the entire intracellular domain of the beta3 integrin subunit to unbiased random mutagenesis and selected it for activated mutants. 25 unique activating mutations were identified in the TM and membrane-proximal cytoplasmic domain. In contrast, no activating mutations were identified in the more distal cytoplasmic tail, suggesting that this region is dispensable for the maintenance of the inactive state. Among the 13 novel TM domain mutations that lead to integrin activation were several informative point mutations that, in combination with computational modeling, suggested the existence of a specific TM helix-helix packing interface that maintains the low affinity state. The interactions predicted by the model were used to identify additional activating mutations in both the alpha and beta TM domains. Therefore, we propose that helical packing of the alpha and beta TM domains forms a clasp that regulates integrin activation.