Transmembrane-domain-driven heterodimer assembly licenses integrin head-domain folding in the endoplasmic reticulum

Integrins are a large family of α/β heterodimeric receptors whose activation is associated with the extension of the ectodomain and disruption of transient transmembrane domain (TMD) interactions. Whether the TMD interaction serves additional function(s) beyond regulating integrin activation remains unclear. Here, we report that three TMD-disruptive mutations within the integrin β1-subunit activate its function while simultaneously reducing its surface expression. While a Gly744Asn (β1-GN) and Gly744Leu (β1-GL) TMD mutation yielded viable homozygous offspring, the more disruptive Leu748Arg (β1-LR) TMD mutation caused embryonic lethality in mice and drastically reduced β1-integrin surface levels. Cell biology and biochemistry studies revealed that TMD-TMD interactions drive efficient α/β-heterodimer assembly, thereby enabling the coupled folding of the ER-luminal βI-domain. Disruption of these TMD interactions leads to endoplasmic reticulum (ER) retention and luminal domain-dependent ER-associated degradation of the monomeric, misfolded β1-subunit. Moreover, mutations that disrupt the α/β TMD interaction of multiple integrin classes in humans similarly impair integrin cell surface expression and cause severe loss-of-function diseases. Together, our findings establish that α/β TMD association serves a previously unrecognized role in integrin biogenesis by promoting βI-domain folding, preventing premature integrin activation in the ER, and ensuring the proper cell surface expression of activatable integrins.