Significance: Kindlin proteins play crucial roles in the integrin-signaling pathway by directly interacting with and activating integrins, which mediate the cell–extracellular matrix adhesion and signaling. Mutations of kindlins lead to diseases, such as Kindler syndrome, associated with skin blistering and atrophy; leukocyte adhesion deficiency; and cancers. However, the molecular basis underlying kindlin-mediated integrin activation remains to be determined. Here, we report the structural basis of the specific interaction between kindlins and integrins. Furthermore, we demonstrate that kindlins synergize integrin activation by forming a dimer, providing a model for understanding integrin signaling. Finally, we interpret disease-causing mutations found in kindlins at the atomic level, which can be useful for understanding and treating these diseases.

Human steroid 5α-reductase 2 (SRD5A2) is an integral membrane enzyme in steroid metabolism and catalyzes the reduction of testosterone to dihydrotestosterone. Mutations in theSRD5A2gene have been linked to 5α-reductase deficiency and prostate cancer. Finasteride and dutasteride, as SRD5A2 inhibitors, are widely used antiandrogen drugs for benign prostate hyperplasia. The molecular mechanisms underlying enzyme catalysis and inhibition for SRD5A2 and other eukaryotic integral membrane steroid reductases remain elusive due to a lack of structural information. Here, we report a crystal structure of human SRD5A2 at 2.8 Å, revealing a unique 7-TM structural topology and an intermediate adduct of finasteride and NADPH as NADP-dihydrofinasteride in a largely enclosed binding cavity inside the transmembrane domain. Structural analysis together with computational and mutagenesis studies reveal the molecular mechanisms of the catalyzed reaction and of finasteride inhibition involving residues E57 and Y91. Molecular dynamics simulation results indicate high conformational dynamics of the cytosolic region that regulate NADPH/NADP⁺exchange. Mapping disease-causing mutations of SRD5A2 to our structure suggests molecular mechanisms for their pathological effects. Our results offer critical structural insights into the function of integral membrane steroid reductases and may facilitate drug development.