Structural elucidation of how ARF small GTPases induce membrane tubulation for vesicle fission.

The ADP-Ribosylation Factor (ARF) small GTPases have been found to act in vesicle fission through a direct ability to tubulate membrane. Here, we have used cryo-electron microscopy (EM) to solve the structure of an ARF6 protein lattice assembled on tubulated membrane to 3.9 Å resolution. ARF6 forms tetramers that polymerize into helical arrays to form this lattice. We identify, and confirm functionally, protein contacts critical for this lattice formation. The solved structure also suggests how the ARF amphipathic helix is positioned in the lattice for membrane insertion, and how a GTPase-activating protein (GAP) docks onto the lattice to catalyze ARF-GTP hydrolysis in completing membrane fission. As ARF1 and ARF6 are structurally conserved, we have also modeled ARF1 onto the ARF6 lattice, which has allowed us to pursue the reconstitution of Coat Protein I (COPI) vesicles to confirm more definitively that the ARF lattice acts in vesicle fission. Our findings are notable for having achieved the first detailed glimpse of how a small GTPase bends membrane and having provided a molecular understanding of how an ARF protein acts in vesicle fission.

Dimerization of ß2-adrenergic receptor is responsible for the constitutive activity subjected to inverse agonism.

Dimerization of beta 2-adrenergic receptor (ß2-AR) has been observed across various physiologies. However, the function of dimeric ß2-AR is still elusive. Here, we revealed that dimerization of ß2-AR is responsible for the constitutive activity of ß2-AR generating inverse agonism. Using a co-immunoimmobilization assay, we found that transient ß2-AR dimers exist in a resting state, and the dimer was disrupted by the inverse agonists. A Gαs preferentially interacts with dimeric ß2-AR, but not monomeric ß2-AR, in a resting state, resulting in the production of a resting cAMP level. The formation of ß2-AR dimers requires cholesterol on the plasma membrane. The cholesterol did not interfere with the agonist-induced activation of monomeric ß2-AR, unlike the inverse agonists, implying that the cholesterol is a specific factor regulating the dimerization of ß2-AR. Our model not only shows the function of dimeric ß2-AR but also provides a molecular insight into the mechanism of the inverse agonism of ß2-AR.