Figure 10.

Speculative model for C-terminal autoinhibition of ATL2. (A and B) Crystal structures of GBP1 (Protein Data Bank accession no. 1F5N; A) and ATL1 (Protein Data Bank accession no. 3Q5E; B) rendered in cartoon in PyMOL. (C) Speculative model for C-terminal autoinhibition. GTP binding by ATL promotes weak G-domain dimerization in trans. In absence of a hypothetical activator, tethering is weak, and an abortive cycle results when GTP is hydrolyzed. If a hypothetical activator is present to overcome C-terminal autoinhibition, dimerization and tethering are strong, resulting in crossover formation and productive fusion. Note that the interactions depicted between the tail amphipathic helix (AH) and 3HB, as well as between the C terminus and the G domain, are speculative and have not been experimentally validated.