Intradimer/Intermolecular interactions suggest autoinhibition mechanism in endophilin A1

Endophilin A1 is a homodimeric membrane-binding endocytic accessory protein with a high dimerization affinity. Its function has been hypothesized to involve autoinhibition. However, the autoinhibition mechanism, as well as the physicochemical basis for the high dimerization affinity of endophilin in solution, have remained unclear. In this contribution, we use a Förster resonance energy transfer (FRET) method to investigate the homodimerization mechanism and intradimer molecular interactions in endophilin. For the endophilin N-BAR domain (which lacks the SH3 domain including a linker region of the full length protein), we observe a large temperature dependence of the dimerization affinity and dimer dissociation kinetics, implying large dimerization enthalpy and dissociation activation enthalpy, respectively. Our evaluation of the protein concentration dependence of dimer dissociation kinetics implies that endophilin reversibly forms monomers via a dissociation/reassociation mechanism. Furthermore, we use a kinetic method that allows us to compare the dissociation kinetics of full-length endophilin to that of truncated mutants. We find that mutants that lack either H0 helix or SH3 domain show significantly faster dissociation kinetics relative to full-length endophilin. This observation supports the presence of an intradimer, intermonomer cross-interaction between H0 helix and SH3 domain from different subunits within a homodimer. Because the H0 helix is known to play a significant role in endophilin's membrane interactions, our measurements support a syngergistic model where these interactions are inhibited in the absence of SH3 domain binding ligands such as dynamin's prolin rich domains, and where the binding of these ligands may be suppressed for non-membrane-bound endophilin.