Date Published: December 7, 2012
Publisher: Public Library of Science
Author(s): Shih Lin Goh, Qi Wang, Laura J. Byrnes, Holger Sondermann, Ludger Johannes. http://doi.org/10.1371/journal.pone.0051628
Endocytosis is a fundamental process in signaling and membrane trafficking. The formation of vesicles at the plasma membrane is mediated by the G protein dynamin that catalyzes the final fission step, the actin cytoskeleton, and proteins that sense or induce membrane curvature. One such protein, the F-BAR domain-containing protein pacsin, contributes to this process and has been shown to induce a spectrum of membrane morphologies, including tubules and tube constrictions in vitro. Full-length pacsin isoform 1 (pacsin-1) has reduced activity compared to its isolated F-BAR domain, implicating an inhibitory role for its C-terminal Src homology 3 (SH3) domain. Here we show that the autoinhibitory, intramolecular interactions in pacsin-1 can be released upon binding to the entire proline-rich domain (PRD) of dynamin-1, resulting in potent membrane deformation activity that is distinct from the isolated F-BAR domain. Most strikingly, we observe the generation of small, homogenous vesicles with the activated protein complex under certain experimental conditions. In addition, liposomes prepared with different methods yield distinct membrane deformation morphologies of BAR domain proteins and apparent activation barriers to pacsin-1’s activity. Theoretical free energy calculations suggest bimodality of the protein-membrane system as a possible source for the different outcomes, which could account for the coexistence of energetically equivalent membrane structures induced by BAR domain-containing proteins in vitro. Taken together, our results suggest a versatile role for pacsin-1 in sculpting cellular membranes that is likely dependent both on protein structure and membrane properties.
Local differences and dynamic changes in curvature are hallmarks of cellular membranes, contributing to the identity of organelles and to mechanisms in membrane trafficking and signaling . Peripheral and integral membrane proteins have been identified that either promote or stabilize membrane curvature at different locations in the cell. For example, endocytosis relies on the coordinated interplay of coat and adaptor proteins to initiate the formation and stabilization of a bud-neck structure, followed by the recruitment of the large G protein dynamin and subsequent fission –. In addition, reorganization of the actin cytoskeleton via the recruitment and activation of Wiskott-Alrich Syndrome proteins (WASP) provides another driving force in this process , .
Bending membranes requires energy because bilayers tend to resist shape changes. Proteins contribute to this energy requirement via electrostatic interactions, scaffolding mechanisms, and the insertion of amphipathic helices or hydrophobic loops. Based on energetic considerations described above, it appears likely that the F-BAR domain of pacsin-1 relies on both scaffolding and hydrophobic insertions to deform membranes, with neither mechanism alone having the capacity to effectively shape membranes into the structures that were observed in our in vitro experiments. Both mechanisms may contribute additively to the deformation of membranes by counteracting the area mismatch between the two leaflets that would arise upon curvature generation, and by stabilizing a preferred membrane topology that is compatible with the shape imposed by the protein structure . A mismatch in geometry between proteins and the membrane would destabilize their interaction, leading to potential disruption of protein lattices and changes in membrane remodeling propensities.