|Associate Professor||Joji MIMA|
We are working on in vitro reconstituted proteoliposomal studies to understand the molecular machineries of membrane tethering, docking, and fusion processes in the endomembrane organelle systems of eukaryotic cells (from yeast to human). Intracellular membrane tethering, docking, and fusion events are fundamental and conserved biological reactions, which are vital for vesicle trafficking between subcellular organelle membrane compartments and plasma membranes, organelle morphology, hormone secretion, and also synaptic neurotransmission. The diverse sets of key protein components for membrane docking/fusion have been identified, including SNARE-family proteins, SNARE-interacting chaperone proteins such as Sec1/Munc18-family proteins, Rab-family small GTPases, Rab-interacting effector proteins, and tethering multisubunit complexes. However, it has still remained elusive how those essential protein factors cooperate to specifically and efficiently mediate membrane tethering, docking, and fusion events. We explored the vital membrane tethering, docking, and fusion machineries by in vitro reconstitution with purified recombinant proteins (SNAREs, Rabs, and their associated protein families) and synthetic lipid bilayers with defined lipid compositions. We found that the functional synergy of SNARE chaperones and phosphoinositides is essential to trigger rapid SNARE-dependent membrane fusion. Moreover, by comprehensively studying 14 purified SNAREs in yeast, which localize at not only vacuoles but also endosomes, Golgi, and endoplasmic reticulum (ER), for their capacity to assemble into QabcR-SNARE complexes and initiate reconstituted proteoliposomal fusion, we uncover the novel concept that SNAREs employ multiple and distinct strategies to confer the specificity of membrane fusion. In addition to those SNARE studies, we recently have developed the in vitro assays to quantitatively analyze membrane tethering of synthetic liposomes in the presence of the protein factors related to physiological tethering processes. Our reconstitution studies now establish that membrane-anchored human Rab GTPases directly and specifically catalyze membrane tethering in a guanine nucleotide-independent manner. This leads us to further study how exactly Rab GTPases and their effectors or tethering factors work together on membranes to mediate membrane tethering for ensuring the directionality of intracellular membrane trafficking.
Current Research Programs
- Understanding the molecular machinery to catalyze membrane tethering, docking, and fusion events in eukaryotic endomembrane systems
- Mechanisms by which miscellaneous sets of SNAREs, Rab GTPases, and their interacting proteins control the directionality of intracellular membrane trafficking
- Multiple and distinct strategies of yeast SNAREs to confer the specificity of membrane fusion. Furukawa N, Mima J (2014) Sci Rep 4, 4277.
- LegC3, an effector protein from Legionella pneumophila, inhibits homotypic yeast vacuole fusion in vivo and in vitro. Bennett TL, Kraft SM, Reaves BJ, Mima J, O’Brien KM, Starai VJ (2013) PLoS One 8, e56798.
- Distinct contributions of vacuolar Qabc- and R-SNARE proteins to membrane fusion specificity. Izawa R, Onoue T, Furukawa N, Mima J (2012) J Biol Chem 287, 3445-3453.
- Minimal membrane docking requirements revealed by reconstitution of Rab GTPase-dependent membrane fusion from purified components. Stroupe C, Hickey CM, Mima J, Burfeind A, Wickner W (2009) Proc Natl Acad Sci USA 106, 17626-17633.
- Phosphoinositides and SNARE chaperones synergistically assemble and remodel SNARE complexes for membrane fusion. Mima J, Wickner W (2009) Proc Natl Acad Sci USA 106, 16191-16196.
- Complex lipid requirements for SNARE- and SNARE chaperone- dependent membrane fusion. Mima J, Wickner W (2009) J Biol Chem 284, 27114-2712.
- Reconstituted membrane fusion requires regulatory lipids, SNAREs and synergistic SNARE chaperones. Mima J, Hickey CM, Xu H, Jun Y, Wickner W (2008) EMBO J 27, 2031-2042.