research

Our long-term goal is to better understand the molecular and cellular mechanisms that underlie the principles for interorganelle communication in the organelle/membrane contact sites. We investigate how protein machinery facilitates organelle specific membrane contacts by determining the atomic resolution structures of protein complexes.

1) Architectural Overview of ER-Mitochondria Contact Sites

Membrane contact sites (MCSs) play an essential role in subcellular communication by exchanging cellular materials and information. Among the various endoplasmic reticulum (ER)-mediated MCSs reported to date, the ER-mitochondria contact site has been the most extensively studied, and an involvement in ion homeostasis, mitochondrial dynamics such as membrane fission and fusion, and cooperative lipid synthesis has been reported. Most importantly, lipid trafficking occurring at the ER-mitochondria MCS is essential for the biogenesis of the mitochondrial membrane, since mitochondria are not connected with the vesicular transport machinery, and essential lipids required for the composition of mitochondrial membrane must therefore be supplied directly from the ER. Our group determined crystal structures of the Mmm1 SMP domain and the Mdm12–Mmm1 binary complex, which are key components of the ER-Mitochondria Encounter Structure (ERMES) complex that play a pivotal role in establishing direct membrane contact sites between the ER and mitochondria.

2) Architectural Overview of Nucleus Vacuole Contact Sites

The nucleus-vacuole junction (NVJ) is the first-identified inter-organellar MCS in the budding yeast Saccharomyces cerevisiae, and its formation depends on the nuclear membrane protein Nvj1 and vacuolar membrane protein Vac8. Although NVJ is suggested to mediate piecemeal microautophagy of the nucleus (PMN) and potentially lipid transfer between the two organelles, details on the molecular mechanisms of these functions are largely uncharacterized. We determined the first crystal structure of the Vac8-Nvj1 complex at 2.4 Å resolution. Based on the crystal structure, together with a series of biochemical experiments and in vivo studies using S. cerevisiae, we made the following findings: (1) Nvj1 binds to the inner groove formed from the 12 Vac8 ARMs in an extensive and antiparallel manner; (2) the Vac8-Nvj1 complex appears to be structurally and functionally conserved with the beta-catenin/E-cadherin complex; (3) Vac8 exclusively and competitively interacts with both Nvj1 and Atg13, a key factor in the cytoplasm-to-vacuole targetting (CVT) pathway, through a conserved cationic triad; and (4) Vac8 forms a homodimer, and this self-association might be regulated by the H1 helix of Vac8 as well as Nvj1 conjugation, providing a working model for NVJ formation and PMN pathway.