Molecular Membrane Biology Laboratory

Outline

The main research theme of our laboratory is to understand mechanisms, dynamics and roles of membrane trafficking in eukaryotic cells. We are aiming at elucidating the molecular mechanisms of vesicle budding and fusion along the secretory, vacuolar and endocytic pathways and are trying to unveil the secrets of molecular recognition and sorting during these processes. We are also interested in the roles of membrane traffic that are important for determination of cell polarity, directions of cell division and elongation, and are pursuing how they contribute to morphogenesis of tissues and organs and to other higher-order physiological functions of multicellular organisms. As experimental systems, we use the budding yeast for detailed molecular and imaging analyses, and the model plant Arabidopsis for extension to multicellular systems. Every kind of methodology, including genetics, biochemistry and cell biology (live imaging in particular), is being employed. We recently developed a super-resolution microscope for live imaging, observed compartmental maturation in the Golgi apparatus, and thus settled a worldwide debate on how proteins are transported in this organelle.

Recent Research Topic

Has the debate on the Golgi apparatus been truly settled?

Fig. 1

Live imaging revealed a mechanism that cisternal maturation with dynamic membrane mixing and segregation is driving transport of cargo.

The Golgi apparatus is an important organelle that fulfills modification, sorting and delivery of proteins. It is usually composed of a stack of cisternae (flattened membrane sacs), which shows a clear polarity from the cis (entry of newly synthesized cargo proteins) to the trans side (exit of mature proteins). There has been a debate about how cargo proteins traverse the stack of cisternae. To address this problem, we developed an ultrahigh-sensitivity, high-speed confocal microscopic system in collaboration with Yokogawa, NHK and Hitachi, which enables live imaging with extraordinary spatial resolution. Using this system, we have demonstrated that the Golgi cisternae mature over time with dynamic mixing and segregation of membranes and thus transport cargo proteins (Fig. 1). This work once appeared to settle the dispute, but some new findings are not totally consistent with this model and an alternative hypothesis has also been proposed. Rekindling of debates in this field drove 13 top scientists in the world to get together and hold a consensus meeting in Barcelona, Spain, in June 2009 (Fig. 2). In this closed meeting, the participants engaged in very heated and constructive discussions for three days and came to an agreement as to what we do and do not know about the Golgi apparatus at the moment, and indicated future directions to address unsolved questions. We published details of the discussions and the consensus reached in J. Cell Biol. The smiling Golgi (Fig. 3) in this report perhaps represents the feelings of the 13 participants. Based on these discussions, our laboratory is now tackling visualization of cargo proteins that traverse the Golgi and development of a microscopic system that live-images multicolor fluorescent proteins at high speed and at high spatial resolution.

Fig. 2 Golgi researchers gathered in Barcelona

From left: S. Emr, C. Rabouille, A. Luini, B. Glick, B. Marsh, J. Lippincott-Schwartz, F. Wieland, G. Warren, S. Pfeffer, J. Rothman, A. Nakano, A. Linstedt, and V. Malhotra.

Fig. 3 Smiling Golgi

References:

Y. Suda, A. Nakano, The yeast Golgi apparatus, Traffic, 2012, in press.

A. Nakano, A. Luini, Passage through the Golgi, Curr. Opin. Cell Biol. 2011, 22, 471.

B. S. Glick, A. Nakano, Membrane traffic within the Golgi complex, Annu. Rev. Cell Dev. Biol. 2009, 24, 113.

S. Emr, et al. Journeys through the Golgi - taking stock in a new era, J. Cell Biol. 2009, 187, 449.