Research Interests

Eukaryotic cells contain membrane-bound compartments, such as cell nucleus, mitochondria, endoplasmic reticulum, Golgi apparatus, peroxisomes, vacuoles/lyososmes, and chloroplasts (for plant). Each of these compartments—termed organelles—has specific functions that are essentialfor cell survival, and malfunctioning organelles cause various types of disease in humans. We are seeking to understand the basic mechanisms of how cells control function and the amount of organelles through a combination of molecular biology, cell biology, and biochemical approaches.

   We are using the unicellular red algae Cyanidioschyzon merolae as a model organism for the following reasons:

- The cells have a simple structure that contains the minimum set of organelles (suitable for microscopy).

- The timing of organelle and cell proliferation can be highly synchronized by light/dark cycles.

- The cells do not have rigid cell walls(good for isolation of organelles, etc.).

- The genome (16.4 Mb) has been completely decoded and has a simple structure.

- Gene targeting is possible.

- Breakthrough findings on organell edivision have been made using C. merolae.

Current projects

1. Molecular mechanisms of organelle–organelle interaction

  In eukaryotes, different types of organelles often interact physically and cooperate to accomplish one function. One of our interests is how such interactions form and what functions are mediated by the association. Fujiwara et al. have found that C. merolae vacuoles bind to mitochondria during mitosis. This binding requires VIG1, a homolog of CHMP5/VPS60 in endosomal sorting complex required for transport (ESCRT). In eukaryotes, ESCRT proteins are involved in various aspects of cell physiology that are related to membrane fusion and scission. We are investigating how VIG1 and other ESCRT proteins function in effecting vacuole-mitochondrial contact and what functions are accomplished by the binding of the two organelles.

2. Mechanisms for organelle proliferation

  Organelles, such as mitochondria and chloroplast, proliferate by division. The machineries that constrict and divide organelle membranes have been studied extensively. To further understand the mechanisms of organelle division, we are examining how proteins that contain organelles behave during division, such as whether and how they are pre-excluded from the site of membrane fission, using synchronous cultures.

3. Metabolism and functions of vacuolar polyphosphate

  C. merolae vacuoles accumulate polyphosphate, a linear chain of phosphate. Polyphosphate exists in a range of cells, from bacteria to human. In bacteria, it is multifunctional, serving as phosphate storage, a regulator of protease, and a signal transducer. In eukaryotes, it is often stored in acidic compartments, including vacuoles and acidocalcisomes—vacuolar compartments that are enriched with polyphosphate and calcium—but their function has not been explored extensively. We are trying to understand the metabolism and functions of polyphosphate using a C. merolae strain that does not store polyphosphate.