Eukaryotic cells have various intracellular organelles with distinct functions that are segregated by organellar membranes. Since organellar membranes, like the plasma membrane, are impermeable to ions, specialized membrane proteins such as ion transporters, pumps and channels play vital roles in the ion homeostasis of intra-organellar spaces as well as the cytosol.
Some proteins are targeted to multiple organelles before being delivered to their final destinations. For example, certain proteins, after being synthesized, travel from the ER (endoplasmic reticulum) to the Golgi apparatus, trans-Golgi network, secretory vesicles and eventually to the plasma membrane (secretory pathway). Furthermore, some proteins in the plasma membrane are internalized to specialized vesicles, called endosomes, before reaching their final location (endocytic pathway). The organellar pH along the secretory and endocytic pathways is tightly regulated, which is important in protein targeting, receptor-ligand interaction, and enzymatic activities in the organelle. It has been also suggested that impaired organellar pH may lead to cell death as well as unregulated proliferation and differentiation. However, the precise molecular mechanism underlying how organellar pH homeostasis is achieved is not yet understood.
My research interest concerns how organellar pH is regulated under physiological and pathological conditions. The Na+/H+ exchangers (NHEs) play pivotal roles in intracellular pH and cell volume regulation by their transporter function, exchanging extracellular Na+ for intracellular H+. Originally, the NHEs were identified in the plasma membrane as regulators of cytosolic pH. Recently, I have isolated human cDNAs encoding novel NHE isoforms localizing to organellar membranes that are involved in maintaining organellar pH and ion homeostasis. We will further characterize them by organellar ion flux assay and pH measurement in the presence of different pharmacological inhibitors. In addition to these cell-based techniques, in vitro reconstitution systems in liposomes will be developed for more precise functional analyses. We will also investigate NHE-interacting proteins through biochemical, cell biological, genetic and immunological techniques. To further define biological roles of NHE proteins as well as the interacting proteins, we are going to employ genetic approaches in different model-organism systems.