Structural and Functional Studies of Organellar Ion Channels

ABSTRACT Ion transfer across biological membranes is central to nerve excitation, muscle cell contraction, signal transduction, and hormone secretion. Ion channels play a vital role by providing a passageway within membranes to allow specific ions to traverse down their electrochemical gradient. The immense physiological importance of ion channels is reflected in the fact that their dysfunction underlies a variety of disabling human diseases including seizures, deafness, ataxia, long QT syndrome, and cardiac arrhythmias. There is a long history of physiological work and a large body of functional and structural data on tetrameric cation channels that are localized to the plasma membrane, including the K+, Ca2+, Na+, TRP and cyclic nucleotide-gated channels; however, relatively little is known about organellar cation channels, partly because of the difficulty in directly measuring their activities in organellar membranes. Currently, there is an emerging research interest in the recently identified organellar cation channels due to their importance in organelle physiology and cell signaling. This Maximizing Investigators' Research Award proposal will be focused on our ongoing efforts to dissect the structural and functional properties of two specific groups of organellar cation channels: the endolysosomal cation channels and the mitochondrial calcium uniporters. The insights gained from the proposed studies will facilitate our understanding of how these organellar channels regulate some basic biological functions of lysosome and mitochondria. Endosomes and lysosomes play crucial roles in many biological processes such as protein and lipid degradation, catabolite export, membrane trafficking, and metabolism-sensing, and defects to these processes can result in lysosomal storage diseases. These acidic organelles contain various ion channels that control endolysosomal pH and ionic homeostasis. One major research direction in my lab is designed to reveal the structural basis of gating and selectivity in endolysosomal cation channels, including two-pore channels (TPCs), transient receptor potential mucolipin channels (TRPMLs), and the non-canonical TMEM175 K+ channels. Mitochondria can take up large amounts of Ca2+ from cytosol, a process that can modulate ATP production, alter cytoplasmic Ca2+ dynamics, and trigger cell death. Mitochondrial calcium uptake is mediated by the mitochondria calcium uniporter (MCU), a highly selective Ca2+ channel that is localized to the inner mitochondrial membrane. In humans, the uniporter functions as a protein complex consisting of at least four components: the pore-forming MCU, the essential membrane-spanning subunit EMRE, and the Ca2+-sensing gate-keeping proteins MICU1 and MICU2. Another major project in the lab aims to reveal the structural basis of the human MCU complex assembly and the channel regulation. Our experimental approach utilizes single particle cryo-electron microscopy (cryo-EM) and protein crystallography to determine the three-dimensional structures of these channels, and electrophysiology to elucidate their biophysical properties.