The Z. Xu lab emphasizes molecular structures of proteins and how structural insights lead to understanding of the underlying molecular mechanisms. We use high-resolution X-ray crystallography to capture molecular images of proteins in action. These images will then serve as guides to help us design additional biochemistry and cell biology experiments to explore how conformational change and molecular interaction within protein structures lead to biological consequences. We have a particular interest in intracellular trafficking. Current research falls into two general areas: (1) Membrane curvature generation and budding by the endosomal sorting complex required for transport (ESCRT); and (2) Import of peroxisomal matrix and membrane proteins by the peroxins.
Membrane curvature generation and budding by the ESCRTs
Biological membranes constantly undergo structural remodeling in response to cellular needs. In the multi-vesicular body (MVB) pathway, endosomal membrane carrying internalized growth factor receptors buds into the lumen of the endosome to form MVBs. This process plays an important role in regulating cell surface signaling by targeting activated receptor-hormone complexes to the lysosome for degradation. In a topologically similar process, HIV viruses bud into the extracellular space to complete their infectious cycle. The ESCRT is a multi-component molecular machinery required for both processes. We are keenly interested in understanding the molecular mechanism that underlies the function of the ESCRT. The ESCRT consists of several protein complexes, each playing specific roles in controlling endosomal membrane dynamics. For example, the ESCRT-III complex and the Vps4/Vta1 complex functions in the later stage of the endosomal sorting pathway by generating membrane curvature and completing vesicle fission. Our recent studies on Vps4 (a AAA-ATPase), Vta1 (a Vps4-binding regulator) and Ist1 (an ESCRT-III protein) have already provided some early mechanistic insights into these proteins. As the function of the ESCRT has clear implications in diseases such as cancer, viral infection and neuro-degeneration, we hope our continued exploration in the molecular mechanism of the ESCRT function will eventually lead to new therapeutic opportunities.
Import of peroxisomal matrix and membrane proteins by the peroxins
Peroxisomes are single-membrane-bound organelles in virtually all eukaryotic cells. Their matrix harbors at least 50 different enzymes that are linked to diverse biochemical pathways, including beta-oxidation of fatty acids and detoxification of hydrogen peroxide. Because of the central role of peroxisomes in lipid metabolism, they are essential for normal human development and physiology. Defects in peroxisomal biogenesis caused by genetic disorders lead to death in early infancy. These genetic disorders have been mapped to genes that encode proteins known as the peroxins. Peroxisomal matrix and membrane proteins are synthesized by the ribosomes in the cytosol and are imported into peroxisomes in a process coordinated by the peroxins. This process can be conceptually divided into four steps: (1) cargo recognition; (2) docking of the cargo-loaded receptors on the membrane; (3) cargo release and translocation; and (4) receptor recycling. Recent studies have indicated that import of the peroxisomal proteins is an ATP-dependent process and ATP is consumed in the final receptor cycling step. Two AAA-ATPase peroxins Pex1 and Pex6 are involved in this step but their mechanism of action is poorly defined. Interestingly, more than 80% of all patients with Zellweger syndrome, the most severe peroxisome biogenesis disorder, carry mutations in Pex1 or Pex6. We have recently begun to explore the structure and function of Pex1 and Pex6, with the eventual goal of achieving a better understanding of the molecular mechanism underlying the function of these two proteins.