Wei Mi

Cells are surrounded by the plasma membranes and a variety of cellular processes, such as substance transport and cell signaling, are carried out by proteins embedded in the membrane. Membrane proteins are targets of more than half of all modern pharmaceutical drugs, and they are notorious for structural determination. Recent breakthroughs in the cryo-electron microscopy (cryo-EM) field provide unprecedented tools to study structures of membrane proteins in their native environment and hence greatly improve our mechanistic views. The Mi laboratory exploits structural, biochemical and biophysical approaches to understand the structures and functions of membrane proteins.

Antibiotic resistance is one of the most pressing public health challenges of our time, demanding new druggable targets, especially for Gram-negative bacteria. Compared with Gram-positive bacteria that have only a plasma membrane, Gram-negative bacteria have an additional unique outer membrane (OM). The OM has an asymmetric lipid structure, with phospholipids in the periplasmic leaflet and lipopolysaccharides (LPS) on the cell surface. Tightly packed LPS molecules form a remarkable permeability barrier against many antibiotics, including large polar molecules and hydrophobic agents. Because of the essential roles played by LPS in antibiotic resistance and bacterial viability, its biogenesis is an attractive target for antibacterial agents. Several inhibitors that target LPS synthesis and transport are under development as promising antibacterial candidates. However, proteins involved in the regulation of LPS biogenesis have not received a great deal of attention mainly because little is known about its regulation. Since very high or low amounts of LPS are lethal to cell viability, regulation of LPS biogenesis provides a potential opportunity for developing valuable antibiotics. By focusing on understanding how LPS biogenesis is regulated in Escherichia coli, we will provide new molecular mechanistic insights and identify novel targets and approaches for antibiotic development.

It has been known for more than two decades that LpxC, a key enzyme in LPS synthesis, determines LPS levels and that FtsH-dependent proteolysis in Escherichia coli primarily regulates LpxC cellular concentrations. However, how FtsH adjusts its protease activity to specifically regulate LpxC turnover rate according to cellular status is not clear. Recently, genetic analyses and in vivo studies suggested that two essential membrane proteins LapB (YciM) and YejM regulate FtsH protease activity, but there have been no in vitro studies on the regulatory mechanisms. The Mi lab established the first assays to reconstitute LpxC degradation in vitro. We have recently determined the cryoEM structure of YejM/LapB complex structure, elucidating the functions of these key membrane proteins in regulating LPS synthesis.