Candida albicans and C. glabrata express exporters of the ATP -binding cassette (ABC) superfamily and address them to their plasma membrane to expel azole antifungals, which cancels out their action and allows the yeast to become multidrug resistant (MDR). In a way to understand this mechanism of defense, we describe the purification and characterization of Cdr1, the membrane ABC exporter mainly responsible for such phenotype in both species. Cdr1 proteins were functionally expressed in the baker yeast, tagged at their C -terminal end with either a His -tag for the glabrata version, cgCdr1-His, or a green fluorescent protein (GFP) preceded by a proteolytic cleavage site for the albicans version, caCdr1-P-GFP. A membrane Cdr1-enriched fraction was then prepared to assay several detergents and stabilizers, probing their level of extraction and the ATPase activity of the proteins as a functional marker. Immobilized metal -affinity and size -exclusion chromatographies (IMAC, SEC) were then carried out to isolate homogenous samples. Overall, our data show that although topologically and phylogenetically close, both proteins display quite distinct behaviors during the extraction and purification steps, and qualify cgCdr1 as a good candidate to characterize this type of proteins for developing future inhibitors of their azole antifungal efflux activity. 

Lipopolysaccharides such as the enterobacterial common antigen are important components of the enterobacterial cell envelope that act as a protective barrier against the environment and are often polymerized by the inner membrane bound Wzy-dependent pathway. By employing cryo-electron microscopy we show that WzzE, the co-polymerase component of this pathway that is responsible for the length modulation of the enterobacterial common antigen, is octameric with alternating up-down conformations of its L4 loops. The alternating up-down nature of these essential loops, located at the top of the periplasmic bell, are modulated by clashing helical faces between adjacent protomers that flank the L4 loops around the octameric periplasmic bell. This alternating arrangement and a highly negatively charged binding face create a dynamic environment in which the polysaccharide chain is extended, and suggest a ratchet-type mechanism for polysaccharide elongation. Cryo-EM structure of bacterial polysaccharide co-polymerase WzzE provides insight into possible mechanisms of lipopolysaccharide elongation and length regulation.

Multidrug ABC transporters translocate drugs across membranes by a mechanism for which the molecular features of drug release are so far unknown. Here, we resolved three ATP-Mg2+-bound outward-facing conformations of the Bacillus subtilis (homodimeric) BmrA by x-ray crystallography and single-particle cryo-electron microscopy (EM) in detergent solution, one of them with rhodamine 6G (R6G), a substrate exported by BmrA when over-expressed in B. subtilis. Two R6G molecules bind to the drug-binding cavity at the level of the outer leaflet, between transmembrane (TM) helices 1-2 of one monomer and TM5'-6' of the other. They induce a rearrangement of TM1-2, highlighting a local flexibility that we confirmed by hydrogen/deuterium exchange and molecular dynamics simulations. In the absence of R6G, simulations show a fast postrelease occlusion of the cavity driven by hydrophobicity, while when present, R6G can move within the cavity, maintaining it open.

Lipopolysaccharides are important components of the bacterial cell envelope that among other things act as a protective barrier against the environment and toxic molecules such as antibiotics. One of the most widely disseminated pathways of polysaccharide biosynthesis is the inner membrane bound Wzy-dependent pathway. Here we present the 3.0 Å structure of the co-polymerase component of this pathway, WzzB from E. coli solved by single-particle cryo-electron microscopy. The overall architecture is octameric and resembles a box jellyfish containing a large bell-shaped periplasmic domain with the 2-helix transmembrane domain from each protomer, positioned 32 Å apart, encircling a large empty transmembrane chamber. This structure also reveals the architecture of the transmembrane domain, including the location of key residues for the Wzz-family of proteins and the Wzy-dependent pathway present in many Gram-negative bacteria, explaining several of the previous biochemical and mutational studies and lays the foundation for future investigations.

Lipoproteins are important components of the cell envelope and are responsible for many essential cellular functions. They are produced by the post-translational covalent attachment of lipids that occurs via a sequential 3-step process controlled by three integral membrane enzymes. The last step of this process, unique to Gram-negative bacteria, is the N-acylation of the terminal cysteine by Apolipoprotein N-acyltransferase (Lnt) to form the final mature lipoprotein. Here we report 2 crystal forms of Lnt from Escherichia coli. In one form we observe a highly dynamic arm that is able to restrict access to the active site as well as a covalent modification to the active site cysteine consistent with the thioester acyl-intermediate. In the second form, the enzyme crystallized in an open conformation exposing the active site to the environment. In total we observe 3 unique Lnt molecules that when taken together suggest the movement of essential loops and residues are triggered by substrate binding that could control the interaction between Lnt and the incoming substrate apolipoprotein. The results provide a dynamic context for residues shown to be central for Lnt function and provide further insights into its mechanism.

In the mycobacterial electron-transport chain, respiratory complex III passes electrons from menaquinol to complex IV, which in turn reduces oxygen, the terminal acceptor. Electron transfer is coupled to transmembrane proton translocation, thus establishing the electrochemical proton gradient that drives ATP synthesis. We isolated, biochemically characterized, and determined the structure of the obligate III2IV2 supercomplex from Mycobacterium smegmatis, a model for Mycobacterium tuberculosis. The supercomplex has quinol:O-2 oxidoreductase activity without exogenous cytochrome c and includes a superoxide dismutase subunit that may detoxify reactive oxygen species produced during respiration. We found menaquinone bound in both the Q(o) and Q(i) sites of complex III. The complex III-intrinsic diheme cytochrome cc subunit, which functionally replaces both cytochrome c(1) and soluble cytochrome c in canonical electron-transport chains, displays two conformations: one in which it provides a direct electronic link to complex IV and another in which it serves as an electrical switch interrupting the connection.

Membrane proteins control a large number of vital biological processes and are often medically important-not least as drug targets. However, membrane proteins are generally more difficult to work with than their globular counterparts, and as a consequence comparatively few high-resolution structures are available. In any membrane protein structure project, a lot of effort is usually spent on obtaining a pure and stable protein preparation. The process commonly involves the expression of several constructs and homologs, followed by extraction in various detergents. This is normally a time-consuming and highly iterative process since only one or a few conditions can be tested at a time. In this article, we describe a rapid screening protocol in a 96-well format that largely mimics standard membrane protein purification procedures, but eliminates the ultracentrifugation and membrane preparation steps. Moreover, we show that the results are robustly translatable to large-scale production of detergent-solubilized protein for structural studies. We have applied this protocol to 60 proteins from an E. coli membrane protein library, in order to find the optimal expression, solubilization and purification conditions for each protein. With guidance from the obtained screening data, we have also performed successful large-scale purifications of several of the proteins. The protocol provides a rapid, low cost solution to one of the major bottlenecks in structural biology, making membrane protein structures attainable even for the small laboratory.