ABC transporters: structures, functions, and reaction mechanisms
Dr. Locher’s research focuses on the structure and mechanism of integral membrane proteins catalyzing active transport reactions of protein N-glycosylation. These reactions, although important for cellular physiology, are generally not well understood at the molecular level because (i) high-resolution structures are scarce, and (ii) functional in vitro assays are challenging to establish. Both are a consequence of the intricacies associated with replacing the native lipid bilayer surrounding membrane proteins with detergent micelles, nanodiscs, bicelles, or lipidic cubic phases for functional and structural analyses. The Locher group has determined several crystal structures that have helped elucidate the reaction mechanisms of ABC transporters and oligosaccharyltransferase, including the first structures of an ABC exporter and type I and type II ABC importers as well as the first structure of a complete bacterial oligosaccharyltransferase.
Our understanding of the reaction mechanisms of membrane-embedded transport proteins or enzymes is insufficient. Dr. Kaspar Locher’s research group investigates two distinct types of membrane proteins, ATP-driven transporters and oligosaccaryltransferase (OST). In the past decade, his lab’s structural studies have defined the transmembrane folds of type 1 and type 2 ABC importers, ABC exporters, and of the catalytic subunit of OST. The first part of the lecture will focus on the mechanism of the bacterial ABC transporter BtuCD-F, which catalyzes vitamin B12 uptake into Gram-negative bacteria. The differences in mechanism to other ABC systems will be discussed, and an outlook on how structural biology might address unanswered mechanistic questions will be attempted. The second half of the lecture deals with OST catalyzing asparagine-linked (N-linked) protein glycosylation. This reaction is essential in eukaryotes and implicated in processes such as protein folding and quality control, organism development, or host-pathogen interactions. Some bacteria, including Campylobacter species, also catalyze N-linked glycosylation. Instead of a multimeric enzyme, they contain a single-subunit OST (PglB protein) that is homologous to the catalytically active subunit (STT3) of the eukaryotic system. The X-ray structure of the complete PglB protein from C. lari revealed the molecular basis of acceptor sequon recognition and provided insight into the reaction mechanism. The presentation will discuss the conclusions from structural mechanistic in vitro studies.
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