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β-barrel membrane proteins populate the outer membrane of Gram-negative bacteria, mitochondria, and chloroplasts, playing significant roles in multiple key cellular pathways. Characterizing the functions of these membrane proteins in vivo is often challenging due to the complex protein network in the periplasm of Gram-negative bacteria (or intermembrane space in mitochondria and chloroplasts) and the presence of other outer membrane proteins. In vitro reconstitution into lipid-bilayer-like environments such as nanodiscs or proteoliposomes provides an excellent method for examining the specific function and mechanism of these membrane proteins in an isolated system. Here, we describe the methodologies employed to investigate Slam, a 14-stranded β-barrel membrane protein also known as the type XI secretion system that is responsible for translocating proteins across the outer membrane of many bacterial species., (© 2024. The Author(s), under exclusive license to Springer Science+Business Media, LLC, part of Springer Nature.)

Total interference reflection fluorescence (TIRF) microscopy of lipid bilayers is an effective technique for studying the lateral movement and ion channel activity of single integral membrane proteins. Here we describe how to integrate the mitochondrial outer membrane preprotein translocase TOM-CC and its β-barrel protein-conducting channel Tom40 into supported lipid bilayers to identify possible relationships between movement and channel activity. We propose that our approach can be readily applied to membrane protein channels where transient tethering to either membrane-proximal or intramembrane structures is accompanied by a change in channel permeation., (© 2024. The Author(s), under exclusive license to Springer Science+Business Media, LLC, part of Springer Nature.)

β-barrel outer membrane proteins (OMPs) are found in the outer membranes (OMs) of all gram-negative bacteria, yet exactly how they are folded and inserted remains unknown. The last decade has provided a wealth of discovery including the identification of the BAM complex, a multicomponent complex responsible for the biogenesis of all OMPs into the OM. It is anticipated that the next decade will further advance our knowledge of how the BAM complex is able to perform its unique and interesting function.

The utility of protein engineering, both the mutation and deletion of specific amino acids, to investigate protein structure and function has been demonstrated time and time again, and intermolecular and intramolecular interactions within the BAM complex and its individual components are no exception. Extensive efforts have probed conserved and unique amino acid sequences of the Bam proteins to define their functional roles. This chapter summarizes efforts as applied to the disulfide cross-link mutants of BamA and describes experimental methods used in our studies to determine that lateral opening of the barrel domain is required for function.

β-barrel membrane proteins are somewhat unique in that their folding states can be monitored using semi-native SDS-PAGE methods to determine if they are folded properly or not. This property, which is commonly referred to as heat modifiability, has been used for many years on both purified protein and on whole cells to monitor folded states of proteins of interest. Additionally, heat modifiability assays have proven indispensable in studying the BAM complex and its role in folding and inserting β-barrel membrane proteins into the outer membrane. Here, we describe the protocol our lab uses for performing the heat modifiability assay in our studies on outer membrane proteins.