The solubilisation of membrane proteins (MPs) necessitates the overlap of two contradictory events; the extraction of MPs from their native lipid membranes and their subsequent stabilisation in aqueous environments. Whilst the current myriad of membrane mimetic systems provide a range of modus operandi, there are no golden rules for selecting the optimal pipeline for solubilisation of a specific MP hence a miscellaneous approach must be employed balancing both solubilisation efficiency and protein stability. In recent years, numerous diverse lipid membrane mimetic systems have been developed, expanding the pool of available solubilisation strategies. This review provides an overview of recent developments in the membrane mimetic field, with particular emphasis placed upon detergents, polymer-based nanodiscs and amphipols, highlighting the latest reagents to enter the toolbox of MP research.

MlaFEDB is a Gram-negative inner membrane protein complex involved in the inter membrane trafficking of phospholipids. Originally proposed to transport phospholipids in a retrograde direction, recent evidence suggests MlaFEDB may actually export phospholipids from the inner membrane to the periplasmic carrier protein, MlaC, potentially suggesting a role in either anterograde trafficking of phospholipids to the outer membrane or bidirectional phospholipid movement. MlaFEDB is part of the ABC transporter superfamily of proteins and has been shown to hydrolyse ATP through the cytoplasmic facing MlaF component. However, the movement of PLs from FEDB to MlaC has been shown to occur in an ATP independent fashion hence the role of ATP hydrolysis within this complex remains unclear. In this study we sought to elucidate the role of ATP and provide evidence to suggest MlaFEDB has flippase activity, utilising ATP hydrolysis to translocate phospholipids from the outer to the inner leaflet of the IM. We also show that in the absence of ATP MlaFEDB mediates the loading of MlaC with phospholipids directly from the inner leaflet only. Our data provides a novel role for MlaFEDB and presents a link between Mla driven phospholipid transport and ATP hydrolysis.

Protein conjugation with the Small Ubiquitin-like Modifier SUMO1 or the related SUMO2/3 drive changes to protein behaviour. Many substrates are found modified by both SUMO1 and SUMO2/3, while others are modified by one or the other. How isoform specificity is directed is poorly understood. Here we examine modification of the catalytic component of the human SUMO Activation Enzyme, SAE2. We find that an acetylated K164-SAE2 analogue preferentially activates SUMO2 in competition with SUMO1, and that K164-SAE2 discriminates paralogues through their C-terminal regions. We find that K164-SAE2 is deacetylated during mitosis. Mitotic defects in cells expressing an acetylated K164-SAE2 analogue can be corrected by over-expression of SUMO1, suggesting SUMO1 conjugation driven by the deacetylated enzyme supports mitotic fidelity. These surprising data reveal that modification of the SUMO-activating enzyme can bias SUMO paralogue conjugation.