Abstract:ABC (ATP Binding Cassette) transporters carry out many vital functions and are involved in numerous diseases, but study of the structure and function of these proteins is often hampered by their large size and membrane location. Membrane protein purification usually utilises detergents to solubilise the protein from the membrane, effectively removing it from its native lipid environment. Subsequently lipids have to be added back and detergent removed to reconstitute the protein into a lipid bilayer. We present here the application of a new methodology for the extraction and purification of ABC transporters without the use of detergent, instead using a styrene maleic acid co-polymer (SMA). SMA inserts in a bilayer and assembles into discrete particles, essentially solubilising the membrane into small discs of bilayer encircled by polymer, termed SMA lipid particles (SMALPs). We show that this polymer can extract several eukaryotic ABC transporters; P-glycoprotein (ABCB1), MRP1 (ABCC1), MRP4 (ABCC4), ABCG2 and CFTR (ABCC7), from a range of different expression systems. The SMALP encapsulated ABC transporters can be purified by affinity chromatography, and are able to bind ligands comparably to those in native membranes or detergent micelles. A greater degree of purity and enhanced stability is seen compared to detergent solubilisation. This study demonstrates that eukaryotic ABC transporters can be extracted and purified without ever being removed from their lipid bilayer environment, opening up a wide range of possibilities for the future study of their structure and function. Summary statement:A styrene maleic acid copolymer can be effectively used to extract and purify large eukaryotic transmembrane proteins in the absence of detergents, forming small bilayer discs encapsulating the protein, which have great potential for future structure & function studies.

What is the significance of the extensive variability observed in individual members of a single-cell phenotype? This question is particularly relevant to the highly differentiated organization of the brain. In this study, for the first time, we analyze the in vivo variability within a neuronal phenotype in terms of input type. We developed a large-scale geneexpression data set from several hundred single brainstem neurons selected on the basis of their specific synaptic input types. The results show a surprising organizational structure in which neuronal variability aligned with input type along a continuum of sub-phenotypes and corresponding gene regulatory modules. Correlations between these regulatory modules and specific cellular states were stratified by synaptic input type. Moreover, we found that the phenotype gradient and correlated regulatory modules were maintained across subjects. As these specific cellular states are a function of the inputs received, the stability of these states represents ''attractor''-like states along a dynamic landscape that is influenced and shaped by inputs, enabling distinct state-dependent functional responses. We interpret the phenotype gradient as arising from analog tuning of underlying regulatory networks driven by distinct inputs to individual cells. Our results change the way we understand how a phenotypic population supports robust biological function by integrating the environmental experience of individual cells. Our results provide an explanation of the functional significance of the pervasive variability observed within a cell type and are broadly applicable to understanding the relationship between cellular input history and cell phenotype within all tissues.

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