One-step site-specific S-alkylation of full-length caveolin-1: Lipidation modulates the topology of its C-terminal domain

Caveolin-1 is an integral membrane protein that is known to acquire a number of posttranslational modifications upon trafficking to the plasma membrane. In particular, caveolin-1 is palmitoylated at three cysteine residues (C133, C143, and C156) located within the C-terminal domain of the protein which could have structural and topological implications. Herein, a reliable preparation of full-length S-alkylated caveolin-1, which closely mimics the palmitoylation observed in vivo, is described. HPLC and ESI-LC-MS analyses verified the addition of the C16 alkyl groups to caveolin-1 constructs containing one (C133), two (C133 and C143), and three (C133, C143, and C156) cysteine residues. Circular dichroism spectroscopy analysis of the constructs revealed that S-alkylation does not significantly affect the global helicity of the protein; however, molecular dynamics simulations revealed that there were local regions where the helicity was altered positively or negatively by S-alkylation. In addition, the simulations showed that lipidation tames the topological promiscuity of the C-terminal domain, resulting in a disposition within the bilayer characterized by increased depth.

Facile production of tagless membrane scaffold protein for nanodiscs

The initial step in the preparation of nanodiscs is to express and purify the membrane scaffold protein (MSP) to homogeneity. Current methods used for the isolation and purification of MSP utilize nickel affinity chromatography. However, the presence of a polyhistidine tag on the MSP often interferes with downstream steps where nanodiscs reconstituted with protein need to be isolated from empty ones. Therefore, one must engage in the finicky process of removing the polyhistidine tag from the MSP using a protease before the formation of nanodiscs. Herein, we describe a robust streamlined approach to produce tagless MSP by expression as inclusion bodies followed by cleavage with cyanogen bromide, and purification by gel filtration chromatography. In addition, the MSP prepared is devoid of tryptophan residues which facilitates tryptophan-based spectroscopic studies of reconstituted proteins. Dynamic light scattering and transmission electron microscopy showed that the tagless MSP produced was competent to produce nanodiscs.

Rapid preparation of nanodiscs for biophysical studies

Nanodiscs, which are disc-shaped entities that contain a central lipid bilayer encased by an annulus of amphipathic helices, have emerged as a leading native-like membrane mimic. The current approach for the formation of nanodiscs involves the creation of a mixed-micellar solution containing membrane scaffold protein, lipid, and detergent followed by a time consuming process (3-12 h) of dialysis and/or incubation with sorptive beads to remove the detergent molecules from the sample. In contrast, the methodology described herein provides a facile and rapid procedure for the preparation of nanodiscs in a matter of minutes (<15 min) using Sephadex® G-25 resin to remove the detergent from the sample. A panoply of biophysical techniques including analytical ultracentrifugation, dynamic light scattering, gel filtration chromatography, circular dichroism spectroscopy, and cryogenic electron microscopy were employed to unequivocally confirm that aggregates formed by this method are indeed nanodiscs. We believe that this method will be attractive for time-sensitive and high-throughput experiments.

Biophysical characterization of full-length oleosin in dodecylphosphocholine micelles

Oleosin is a hydrophobic protein that punctuates the surface of plant seed lipid droplets, which are 20 nm-100 μm entities that serve as reservoirs for high-energy metabolites. Oleosin is purported to stabilize lipid droplets, but its exact mechanism of stabilization has not been established. Probing the structure of oleosin directly in lipid droplets is challenging due to the size of lipid droplets and their high degree of light scattering. Therefore, a medium in which the native structure of oleosin is retained, but is also amenable to spectroscopic studies is needed. Here, we show, using a suite of biophysical techniques, that dodecylphosphocholine micelles appear to support the tertiary structure of the oleosin protein (i.e., hairpin conformation) and render the protein in an oligomeric state that is amenable to more sophisticated biophysical techniques such as NMR.

Preparation and characterization of neutrally-buoyant oleosin-rich synthetic lipid droplets

Lipid droplets also known as oil bodies are found in a variety of organisms and function as stores of high-energy metabolites. Recently, there has been interest in using lipid droplets for protein production and drug delivery. Artificial lipid droplets have been previously prepared, but their short lifetime in solution and inhomogeneity has severely limited their applicability. Herein we report an improved methodology for the production of synthetic lipid droplets that overcomes the aforementioned limitations. These advancements include: 1) development of a methodology for the expression and purification of high-levels of oleosin, a crucial lipid droplet component, 2) preparation of neutrally-buoyant synthetic lipid droplets, and 3) production of synthetic lipid droplets of a specific size. Together, these important enhancements will facilitate the advancement of lipid droplet science and its application in biotechnology.