A facile method for isolation of recombinant human apolipoprotein A-I from E. coli

Purification and HDL-like particle formation of apolipoprotein A-I after co-expression with the EDDIE mutant of Npro autoprotease

Apolipoprotein A-I (ApoA-I) is the major protein constituent of high-density lipoprotein particles, and as such is involved in cholesterol transport and activation of LCAT (the lecithin:cholesterol acyltransferase). It may also form amyloidal deposits in the body, showing the multifaceted interactions of ApoA-I. In order to facilitate the study of ApoA-I in various systems, we have developed a protocol based on recombinant expression in E. coli. ApoA-I is protected from degradation by driving its expression to inclusion bodies using a tag: the EDDIE mutant of Npro autoprotease from classical swine fever virus. Upon refolding, EDDIE will cleave itself off from the target protein. The result is a tag-free ApoA-I, with its N-terminus intact. ApoA-I was then purified using a five-step procedure composed of anion exchange chromatography, immobilized metal ion affinity chromatography, hydrophobic interaction chromatography, boiling and size exclusion chromatography. This led to protein of high purity as confirmed with SDS-PAGE and mass spectrometry. The purified ApoA-I formed discoidal objects in the presence of zwitterionic phospholipid DMPC, showing its retained function of interacting with lipids. The protocol was also tested by expression and purification of two ApoA-I mutants, both of which could be purified in the same manner as the wildtype, showing the robustness of the protocol.

Single-Molecule 3D Images of "Hole-Hole" IgG1 Homodimers by Individual-Particle Electron Tomography

The engineering of immunoglobulin-G molecules (IgGs) is of wide interest for improving therapeutics, for example by modulating the activity or multiplexing the specificity of IgGs to recognize more than one antigen. Optimization of engineered IgG requires knowledge of three-dimensional (3D) structure of synthetic IgG. However, due to flexible nature of the molecules, their structural characterization is challenging. Here, we use our reported individual-particle electron tomography (IPET) method with optimized negative-staining (OpNS) for direct 3D reconstruction of individual IgG hole-hole homodimer molecules. The hole-hole homodimer is an undesired variant generated during the production of a bispecific antibody using the knob-into-hole heterodimer technology. A total of 64 IPET 3D density maps at ~15 Å resolutions were reconstructed from 64 individual molecules, revealing 64 unique conformations. In addition to the known Y-shaped conformation, we also observed an unusual X-shaped conformation. The 3D structure of the X-shaped conformation contributes to our understanding of the structural details of the interaction between two heavy chains in the Fc domain. The IPET approach, as an orthogonal technique to characterize the 3D structure of therapeutic antibodies, provides insight into the 3D structural variety and dynamics of heterogeneous IgG molecules.

Optimized Negative-Staining Protocol for Lipid-Protein Interactions Investigated by Electron Microscopy

A large number of proteins are capable of inserting themselves into lipids, and interacting with membranes, such as transmembrane proteins and apolipoproteins. Insights into the lipid-protein interactions are important in understanding biological processes, and the structure of proteins at the lipid binding stage can help identify their roles and critical functions. Previously, such structural determination was challenging to obtain because the traditional methods, such as X-ray crystallography, are unable to capture the conformational and compositional heterogeneity of protein-lipid complexes. Electron microscopy (EM) is an alternative approach to determining protein structures and visualizing lipid-protein interactions directly, and negative-staining (OpNS), a subset of EM techniques, is a rapid, frequently used qualitative approach. The concern, however, is that current NS protocols often generate artifacts with lipid-related proteins, such as rouleaux formation from lipoproteins. To overcome this artifact formation, Ren and his colleagues have refined early NS protocols, and developed an optimized NS protocol that validated by comparing images of lipoproteins from cryo-electron microscopy (cryo-EM). This optimized NS protocol produces "near native-state" particle images and high contrast images of the protein in its native lipid-binding state, which can be used to create higher-quality three-dimensional (3D) reconstruction by single-particle analysis and electron tomography (e.g. IPET). This optimized protocol is thus a promising hands-on approach for examining the structure of proteins at their lipid-binding status.

High-efficient bacterial production of human ApoA-I amyloidogenic variants

Apolipoprotein A-I (ApoA-I)-related amyloidosis is a rare disease caused by missense mutations in the APOA1 gene. These mutations lead to protein aggregation and abnormal accumulation of ApoA-I amyloid fibrils in heart, liver, kidneys, skin, nerves, ovaries, or testes. Consequently, the carriers are at risk of single- or multi-organ failure and of need of organ transplantation. Understanding the basic molecular structure and function of ApoA-I amyloidogenic variants, as well as their biological effects, is, therefore, of great interest. However, the intrinsic low stability of this type of proteins makes their overexpression and purification difficult. To overcome this barrier, we here describe an optimized production and purification procedure for human ApoA-I amyloidogenic proteins that efficiently provides between 46 mg and 91 mg (depending on the protein variant) of pure protein per liter of Escherichia coli culture. Structural integrity of the amyloidogenic and native ApoA-I proteins were verified by circular dichroism spectroscopy and intrinsic fluorescence analysis, and preserved functionality was demonstrated by use of a lipid clearance assay as well as by reconstitution of high-density lipoprotein (HDL) particles. In conclusion, the use of the described high-yield protein production system to obtain amyloidogenic ApoA-I proteins, and their native counterpart, will enable molecular and cellular experimental studies aimed to explain the molecular basis for this rare disease.