Recombinant locust apolipophorin III: characterization and NMR spectroscopy

Apolipophorin III (apoLp-III) from the locust Locusta migratoria is an exchangeable apolipoprotein that reversibly binds to lipoproteins. During lipid binding the protein has been proposed to undergo a major conformational change. To study the mechanism of lipid binding we have cloned and expressed recombinant protein in bacteria, permitting stable isotope enrichment for heteronuclear NMR spectroscopy and site-directed mutagenesis. The cDNA coding for apoLp-III was subcloned into the PET expression vector and transformed into Escherichia coli cells. Induction of expression resulted in the specific appearance of apoLp-III in the cell culture medium, indicating it escaped the bacteria without lysis. The protein was purified from the cell-free supernatant by reversed-phase HPLC, characterized and compared to the natural protein isolated from locust hemolymph, SDS-PAGE revealed the recombinant protein has a molecular mass of approximately 17 kDa, similar to that of deglycosylated natural apoLp-III, Monoclonal antibodies were used to detect recombinant apoLp-III in the cells as well as in cell-free medium of induced bacterial cultures. Amino acid sequencing and analysis confirmed the identity of the recombinant protein as L. migratoria apoLp-III. Circular dichroism spectroscopy of recombinant and natural apoLp-III showed similar spectra, both displaying high contents of alpha-helical secondary structure. Denaturation studies of lipid-free apoLp-III with guanidine hydrochloride showed that both proteins have similar denaturation midpoints and Delta G values indicating similar protein stability. The natural and recombinant protein were functional in lipoprotein binding assays. Using recombinant protein, uniformly and specifically labeled with N-15-amino acids, two dimensional H-1-N-15 heteronuclear single quantum correlation spectra were obtained. The spectra revealed excellent chemical shift dispersion in both the H-1 and N-15 dimensions with a well defined resonance pattern. Studies with N-15-leucine specifically labeled apoLp-III in the presence and absence of the micelle forming lipid, dodecylphosphocholine, provided evidence for a significant conformational change upon lipid association. (C) 1998 Elsevier Science B.V. All rights reserved.