An improved protocol for amino acid type-selective isotope labeling in insect cells.

An improved expression protocol is proposed for amino acid type-specific [13C], [15N]-isotope labeling of proteins in baculovirus-infected (BV) insect cell cultures. This new protocol modifies the methods published by Gossert et al. (J Biomol NMR 51(4):449-456, 2011) and provides efficient incorporation of isotopically labeled amino acids, with similar yields per L versus unlabeled expression in rich media. Gossert et al. identified the presence of unlabeled amino acids in the yeastolate of the growth medium as a major limitation in isotope labeling using BV-infected insect cells. By reducing the amount of yeastolate in the growth medium ten-fold, a significant improvement in labeling efficiency was demonstrated, while maintaining good protein expression yield. We report an alternate approach to improve isotope labeling efficiency using BV-infected insect cells namely by replacing the yeast extracts in the medium with dialyzed yeast extracts to reduce the amount of low molecular weight peptides and amino acids. We report the residual levels of amino acids in various media formulations and the amino acid consumption during fermentation, as determined by NMR. While direct replacement of yeastolate with dialyzed yeastolate delivered moderately lower isotope labeling efficiencies compared to the use of ten-fold diluted undialized yeastolate, we show that the use of dialyzed yeastolate combined with a ten-fold dilution delivered enhanced isotope labeling efficiency and at least a comparable level of protein expression yield, all at a scale which economizes use of these costly reagents.