In Meso Crystallization of the Integral Membrane Glycerol 3-Phosphate Acyltransferase with Substrates

Here we retrospectively report the successful application of the host lipid screening strategy in the in meso crystallization of PlsY, a bacterial glycerol 3-phosphate (G3P) acyltransferase that catalyzes the committed step in phospholipid biosynthesis. Two monoacylglycerols (MAGs) with different chain lengths, namely, the 9.9 MAG (monoolein, nine carbons on either side of the cis-double bond in the acyl chain) and the 7.8 MAG, were the two main host lipids used in this study. The native PlsY crystals growing in 9.9 MAG diffracted to 1.62 angstrom. However, the selenomethioninyl crystals growing in the same host lipid under similar precipitant conditions diffracted poorly (4.3 angstrom). Switching the host lipid to 7.8 MAG dramatically improved crystal quality in both size and diffraction, yielding a 2.0-angstrom data set, with which the structure was solved. Along with low-temperature crystallization, 7.8 MAG was also critical for cocrystallization of PlsY with the unstable lipid substrate, acyl phosphate (acylP). Interestingly, however, 7.8 MAG was unsuitable for PlsY-G3P cocrystallization as multiple crystal forms obtained from an extensive screening did not reveal electron density for this water-soluble substrate. In contrast, 9.9 MAG supported the cocrystallization, yielding a 2.37-angstrom data set for the PlsY-G3P complex. We hypothesize that, under crystallization conditions, G3P binding which occurs at the membrane boundary favors a more rigid mesophase lipid bilayer formed by the longer-chain 9.9 MAG.