Imperfect pseudo-merohedral twinning in crystals of fungal fatty acid synthase

Simon Jenni,Nenad Ban

doi:10.1107/s0907444909000778

Abstract

The recent high-resolution structures of fungal fatty acid synthase (FAS) have provided new insights into the principles of fatty acid biosynthesis by large multifunctional enzymes. The crystallographic phase problem for the 2.6 MDa fungal FAS was initially solved to 5 A resolution using two crystal forms from Thermomyces lanuginosus. Monoclinic crystals in space group P2(1) were obtained from orthorhombic crystals in space group P2(1)2(1)2(1) by dehydration. Here, it is shown how this space-group transition induced imperfect pseudo-merohedral twinning in the monoclinic crystal, giving rise to a Moiré pattern-like interference of the two twin-related reciprocal lattices. The strategy for processing the twinned diffraction images and obtaining a quantitative analysis is presented. The twinning is also related to the packing of the molecules in the two crystal forms, which was derived from self-rotation function analysis and molecular-replacement solutions using a low-resolution electron microscopy map as a search model.

Highlights

Eukaryotic fatty acid synthases (FASs) are large multifunctional enzymes that catalyze all the reaction steps in the essential biosynthesis of fatty acids
This EM map was used as a search model to derive molecular-replacement solutions for several crystal forms of fungal FAS (Jenni et al, 2006; Xiong, 2008). (c) 8 Aresolution electrondensity map of T. lanuginosus FAS after crystal derivatization with Ta6Br12 heavy-atom clusters and SAD phasing in space group P212121 (Jenni et al, 2006). (d) Electron-density map in space group P21 of T. lanuginosus FAS phased at 5 Aresolution by cross-crystal and noncrystallographic symmetry (NCS) density averaging starting with phases from (c)
We describe how we identified the twinning in the monoclinic crystal form and how we processed the diffraction images acquired from twinned crystals, which allowed us to calculate accurate intensity statistics for a quantitative description of the twinning

Summary

Introduction

Eukaryotic fatty acid synthases (FASs) are large multifunctional enzymes that catalyze all the reaction steps in the essential biosynthesis of fatty acids. (b) Threedimensional single-particle reconstruction at 21 Aresolution from frozen-hydrated particles (Kolodziej et al, 1997) This EM map was used as a search model to derive molecular-replacement solutions for several crystal forms of fungal FAS (Jenni et al, 2006; Xiong, 2008). (d) Electron-density map in space group P21 of T. lanuginosus FAS phased at 5 Aresolution by cross-crystal and noncrystallographic symmetry (NCS) density averaging starting with phases from (c) At this resolution, molecular features were recognized and the map was interpreted by rigid-body fitting of structures from homologous individual enzymes (Jenni et al, 2006). We show how molecular-replacement solutions were obtained for the two crystal forms at very low resolution, which established the packing of the FAS molecules and rationalized the observed twinning

Crystallization and data collection

Data processing

Molecular-replacement solution in the P212121 and P21 crystals

Phenomenology of imperfect pseudo-merohedral twinning

Processing of data from twinned crystals