research papers 1428 Prince et al.  Integral membrane LH2 complex Acta Cryst. (1999). D55, 1428±1431 Acta Crystallographica Section D Biological Crystallography ISSN 0907-4449 Derivative manipulation in the structure solution of the integral membrane LH2 complex S. M. Prince, a G. McDermott, a ² A. A. Freer, a M. Z. Papiz, c A. M. Lawless, c R. J. Cogdell b and N. W. Isaacs a * a Department of Chemistry, University of Glasgow, Glasgow G12 8QQ, Scotland, b Division of Biochemistry and Molecular Biology, University of Glasgow, Glasgow G12 8QQ, Scotland, and c CCLRC Daresbury Laboratory, Daresbury, Warrington WA4 4AD, England ² Present address: Macromolecular Crystallo- graphy Facility, Lawrence Berkeley National Laboratory, Mailstop 6-2100, 1 Cyclotron Road, Berkeley, CA 94720, USA. Correspondence e-mail: neil@chem.gla.ac.uk # 1999 International Union of Crystallography Printed in Denmark ± all rights reserved The structure of the peripheral light-harvesting complex from Rhodopseudomonas acidophila strain 10050 was determined by multiple isomorphous replacement methods. The deriva- tization of the crystals was augmented by the addition of a backsoaking stage. The soak/backsoaked data comparison had greater isomorphism and showed simpler Patterson maps than the standard native/soak comparison. Amplitudes from the derivatized then backsoaked crystals and from the derivatized crystals were compared in order to extract a subset of heavy- atom sites. Using this information, the full array of sites were found from a derivative/native comparison, eventually leading to excellent electron-density maps. Received 19 February 1999 Accepted 7 May 1999 1. Introduction The structure determination of integral membrane proteins (IMPs) by multiple isomorphous replacement and anomalous scattering (MIRAS) techniques is generally complicated by detergent-related factors. Partial encapsulation of the protein by a detergent micelle (Garavito et al. , 1986) reduces the effective protein surface area for heavy-atom binding, and heavy-atom salts may interact with detergent causing disrup- tion to the crystalline lattice. The surface available for binding a derivative in an isomorphous manner is further reduced as the hydrophilic extra-membraneous domains of IMPs are often involved in crystal contacts. The light-harvesting complex from Rhodopseudomonas acidophila strain 10050 (LH2) (McDermott et al., 1995) has small extra-membraneous domains (Hawthornthwaite & Cogdell, 1991), and a large proportion of the surface of the protein is thought to be encapsulated by a detergent micelle. The crystals are type II IMP crystals (Michel, 1983), in which all crystal contacts are made between hydrophilic regions. Prior to the structure determination of the LH2 complex, the structures of only two other types of integral trans- membrane proteins had been determined to high resolution by MIR methods: the bacterial reaction centre (Diesenhofer et al., 1985) and porin (Weiss et al. , 1991), along with the struc- ture of the membrane-associated protein prostaglandin H2 synthase (Picot et al., 1994). In each of these cases, the deri- vatization problems were alleviated by the presence of extensive hydrophilic domains. In the case of the bacterial reaction centre from Rps. viridis, large extramembranous domains are formed on either side of the membrane by a subunit of the reaction-centre complex and a tightly bound cytochrome. The porin trimer has extensive extracellular loop structures and the interior of each monomer is hydrophilic. Prostaglandin H2 synthase has extensive extra-membranous domains.