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WILLIAM CRAMER
| Henry Koffler Distinguished Professor of Biological Sciences LILY B-410A 494-4956 CV: Link |
Structure-Function of Membrane Proteins
I. Structure-Function of the Cytochrome b6f Complex
Although there are approximately 50,000 independent structures of soluble proteins, and somewhat more than 150 independent structures of integral membrane proteins (IMP) in the Protein Data Bank, there are only about 2 dozen structures of hetero-oligomeric integral membrane proteins that have been solved to a resolution ≤ 3.0 Å. Although these membrane proteins are of great interest in health and disease-related studies, most of these hetero-oligomeric structures are of protein complexes that function in energy transducing membranes. A high resolution (3.0 Å) structure in the presence of Cd2+ ion has been obtained of one of the three major hetero-oligomeric membrane protein complexes, the cytochrome b6f complex, in the electron transport chain of oxygenic photosynthesis.
The dimeric 220 kDa complex in cyanobacteria contains 8 polypeptide subunits (Science, 302, 1009-, 2003; Ann. Rev. Biochem., 75, 769-, 2006, J. Mol. Biol., 370: 59-72, 2007), 13 trans-membrane helices, and 7 prosthetic groups (4 hemes, 1 FeS cluster, 1 Chl a, 1 beta-carotene) per monomer. Several aspects of this structure complex are novel, including the single chlorophyll a, beta-carotene, and unique covalently bound heme cn, which is 5-coordinate, with a water as the 5th ligand, but unique without any amino acid side chain to serve as an axial ligand. Crystal structures with quinone analogue inhibitors imply that heme cn is the electron donor to plastoquinone on the n- or stromal side of the membrane. The structures provide insight into the pathways of electron and H + transport, and also describe the labyrinthine transfer pathways of the hydrophobic plastoquinone and -quinol across the b6f complex.
II. Protein Import:The Colicin Translocon
Concept for the pathway and mechanisms of import into E. coli of the cytotoxic E colicins are based on structures of the outer membrane vitamin B12 receptor (BtuB; panels A, B, side and top views; J. Mol. Biol. 364: 716-734, 2006) and OmpF translocator (see below, E; EMBO J., 27, 2171-2180, 2008) which the colicins parasitize for their import. A 2.75 Å structure of the complex of the receptor-binding domain of the endoribonucleolytic colicin E3 (panel C) showed the elongate 100 Å long colicin domain to be bound in an oblique mode, in which it can ‘fish’ for a second (OmpF) outer membrane translocator (Nat Struct Biol, 10, 948-954, 2003 ). A very similar structure was obtained for a complex of the receptor-binding domain of colicin E2 and BtuB (panel D) [J. Biol. Chem., 282: 2171-2180]. Circular dichroism in the far UV has been used to characteracterize the "unfolded" secondary structure of the N- and C-terminal peptides of colicin E3 that interact with the outer membrane receptors, BtuB and OmpF (Biochemistry, 45, 10199-, 2006).
III. Discrete Ion Channel Formation by Alpha-Synuclein
Alpha-Synuclein, a 140 amino acid cytosolic protein (Fig. below), implicated in the pathogenesis of Parkinson's Disease (PD), can exert its cellular function through interaction with membranes. This interaction has been studied mostly with oligomeric or aggregated "protofibrillar" forms of synuclein. In contrast to the view prevalent in the literature that the membrane-interactive form of alphaSynuclein is the oligomeric beta-stranded form that permeabilizes membranes, studies that we have carried out with J. - C. Rochet showed that monomeric synuclein in an alpha-helical conformation formed specific ion channels in planar bilayer membranes having a physiological lipid composition (Zakharov et al., Biochemistry, 2007). These channels, formed by insertion into the membrane bilayer, must result from the formation of a trans-membrane helical dimer or higher order oligomer. The synuclein trans-membrane channels could have a positive function in the metabolism of synaptic membranes through transport of biogenic amines.
Education
B. S., Physics, Massachusetts Institute of Technology
Ph. D., Biophysics, University of Chicago;
Postdoctoral, University of California/San Diego
Teaching:
- Bioenergetics: Energy Transduction in Biological Membranes: Course Outline
- Membrane Proteins (with D. Yernool) Course Outline
- Ethics: Life and Times in Academic Research Labs Course Outline
Professional Faculty Research
(Biochemistry; Biophysics) Membrane biochemistry, biophysics; structure-function of membrane proteins.