Headlines
William Cramer named Fellow of national society. full
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Protein-hungry cells 'go fishing,' report Purdue biologists. full
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Purdue biologists' spotlight solves mysteries of photosynthesis, metabolism. full
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Purdue biologists crystallize technique to expand
protein research. full
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Research Interests
Structure-Function of the
Cytochrome b6f Complex
Although there are approximately 45,000 independent structures of soluble
proteins, and somewhat more than 150 such structures of integral membrane
proteins (IMP) in the Protein Data Bank, there are only about 2 dozen structures
of hetero-oligomeric oligomeric structures 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 electron transfer in energy transducing membranes. A
high resolution (3.0 Å) structure has been obtained of one of the three
major hetero-oligomeric membrane protein complexes, the cytochrome b6f complex,
the electron and proton transferring complex that connects the two reaction
centers in photosynthetic electron transport. The dimeric 220 kDa complex
contains 8 polypeptide subunits (Science, 302, 1009-, 2003; Ann. Rev. Biochem.,
75, 769-, 2006) 13 trans-membrane helices, and 7 prosthetic groups (4 hemes,
1 FeS cluster, 1 Chl a, 1 beta-carotene)
per monomer. Several structure and function aspects of this complex are novel,
including the single chlorophyll a, beta-carotene, and unique covalently
bound heme cn, which is 5-coordinate, with two waters as the ligand,
but unique because there is no amino acid side chain that serves 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 (Yamashita et al., J. Mol. Biol., 370:59-72, 2007). The
details of the structure are of interest not only for the insight they provide
to electron transport and energy transduction, but also for their description
of the labyrinthine transfer pathways of the hydrophobic plastoquinone and –quinol
across the b6f complex
Protein Import:The Cytotoxic Colicins
Hypothesesfor the pathway and mechanisms of import into E. coli of
the cytotoxic E colicins are partly based on structures of a complex of
the colicin with its outer membrane vitamin B12 receptor (BtuB), which
the colicins parasitize for import. A 2.75 Å structure of the complex
of the receptor-binding domain of the endoribonucleolytic colicin E3 showed
the elongate 100 Å long colicin domain to be bound in an oblique
mode, in which it appears to be ‘fishing,’ for a second outer
membrane receptor-translocator (Nature Structural Biology , 10,
948-954, 2003). A similar structure has been obtained with the receptor
binding domain of colicin E2 (Sharma et al., JBC, 2007). Circular dichroism
and electrophysiological studies imply that the colicin is first bound
tightly to the extracellular surface of BtuB, is unfolded upon binding,
and then uses another very abundant outer membrane protein, OmpF, for translocation
(Biochemistry, 45:10199-10207, 2006, . Biol. Chem, J. Biol. Chem. 2007. 282: 23163-23170). It is proposed that colicin import
across the outer membrane involves formation of a ‘translocon’ between
BtuB and OmpF. A 1.6 Å structure has recently been obtained
for OmpF (Yamashita et al., in preparation, 2007). A structure of the BtuB
vitamin B12 receptor has recently been obtained, in collaboration with
M. Caffrey, by crystallization in the lipid cubic phase (Cherezov et al.,
2006). At 1.95 Å resolution, it is the highest resolution structure
of a membrane protein not related to bacteriorhodopsin to be obtained by
the in meso approach to crystallization.
