Membrane Protein Structure Overview
present and future
D.C. Rees
Caltech/HHMI
Membrane Proteins - Interests and Challengesparticipate in transport and transduction processes that mediate the flow of matter, energy and information across the membrane bilayer
are poorly characterized relative to water-soluble proteins due to experimental challenges of mimicking the membrane and water-bilayer interfaces
constitute an estimated ~20% of all proteins, yet only ~40 distinct structures are available
membrane protein structure websites:
http://blanco.biomol.uci.edu/Membrane_Proteins_xtal.html
http://www.mpibp-frankfurt.mpg.de/michel/public/memprotstruct.html
Liu & Rost Prot. Sci. 10, 1970 (2001)15-30% total, 10-20% > 2 TM
membrane protein abundances
Membrane protein structure: Status
bioenergetic systemsphotosynthetic reaction centers (bacterial, PSI, PSII)bacteriorhodopsin (BR, HR, SR)respiratory complexes (II, III, IV and V)FDH, NarGHIlight harvesting complexesADP/ATP carrier
transportersCa+2 ATPaseAcrBABC transporters (MsbA, BtuCD)MFS (LacY, GlpT)
channels/receptorsK+ (KcsA, MthK, KirBac1.1, KvAP)mechanosensitive (MscL, MscS)aquaporins (GlpT, AQP)ClCAchRrhodopsin
-barrel 8,10,12,14,16,18,22 -strands
Structural analysis of membrane proteins
natural sources or cloning & expression (recombinant)
solubilization (detergents)
purification
crystallization
structure determination
QuickTime™ and aSGI decompressor
are needed to see this picture.
10-30 homologs
membrane protein crystallization: approaches
detergents/additives - RClipidic phases - BRFab complexes - cytochrome c oxidaseproteolysis - KcsAsequence engineering - OmpXhomologues - MscL
BtuCD structure determination
3.2 Å resolution
Locher et al. Science 296, 1091 (2002)
BtuCD: molecular architecture of an ABC transporter
periplasm
cytoplasm
BtuC BtuC
BtuC BtuD
two-fold NCS
surface polarity of membrane proteins reflectsthe solvent environment
Nagle & Tristram-Nagle, COSB 10, 474 (2000)
surface hydrophobicity profile for membrane proteins
ARBB 31, 207 (2002)
endpoints of transmembrane helices
helix tilt and helix-helix crossing angle distributions
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observed probability
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ARBB 31, 207 (2002)
average tilt = 23˚ + 10˚
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Asp Ile
hydrophobicity
buriedsurface
water-soluble proteins
buried / surface
Hydrophobicities of buried/surface residuesmembrane proteins vs water-soluble proteins
TM region, 16 -helical mps
Eisenberg et al. hydrophobicity scaleFaraday Symp. Chem. Soc. 17, 109 (1982)
Science 245, 510 (1989)ARBB 31, 207 (2002)
Membrane protein - water-soluble protein comparison
membrane proteins and water soluble proteins have similarinterior apolaritiespacking densitiessurface areasstabilities (?)
and differ in surface polaritieshelix-helix packing distributionstertiary folds
what is the role of solvent in defining protein structures?
water membrane
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progress in membrane protein structure determinations parallels that of water-soluble proteins with a ~25 year offset
photosynthetic reaction center
myoglobin
http://www.mpibp-frankfurt.pg.de/michel/public/memprotstruct.html
B.W. Matthews Ann. Rev. Phys. Chem. 27, 493 (1976)
“Should we designate a certain percentage of targetsto membrane proteins and/or protein complexes?”
Membrane protein challenges:
overexpression of eukaryotic proteins
solubilization (detergents)
state-specific stabilizers
YES!!!
Acknowledgments (~t-20 years to present)
RC: J. Allen, T. Yeates, A. Chirino, A. Yeh,H. Axelrod, G. Feher
FRD: T. Iverson, C. Luna-Chavez, G. Cecchini
MscL: G. Chang, R. Spencer, A. Lee, M. Barclay, R. Bass, P. Strop, J. Choe, S. Steinbacher
MscS: R. Bass, P. Strop, M. Barclay, Y. Poon
BtuCD: K. Locher, A. Lee, L. Borths
general: R. Spencer, L. DeAntonio, D. Eisenberg