The directed cooperative assembly of proteorhodopsin into 2D and 3D polarized arrays

doi:10.1073/pnas.0702336104

The directed cooperative assembly of proteorhodopsin into 2D and 3D polarized arrays

*Department of Chemistry and Biochemistry, University of California, Santa Barbara, CA 93106; and
^† Genencor International, Inc., Palo Alto, CA 94304

Communicated by Jacob N. Israelachvili, University of California, Santa Barbara, CA, March 15, 2007 (received for review January 1, 2007)

Abstract

Proteorhodopsin is the membrane protein used by marine bacterioplankton as a light-driven proton pump. Here, we describe a rapid cooperative assembly process directed by universal electrostatic interactions that spontaneously organizes proteorhodopsin molecules into ordered arrays with well defined orientation and packing density. We demonstrate the charge density-matching mechanism that selectively controls the assembly process. The interactions among different components in the system are tuned by varying their charge densities to yield different organized transmembrane protein arrays: (i) a bacteriorhodopsin purple membrane-like structure where proteorhodopsin molecules are cooperatively arranged with charged lipids into a 2D hexagonal lattice; (ii) selected liquid-crystalline states in which crystalline lamellae made up of the coassembled proteorhodopsin and charged lipid molecules are coupled three-dimensionally with polarized proteorhodopsin orientation persisting through the macroscopic scale. Understanding this rapid electrostatically driven assembly process sheds light on organizing membrane proteins in general, which is a prerequisite for membrane protein structural and mechanistic studies as well as in vitro applications.

Footnotes

^§To whom correspondence should be addressed. E-mail: stucky{at}chem.ucsb.edu
Author contributions: H.L. and G.D.S. designed research; H.L. and C.N. performed research; G.W. contributed new reagents/analytic tools; H.L., G.W., C.N. and G.D.S. analyzed data; and H.L., G.W., and G.D.S. wrote the paper.
↵ ^‡Present address: Department of Chemistry and Life Science, U.S. Military Academy, West Point, NY 10996.
The authors declare no conflict of interest.
This article contains supporting information online at www.pnas.org/cgi/content/full/0702336104/DC1.
Abbreviations:

MP,

membrane protein;

PR,

proteorhodopsin;

bR,

bacteriorhodopsin;

CL–PR complexes,

cationic lipid-proteorhodopsin complexes;

XRD,

x-ray diffraction;

TEM,

transmission electron microscopy;

DDM,

n-dodecyl β-D-maltoside;

CMC,

critical micelle concentration.