Powering Molecular Shuttles through an Artificial Light Harvesting System
H. Hessa, K. Jardineb, J. Clemmensa, T. A. Mooreb, A. L. Mooreb, A. Primakb, J. Howardc, D. Gustb, Viola Vogel*, a
aDepartment of Bioengineering and Center for Nanotechnology, University of Washington,
Seattle, WA 98195 USA bDepartment of Chemistry and Biochemistry, Arizona State University cDepartment of Physiology and Biophysics and Center for Nanotechnology, University of Washington
Complexity is one of the defining aspects of life. In a cell, supramolecular assemblies and organelles perform tightly regulated and integrated tasks. Many of the individual building blocks are now understood structurally and functionally, but a button-up co-assembly of such interacting supramolecular systems poses major challenges. Here we present a proof-of-principle experiment demonstrating how to power motor proteins by light exposure, thus coupling the following key functions. An artificial light harvesting system is employed to power proton transport across a liposomal bilayer with light energy. The resulting transmembrane protein gradient then drives the synthesis of adenosine triphosphate (ATP) by the membrane-anchored ATP synthase enzyme (1,2). This "power plant" which converts light into chemical energy is co-assembled with a "molecular shuttle" system where microtubules are moved along linear tracks by the motor protein kinesin (3,4). In this integrated system we can therefore nonintrusively control the motion of the microtubules through light exposure.
The experimental setup consists of a flow cell mounted on an epi-fluorescence optical microscope and illuminated by a laser diode. The surface of the flow cell was patterned with parallel grooves spaced between 30 nm and 1 um apart by shear-deposition of a Teflon film (5). The motor protein kinesin (6) adsorbed preferentially along the grooves providing "tracks" for the motion of the microtubules. The microtubules were fluorescently labeled and bound to the motor proteins in the absence of ATP. The ATP-generating vesicles floated freely in the buffer solution. Illumination of the sample with light was followed by motion of the microtubules. The motion was mainly directed along the direction of shear of the underlying Teflon film. This experiment thus demonstrates how to co-assemble supramolecular systems that cooperate functionally all the way from light harvesting to charge separation across a lipid membrane, ATP-synthesis, and ATP-hydrolysis by kinesin, which finally results in directed motion of microtubules on uniaxially aligned kinesin tracks.
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