Our research goal is to understand an optimized integrative behavior of an individual biological cell. We use, as a model, the phototaxis system of Chlamydomonas, a single celled organism which swims, rotates along a helical path, and steers with the beating of a pair of cilia.

J. Vidyadharan and K. Foster

In addition to our ability to record realtime motion of its two cilia independently and the convenience of light stimulation, the whole genome has been sequenced. We began our studies by identifying the eye of Chlamydomonas as a quarter-wave stack structure that uses constructive interference to optimize the capture of light and control the eye's field of view.

We then studied light receptor activation in which we identified the photoreceptor to be a rhodopsin. We found there is an activation step prior to isomerization which is necessary and sufficient for photoactivation. This step involves charge separation along the chromophore (the molecule that absorbs the light) and coupling to the adjacent polarizable amino acid residues.

We have shown how intracellular processing of light signals alters transmembrane electric currents that results in transmembrane electric field changes in the cilia that in turn differentially control the responses of the two cilia steering the cell.

The intracellular processing determines the ciliary behavior such as the direction of phototaxis and how the tracking of a light source is optimized. Further we have studied the effect on ciliary beating of energy (ATP) supplied from the cell body including its feedback control and energy synthesized in the cilium.

In addition to this work with Chlamydomonas we have done comparative work with rhodopsin activation and photo-movement in other organisms from an evolutionary point of view.

Presently we are addressing with the Chlamydomonas system, 1) how biological sensors -- in particular light sensors and their chemical sensing cousins, G-protein receptors--work, and how they have evolved over the last few billion years to be the number one human receptor machine; 2) how cilia perform self organizing beating; 3) how cilia convert what was an evolutionary helical beat to the observed planar beat; and 4) how multiple sensor inputs, both internal and in response to many environmental stimuli, are processed by a molecular network to give robust integrated cell responses such as steering toward or away from a light source with apparently too few intermediate messengers.