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Biophysical Journal 86:2342-2349 (2004)
© 2004 The Biophysical Society

Chlamydomonas Sensory Rhodopsins A and B: Cellular Content and Role in Photophobic Responses

Elena G. Govorunova, Kwang-Hwan Jung, Oleg A. Sineshchekov and John L. Spudich

Center for Membrane Biology, Department of Biochemistry and Molecular Biology and Department of Microbiology and Molecular Genetics, University of Texas Medical School, Houston, Texas

Correspondence: Address reprint requests to John L. Spudich, Center for Membrane Biology, University of Texas Medical School, MSB 6.130, 6431 Fannin St., Houston, TX 77030. Tel.: 713-500-5473; Fax: 713-500-0545; E-mail: John.L.Spudich{at}uth.tmc.edu.

Two retinylidene proteins, CSRA and CSRB, have recently been shown by photoelectrophysiological analysis of RNAi-transformants to mediate phototaxis signaling in Chlamydomonas reinhardtii. Here we report immunoblot detection of CSRA and CSRB apoproteins in C. reinhardtii cells enabling assessment of the cellular content of the receptors. We obtain 9 x 104 CSRA and 1.5 x 104 CSRB apoprotein molecules per cell in vegetative cells of the wild-type strain 495, a higher value than that for functional receptor cellular content estimated previously from photosensitivity measurements and retinal extraction yields. Exploiting our ability to control the CSRA/CSRB ratio by transformation with receptor gene-directed RNAi, we report analysis of the CSRA and CSRB roles in the photophobic response of the organism by action spectroscopy with automated cell tracking/motion analysis. The results show that CSRA and CSRB each mediate the photophobic swimming response, a second known retinal-dependent photomotility behavior in C. reinhardtii. Due to the different light saturation and spectral properties of the two receptors, CSRA is dominantly responsible for photophobic responses, which appear at high light intensity.




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