Function and processing of Chlamydomonas phototropin
Andre Greiner1, Yinghong Lu2, and Peter Hegemann1
1Institute for Experimental Biophysics, Humboldt-University, 10115 Berlin, Germany 2Max-Planck-Institute for Plant Physiology, Am Mühlenberg 1, 14476 Golm, Germany
Chlamydomonas reinhardtii Phototropin (Phot) is a flavin-based sensory photoreceptor. We show that Phot (80 kDapp) is within 2 minutes of blue light irradiation proteolytically degraded into a defined Phot-B species of 60 kDapp. This cleavage process is completely prevented by inactivation of both photoreceptor LOV-domains. Its kinase domain activation loop (A-loop) region, a highly charged Lys, Ser-rich and evolutionary variable region within Phot-kinases is identified as the cleavage site for this autocatalytic process. We recently reported the successful deletion of C.r. phototropin by ssDNA homologous recombination. The resultant strain exhibits reduced chlorophyll content, which could be complemented by over-expression of full-length phototropin or the C-terminal kinase part. On the contrary, over-expression of the LOV-domains alone caused a further decrease of pigmentation. Phototropin LOV-domains were presumed to be exclusively responsible for photosensory acti vation and deactivation of the kinase part. Here we show that besides this action, the LOV-domains themselves play a decisive physiological role. We propose that the proteolytic cleavage within the kinase domain is a mechanism to eliminate enzyme activity while the dark function of the LOV-domains is persisting.
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