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Plant Cell, Vol. 10, 91-104, Copyright © 1998, American Society of Plant Physiologists

Light- and Ca2+-Modulated Heterotrimeric GTPases in the Eyespot Apparatus of a Flagellate Green Alga

Michael Calenberga, Uwe Brohsonna, Marlies Zedlachera, and Georg Kreimera
a Botanisches Institut, Universität zu Köln, Lehrstuhl I, Gyrhofstrasse 15, D-50931 Cologne, Germany

Correspondence to: Georg Kreimer, gkreimer{at}novell.biolan.uni-koeln.de (E-mail), 49-221-470-5181 (fax).

Little is known about phototactic signal transduction in flagellate green algae; therefore, eyespot apparatuses, which are the light-sensitive "organelles" involved in photoorientation of these algae, were isolated and analyzed for the presence of heterotrimeric guanine nucleotide binding proteins (G proteins) and their coupling to the retinal-based photoreceptor. Specific high-affinity 35S-GTP-{gamma}-S binding and GTPase activity, with sensitivity toward antibodies raised against vertebrate/invertebrate G{alpha} subunits and fluoroaluminates, were detected. In one- and two-dimensional immunoblot analyses, an antiserum directed against Gi{alpha}-type subunits exhibited cross-reactivity at 42 kD, whereas a 43-kD protein cross-reacted with antisera directed against Gq{alpha} subunits. Green light below 1 µE m-2 sec-1 suppressed cholera toxin-dependent ADP ribosylation at these apparent molecular masses and modulated a significant proportion of the GTPase activity in a reversible manner. Antisera against Chlamydomonas rhodopsin and the G{alpha} subunits completely impaired light modulation. Both light sensitivity and dark recovery of the GTPase were affected by changes in free Ca2+. Dissociation of the putative G{alpha} subunits from the eyespot membranes was not observed when the membranes were illuminated. Our results emphasize the regulatory potential of G{alpha} subunits in rhodopsin-based signaling of flagellate green algae.




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