The nature of rhodopsin-triggered photocurrents in Chlamydomonas. I. Kinetics and influence of divalent ions.
In the green alga Chlamydomonas chlamyrhodopsin fulfills its role as a light sensor by absorbing light and activating photoreceptor channels within the eyespot area. At intense light stimuli, the photoreceptor (P) current triggers a fast and a slow flagellar current that finally leads to backward swimming (stop response). Here we report about probing the photoreceptor current directly at the eyespot. This allows the detection of the whole P current with a size of above 50 pA. The P current appears with a delay of less than 50 microseconds, suggesting that rhodopsin and the P channel are closely coupled or form one ion channel complex. The Ca2+ dependence of the P current has been demonstrated with the established suction technique in a capacitive mode. The P current shows the maximum amplitude at only 300 nM Ca2+, and it gradually declines at higher Ca2+. In addition to Ca2+, the photoreceptor and the fast flagellar current can be carried by Sr2+ and Ba2+. Mg2+ is conducted less efficiently and at high concentrations blocks the photoreceptor channel. A motion analysis of the cells shows that only Ca2+ and Sr2+ can induce physiological stop responses, whereas the large Ba2+ currents cause abnormal long-lasting cell spiraling.
Holland
E M
EM
Institut für Biochemie I, Regensburg, Germany.
Braun
F J
FJ
Nonnengässer
C
C
Harz
H
H
Hegemann
P
P
D000818:Animals; D055592:Biophysical Phenomena; D001703:Biophysics; D002118:Calcium / Q000378:metabolism; D002413:Cations, Divalent / Q000378:metabolism; D002465:Cell Movement / Q000528:radiation effects; D002696:Chlamydomonas / Q000737:chemistry / Q000378:metabolism / Q000528:radiation effects*; D004563:Electrochemistry; D007473:Ion Channels / Q000378:metabolism / Q000528:radiation effects; D007700:Kinetics; D008954:Models, Biological; D018462:Photobiology; D010777:Photochemistry; D017956:Photoreceptor Cells, Invertebrate / Q000737:chemistry / Q000528:radiation effects; D012243:Rhodopsin / Q000737:chemistry / Q000378:metabolism / Q000528:radiation effects*
D002413:Cations, Divalent; D007473:Ion Channels; D012243:Rhodopsin; D002118:Calcium