Matsuda A, Yoshimura K, Sineshchekov O A, Hirono M, Kamiya R
Department of Biological Sciences, Graduate School of Science, University of Tokyo, Japan.
Cell Motil Cytoskeleton. 1998;41(4):353-62. doi: 10.1002/(SICI)1097-0169(1998)41:4<353::AID-CM7>3.0.CO;2-0.
The unicellular green alga Chlamydomonas displays two distinct kinds of behavioral response to light: phototaxis, in which cells swim toward or away from the light source under constant illumination; and photophobic responses (also called stop responses or photoshock responses), in which cells transiently convert their flagellar waveform and swim backward upon sudden increase in light intensity. It has been suggested that the two responses partly share a common signal transduction pathway, but exactly how the different responses are produced has not been established. In this study, to help understand the molecular and cellular mechanisms that bring about the photophobic response, we isolated novel mutants (ppr1, ppr2, ppr3, and ppr4) that do not show the photophobic response. Importantly, these mutants retain the ability to display phototaxis, with almost the same sensitivities as in the wild type cell. Demembranated and reactivated flagellar axonemes of the ppr mutants were found to convert the bending patterns depending on the Ca2+ concentration, indicating that the axonemal mechanism for waveform conversion required for the photophobic response was unaffected by the mutations. In addition, measurements of electric currents in cell suspensions showed that these mutants generate normal photoreceptor currents (PRC) upon photostimulation, suggesting that they retain the normal activity of photoreception and the ionic channels that produce PRCs. However, the all-or-none flagellar current (FC), a Ca2+ current generated by PRC-induced depolarization of flagellar membrane, was absent or seriously impaired in the mutants. These findings clearly indicate that the all-or-none FC is necessary for the photophobic response but not for phototaxis. The isolation of the four genetically independent ppr mutants suggests that the generation of the FC is based on multiple components that are not used in the mechanism for phototaxis, and implies that the Chlamydomonas flagellar membrane possesses a voltage-dependent Ca2+-channel specifically used for generation of photophobic responses.
趋光性，即在持续光照下细胞朝着或远离光源游动；以及避光反应（也称为停止反应或光冲击反应），即细胞在光强度突然增加时短暂改变其鞭毛波形并向后游动。有人提出这两种反应部分共享一条共同的信号转导途径，但不同反应究竟是如何产生的尚未明确。在本研究中，为了帮助理解引发避光反应的分子和细胞机制，我们分离出了不表现避光反应的新型突变体（ppr1、ppr2、ppr3和ppr4）。重要的是，这些突变体保留了表现趋光性的能力，其敏感性与野生型细胞几乎相同。发现ppr突变体的去膜并重新激活的鞭毛轴丝会根据Ca2+浓度改变弯曲模式，这表明避光反应所需的波形转换的轴丝机制不受这些突变的影响。此外，细胞悬液中的电流测量表明，这些突变体在光刺激时会产生正常的光感受器电流（PRC），这表明它们保留了正常的光感受活性以及产生PRC的离子通道。然而，突变体中不存在或严重受损的全或无鞭毛电流（FC）是由PRC诱导的鞭毛膜去极化产生的Ca2+电流。这些发现清楚地表明，全或无的FC对于避光反应是必需的，但对于趋光性则不是。这四个遗传上独立的ppr突变体的分离表明，FC的产生基于趋光性机制中未使用的多个成分，这意味着衣藻鞭毛膜拥有一个专门用于产生避光反应的电压依赖性Ca2+通道。
