Hartzell L B, Hartzell H C, Quarmby L M
Department of Physiology, Emory University School of Medicine, Atlanta, Georgia 30322.
Exp Cell Res. 1993 Sep;208(1):148-53. doi: 10.1006/excr.1993.1232.
It has previously been shown that exposure of Chlamydomonas to low pH induces the cells to shed their flagella. In the present paper we report that a 30-s treatment with 20 mM acetic, carbonic, formic, or benzoic acid at pH 4.0 will induce flagellar excision. In contrast, 20 mM concentrations of the stronger aspartic, phosphoric, citric, and tartaric acids (pH 4) do not induce excision. Further, the excision efficacy of acetate is a function of the concentration of protonated acetate. Thus, excision correlates with the presence of a protonated, membrane-permeant species of acid. Relative to acetate, the more permeant benzoate induces excision at a much lower concentration of protonated acid. We conclude that a flux of acid into the cell is the signal for excision. Previous work has shown that detergent-permeabilized cells excise their flagella in response to calcium but not in response to low pH. This suggests that the acidification of intact cells triggers excision by stimulating an increase in intracellular calcium. We have previously reported that the source of this calcium might be IP3-sensitive. In our model for the mechanism of pH-induced flagellar excision, a flux of acid into the cell activates phospholipase C, leading to IP3 production, the activation of an IP3-gated calcium channel (located on either an intracellular or surface membrane), and an increase in cytosolic calcium, which is the trigger of flagellar excision.
先前的研究表明，将衣藻暴露于低pH环境会诱导细胞脱落鞭毛。在本文中，我们报告称，在pH 4.0条件下用20 mM乙酸、碳酸、甲酸或苯甲酸处理30秒会诱导鞭毛切除。相比之下，20 mM浓度的更强的天冬氨酸、磷酸、柠檬酸和酒石酸（pH 4）不会诱导切除。此外，乙酸盐的切除效力是质子化乙酸盐浓度的函数。因此，切除与质子化的、可透过膜的酸的存在相关。相对于乙酸盐，更易透过的苯甲酸盐在低得多的质子化酸浓度下诱导切除。我们得出结论，酸流入细胞是切除的信号。先前的工作表明，用去污剂通透处理的细胞会响应钙而切除鞭毛，但不响应低pH。这表明完整细胞的酸化通过刺激细胞内钙的增加来触发切除。我们先前曾报道，这种钙的来源可能对肌醇三磷酸（IP3）敏感。在我们关于pH诱导鞭毛切除机制的模型中，酸流入细胞会激活磷脂酶C，导致IP3产生，激活IP3门控钙通道（位于细胞内或表面膜上），并使胞质钙增加，这是鞭毛切除的触发因素。
