Cyclic Nucleotide-Sensitive Ion Channels In Olfactory Receptor Cell Membrane.

有人提出，腺苷或鸟苷环化酶级联参与了嗅觉受体分子的气味激活与电导变化之间的耦合，从而产生嗅觉受体电位。环核苷酸被认为作为细胞内的信使作用于电导。虽然腺苷酸环化酶对气味的激活已经被广泛研究，但级联中蛋白质的分子性质以及这些蛋白质作为一个整体如何调节电导的机制还没有完全了解。我们重点研究了环核苷酸激活离子通道的电学性质和分子性质，发现该通道具有以下特征：(1)通道既可被环AMP激活，又可被环GMP激活。这表明通道分子的磷酸化不是激活所必需的。有一种可能性是嗅腺苷环化酶不是单一种类的酶，还是…更多的是没有底物特异性来产生不同的环核苷酸。(2)通道具有较低的阳离子选择性。许多一价和二价阳离子可以通过通道。因此，这些通道可以归类为一种“阳离子通道”。(3)通道分子至少与环核苷酸有两个协同结合部位，它们对环核苷酸的亲和力在3~5微米之间。(4)通道的单位电导为24ps。航道开放时间的分布用两个指数之和表示。在1微米处，时间常数分别为10.1毫秒和0.9毫秒。通道可能以一种电压依赖的方式被二价阳离子阻断。这增加了二价阳离子在适应过程中发挥作用的可能性，无论它们是直接阻断通道，还是激活或抑制磷酸二酯酶DR腺苷环化酶的活性。(5)通道除分布于嗅觉纤毛外，还分布于树突和胞体膜上。(6)候选通道分子为分子质量为35-39 kDa的蛋白质。较少

It has been proposed that an adenylate or guanylate cyclase cascade is involved in coupling odorant-activation of olfactory receptor molecule with conductance changes, giving rise to olfactory receptor potentials. Cyclic nucleotides have been thought to act on the conductance as intracellular messengers. Although the odorant-activation of adenylate cyclase has been examined extensively, molecular nature of the proteins in the cascade and the mechanism how these as a whole regulate the conductance have not been fully understood. We have focused on the studies on electrical properties and molecular nature of the cyclic nucleotide-activated ion channels, and have found that the channels have the following characteristics: (1)The channels are activated directly by both cyclic AMP and cyclic GMP. This indicates that the phosphorlylation of channel molecule is not required in its activation. There is a possibility whether olfactory adenylate cyclase is not a single species of enzyme or does … More not have a substrate specificity for production of different cyclic nucleotides. (2)The channels have a low cationic selectivity. Many monovalent and divalent cations can pass through the channels. Thus, the channels may be categorized into a species of the "cation channels". (3)The channel molecule has at least two cooperative binding sites for the cyclic nucleotides and their affinity (K_<1/2>) to the cyclic nucleotides ranges between 3 and 5 uM. (4)The channels have a unit conductance of 24 pS. The distribution of channel open times is expressed as a sum of two exponentials. The time constants are 10.1 msec and 0.9 msec at 1 uM cAMP. The channels are blocked by divalent cations likely in a voltage-dependent manner. This raises the possibility that divalent cations play roles in the adaptation process through whether their direct blockage of the channels or their activation or inhibition of phosphodiesterase dr adenylate cyclase activity. (5)The channels are distributed in not only olfactory cilia but also dendrite and soma membrane. (6)The candidate channel molecule is a protein with a molecular mass of 35-39 kDa. Less