Yuri Nishino

Chloroplast-actin (cp-actin) filaments are crucial for light-induced chloroplast movement, and appear in the front region of moving chloroplasts when visualized using GFP-mouse Talin. They are short and thick, exist between a chloroplast and the plasma membrane, and move actively and rapidly compared to cytoplasmic long actin filaments that run through a cell. The average period during which a cp-actin filament was observed at the same position was less than 0.5 s. The average lengths of the cp-actin filaments calculated from those at the front region of the moving chloroplast and those around the chloroplast periphery after stopping the movement were almost the same, approximately 0.8 µm. Each cp-actin filament is shown as a dotted line consisting of 4-5 dots. The vector sum of cp-actin filaments in a moving chloroplast is parallel to the moving direction of the chloroplast, suggesting that the direction of chloroplast movement is regulated by the vector sum of cp-actin filaments. However, once the chloroplasts stopped moving, the vector sum of the cp-actin filaments around the chloroplast periphery was close to zero, indicating that the direction of movement was undecided. To determine the precise structure of cp-actin filaments under electron microscopy, Arabidopsis leaves and fern Adiantum capillus-veneris gametophytes were frozen using a high-pressure freezer, and observed under electron microscopy. However, no bundled microfilaments were found, suggesting that the cp-actin filaments were unstable even under high-pressure freezing.

Nicotinic acetylcholine receptors (nAChRs) are ligand-gated ion channels that play an important role in signal transduction at the neuromuscular junction (NMJ). Movement of the nAChR extracellular domain following agonist binding induces conformational changes in the extracellular domain, which in turn affects the transmembrane domain and opens the ion channel. It is known that the surrounding environment, such as the presence of specific lipids and proteins, affects nAChR function. Diffracted X-ray tracking (DXT) facilitates measurement of the intermolecular motions of receptors on the cell membranes of living cells, including all the components involved in receptor function. In this study, the intramolecular motion of the extracellular domain of native nAChR proteins in living myotube cells was analyzed using DXT for the first time. We revealed that the motion of the extracellular domain in the presence of an agonist (e.g., carbamylcholine, CCh) was restricted by an antagonist (i.e., alpha-bungarotoxin, BGT).