One of the major topics in biophysics and physicobiology is to understand and utilize biological functions using various techniques. Rhodopsin is a photoreactive membrane-embedded protein that uses a retinal pigment (vitamin-A aldehyde) as a chromophore. By taking advantages of its photoreactivity, structure-function relationship of rhodopsins has been widely investigated with a variety of methods. Of note, recent rhodopsin research provided unexpected information regarding (i) wide distribution in various organisms, (ii) rich functional diversity and (iii) great potential for optogenetics. Here we review these three topics with future directions.

Study on transient folding intermediates is one of challenging issues in protein folding. In addition to the use of fluorescence and circular dichroism combined with rapid mixing apparatus for kinetic experiments, quench-flow experiments followed by monitoring amide-proton exchange and chemical modification were used for atomic level analyses of apomyoglobin. The topics for this review include 1) possible order of the folding of helices, 2) non-native and native-like structures in the folding intermediate, 3) mutations that stabilized or destabilized intermediates, and 4) mechanism of the switch between intermediate and native structures regulated by H24 and H119 interaction.

Fusion of myoblast cells and subsequent formation of multinucleated cells (called myotubes) are fundamental steps for generation of myofibers. In this review we focus on a role of phospholipid flippase complex ATP11A/CDC50A, a phospholipid translocase that transports phospholipids from the outer to the inner leaflets, during myotube formation. We show that translocation of phosphoatidylserine (PS) via phospholipid flippase is essential for positive regulation of PIEZO1, a mechanosensitive ion channel that is activated by membrane tension, thereby promoting RhoA/ROCK-mediated actomyosin formation that is critical for determination of myotube morphology.

We recently established the intravital imaging system for visualizing the bone tissues in living mice using two-photon excitation microscopy. By means of this system, we succeeded in visualization of direct cell-cell contact between osteoclasts and osteoblasts. In addition, we revealed that this intercellular interaction could regulate the bone remodeling. Our intravital bone imaging system would be useful for the analysis of pathology of various bone diseases and the evaluation of the effect of new drug.

Motility is a major characteristic of living things. We categorized all motilities reported so far based on the structure of the force-generating unit, and revealed that they can be classified into 18 types. Furthermore, we have proposed the origin and evolution of these 18 motilities on a phylogenetic tree constructed from new genomic information.

Propagation of amyloid is achieved by the combination of amyloid formation and disaggregation. Although both processes in the cell are regulated by molecular chaperones such as Hsp104 and Hsp70, the underlying molecular mechanism has remained elusive. Here we review a recent progress of metabolic regulation of amyloid fibrils, and discuss about the effects of dynamic conformational fluctuation of monomeric proteins on structural polymorphism of amyloid. The detailed analysis of the processes of amyloid formation and disaggregation by biophysical methods will provide important mechanistic insights into the cellular process responsible for amyloid propagation.