The polytopic plasma membrane protein Rim21 senses both the elevation of ambient pH and alterations in plasma membrane lipid asymmetry in the Rim101 pathway in budding yeast. Rim21 is known to undergo N-glycosylation, but the site and function of the glycosylation modification is not known. Using a systematic mutation analysis, we found that Rim21 is N-glycosylated at an unconventional motif located in the N-terminal extracellular region. The Rim21 mutant protein that failed to receive N-glycosylation showed prolonged protein lifetime compared to that of WT Rim21 protein. Although both the WT and mutant Rim21 localized to the plasma membrane, they exhibited different biochemical fractionation profiles. The mutant Rim21, but not WT Rim21, was mainly fractionated into the heavy membrane fraction. Further, compared to WT Rim21, mutant Rim21 was more easily solubilized with digitonin but was conversely more resistant to solubilization with Triton X-100. Despite these different biochemical properties from WT Rim21, mutant Rim21 protein could still activate the Rim101 pathway in response to external alkalization. Collectively, N-glycosylation of Rim21 is not indispensable for its activity as a sensor protein, but modulates the residence of Rim21 protein to some microdomains within the plasma membrane with distinct lipid conditions, thereby affecting its turnover.

Key words: plasma membrane, lipid asymmetry, N-linked glycosylation, microdomain, Saccharomyces cerevisiae

ATF6α is an endoplasmic reticulum (ER)-embedded transcription factor which is rapidly activated by ER stress, and a major regulator of ER chaperone levels in vertebrates. We previously suggested that ATF6α occurs as a monomer, dimer and oligomer in the unstressed ER of Chinese hamster ovary cells due to the presence of two evolutionarily conserved cysteine residues in its luminal region (C467 and C618), and showed that ATF6α is reduced upon ER stress, such that only reduced monomer ATF6α is translocated to the Golgi apparatus for activation by proteolysis. However, mutagenesis analysis (C467A and C618A) revealed that the C618A mutant behaves in an unexpected manner (monomer and oligomer) during non-reducing SDS-PAGE, for reasons which remained unclear. Here, we used human colorectal carcinoma-derived HCT116 cells deficient in ATF6α and its relevant ATF6β, and found that ATF6α dimer and oligomer are both dimers, which we designated C618-dimer and C467-dimer, respectively. We demonstrated that C467-dimer (previously considered an oligomer) behaved bigger than C618-dimer (previously considered a dimer) during non-reducing SDS-PAGE, based on their disulfide-bonded structures. Furthermore, ATF6α monomer physically associates with another ATF6α monomer in the absence of disulfide bonding, which renders two C467 residues in close proximity so that formation of C467-dimer is much easier than that of C618-dimer. In contrast, C618-dimer is more easily reduced upon ER stress. Thus, our analysis revealed that all forms of ATF6α, namely monomer, C618-dimer and C467-dimer, are activated by single reduction of a disulfide bond in response to ER stress, ensuring the rapidity of ATF6α activation.

Key words: disulfide-bonded structure, endoplasmic reticulum, membrane-bound transcription factor, non-reducing SDS-PAGE, unfolded protein response

Three types of transmembrane protein, IRE1α/IRE1β, PERK, and ATF6α/ATF6β, are expressed ubiquitously in vertebrates as transducers of the unfolded protein response (UPR), which maintains the homeostasis of the endoplasmic reticulum. IRE1 is highly conserved from yeast to mammals, and transmits a signal by a unique mechanism, namely splicing of mRNA encoding XBP1, the transcription factor downstream of IRE1 in metazoans. IRE1 contains a ribonuclease domain in its cytoplasmic region which initiates splicing reaction by direct cleavage of XBP1 mRNA at the two stem loop structures. As the UPR is considered to be involved in the development and progression of various diseases, as well as in the survival and growth of tumor cells, UPR inhibitors have been sought. To date, IRE1 inhibitors have been screened using cell-based reporter assays and fluorescent-based in vitro cleavage assays. Here, we used medaka fish to develop an in vivo assay for IRE1α inhibitors. IRE1α, IRE1β, ATF6α and ATF6β are ubiquitously expressed in medaka. We found that IRE1α/ATF6α-double knockout is lethal, similarly to IRE1α/IRE1β- and ATF6α/ATF6β-double knockout. Therefore, IRE1 inhibitors are expected to confer lethality to ATF6α-knockout medaka but not to wild-type medaka. One compound named K114 was obtained from 1,280 compounds using this phenotypic screening. K114 inhibited ER stress-induced splicing of XBP1 mRNA as well as reporter luciferase expression in HCT116 cells derived from human colorectal carcinoma, and inhibited ribonuclease activity of human IRE1α in vitro. Thus, this phenotypic assay can be used as a quick test for the efficacy of IRE1α inhibitors in vivo.

Key words: endoplasmic reticulum, inhibitor screening, mRNA splicing, phenotypic assay, unfolded protein response

Cellular durotaxis has been extensively studied in the field of mechanobiology. In principle, asymmetric mechanical field of a stiffness gradient generates motile polarity in a cell, which is a driving factor of durotaxis. However, the actual process by which the motile polarity in durotaxis develops is still unclear. In this study, to clarify the details of the kinetics of the development of durotactic polarity, we investigated the dynamics of both cell-shaping and the microscopic turnover of focal adhesions (FAs) for Venus-paxillin-expressing fibroblasts just crossing an elasticity boundary prepared on microelastically patterned gels. The Fourier mode analysis of cell-shaping based on a persistent random deformation model revealed that motile polarity at a cell-body scale was established within the first few hours after the leading edges of a moving cell passed through the boundary from the soft to the stiff regions. A fluorescence recovery after photobleaching (FRAP) analysis showed that the mobile fractions of paxillin at FAs in the anterior part of the cells exhibited an asymmetric increase within several tens of minutes after cells entered the stiff region. The results demonstrated that motile polarity in durotactic cells is established through the hierarchical step-wise development of different types of asymmetricity in the kinetics of FAs activity and cell-shaping with a several-hour time lag.

Key words: Microelasticity patterned gel, durotaxis, cell polarity, focal adhesions, paxillin

Keratinocytes uptake melanosomes from melanocytes and retain them in the perinuclear region, where they form melanin caps. Although these processes are crucial to protecting nuclear DNA against ultraviolet injury, the molecular basis of melanosome uptake and decomposition in keratinocytes is poorly understood. One of the major reasons for its being poorly understood is the lack of a specific marker protein that can be used to visualize or monitor melanosomes (or melanosome-containing compartments) that have been incorporated into keratinocytes. In this study, we performed a comprehensive localization screening for mammalian Rab family small GTPases (Rab1–45) and succeeded in identifying 11 Rabs that were enriched around melanosomes that had been incorporated into keratinocytes. We also established a new assay by using a recently developed melanosome probe (called M-INK) as a means of quantitatively assessing the degradation of proteins on incorporated melanosomes in control and each of a series of Rab-knockdown keratinocytes. The results showed that knockdown or CRISPR/Cas9-mediated knockout of Rab7B (also identified as Rab42) in keratinocytes caused strong inhibition of protein degradation on melanosomes. Our findings indicated that Rab7B/42 is recruited to melanosome-containing compartments and that it promotes protein degradation on melanosomes in keratinocytes.

Key words: degradation, keratinocytes, melanocytes, melanosome, Rab small GTPase

Centrosomes are highly conserved organelles that act as the major microtubule-organizing center (MTOC) in animal somatic cells. Through their MTOC activity, centrosomes play various roles throughout the cell cycle, such as supporting cell migration in interphase and spindle organization and positioning in mitosis. Various approaches for removing centrosomes from somatic cells have been developed and applied over the past few decades to understand the precise roles of centrosomes. Centrinone, a reversible and selective PLK4 (polo-like kinase 4) inhibitor, has recently emerged as an efficient approach to eliminate centrosomes. In this review, we describe the latest findings on centrosome function that have been revealed using various centrosome-eliminating approaches. In addition, we discuss our recent findings on the mechanism of centrosome-independent spindle bipolarization, discovered through the use of centrinone.

Key words: centrosome, centrinone, mitotic spindle, bipolarity, NuMA

It is often assumed that α-subunit phosphorylation of the eukaryotic translation initiation factor 2 (eIF2) complex is just a mechanism to control protein synthesis. However, eIF2α phosphorylation induced by multiple kinases can recognize various intracellular and extracellular stress conditions, and it is involved in various other cellular processes beyond protein synthesis. This review introduces the roles of eIF2α phosphorylation in translational regulation, the generation of reactive oxygen species, changes in mitochondria structure and shape, and mitochondrial retrograde signaling pathways in response to diverse stress conditions.

Key words: eIF2α phosphorylation, Translation, Unfolded Protein Response, Reactive Oxygen Species, Mitochondria

Endocytic cargos are transported to recycling endosomes (RE) but how these sorting platforms are generated is not well understood. Here we describe our biochemical and live imaging studies of the conserved MON2-DOPEY complex in RE formation. MON2 mainly co-localized with RE marker RAB4B in peripheral dots and perinuclear region. The peripheral RE approached, interacted with, and separated from sorting nexin 3 (SNX3)-positive early endosomes (EE). Membrane-bound DOPEY2 was recruited to RE dependent upon MON2 expression, and showed binding abilities to kinesin and dynein/dynactin motor proteins. MON2-knockout impaired segregation of RE from EE and led to a decreased tubular recycling endosomal network, whereas RE was accumulated at perinuclear regions in DOPEY2-knockout cells. MON2 depletion also impaired intracellular transferrin receptor recycling, as well as retrograde transport of Wntless during its passage through RE before delivery from EE to the Golgi. Together, these data suggest that the MON2 drives separation of RE from EE and is required for efficient transport of endocytic cargo molecules.

Key words: membrane trafficking, MON2, recycling endosomes, Wntless