Edited by Karen G. Fleming Mechanosensitive channels play an important role in the adaptation of cells to hypo-osmotic shock. Among members of this channel family in Escherichia coli, the exact function and physiological role of the mechanosensitive channel homolog YbdG remain unclear. Characterization of YbdG's physiological role has been hampered by its lack of measurable transport activity. Using a nitrosoguanidine mutagenesis-aided screen in combination with next-generation sequencing, here we isolated a mutant with a point mutation in ybdG. This mutation (resulting in a I167T change) conferred sensitivity to high osmotic stress, and the mutant cells differed from WT cells in morphology during hyperosmotic stress at alkaline pH. Interestingly, unlike the cells containing the I167T variant, a null-ybdG mutant did not exhibit this sensitivity and phenotype. Although I167T was located near the putative ion-conducting pore in a transmembrane region of YbdG, no change in ion channel activities of YbdG-I167T was detected. Of note, introduction of the WT C-terminal cytosolic region of YbdG into the I167T variant complemented the osmo-sensitive phenotype. Co-precipitation of proteins interacting with the C-terminal YbdG region led to the isolation of HldD and FbaA, whose overexpression in cells containing the YbdG-I167T variant partially rescued the osmo-sensitive phenotype. This study indicates that YbdG functions as a component of a mechanosensing system that transmits signals triggered by external osmotic changes to intracellular factors. The cellular role of YbdG uncovered here goes beyond its predicted function as an ion or solute transport protein. cro ARTICLE

Over-expression and aberrant activation of tyrosine kinases occur frequently in human cancers. Various tyrosine kinase inhibitors (TKIs) are under clinical use, but acquisition of resistance to these drugs is a major problem. Here, we studied the interaction between two drug-resistant mutants of fibroblast growth factor receptor 1 (FGFR1), N546K and V561M, and four ATP-competitive inhibitors, ponatinib, dovitinib, PD173074 and BGJ-398. Among these protein-drug systems, the only marked reduction in affinity was that of PD173074 for the V561M mutant. We also examined the interaction of these FGFR1 variants to AMP-PNP, a nonhydrolyzable analogue of ATP, and showed that N546K showed increased affinity for the ATP analogue as compared with the wild type. These findings will help to clarify the mechanism of drug resistance in mutant tyrosine kinases.

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Tyrosine kinases are key enzymes that play critical roles in growth signaling, the abnormal activation of which is associated with various human cancers. Activation of tyrosine kinases is mediated by tyrosine phosphorylation in the activation‐loop, which transforms the catalytic domain to the active state conformation. Cancer mutations are supposed to transform the conformation of the catalytic domain into the active‐form independent of the phosphorylation state of the activation‐loop. Here, we report structural and biophysical analyses of cancer mutations of the tyrosine kinase domain of fibroblast growth factor receptor 1 (FGFR1). Based on the nuclear magnetic resonance analyses, phosphorylation of the activation‐loop exhibited cooperative structural transition in the activation‐loop, C‐helix and P‐loop regions, whereas cancer mutations induced structural transformation at either one or two of these regions.

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