The molecular chaperone heat shock protein 90 (HSP90) has been used by cancer cells to facilitate the function of numerous oncoproteins, and it can be argued that cancer cells are 'addicted' to HSP90. However, although recent reports of the early clinical efficacy of HSP90 inhibitors are encouraging, the optimal use of HSP90-targeted therapeutics will depend on understanding the complexity of HSP90 regulation and the degree to which HSP90 participates in both neoplastic and normal cellular physiology.

Client protein activation by Hsp90 involves a plethora of cochaperones whose roles are poorly defined. A ubiquitous family of stress-regulated proteins have been identified (Aha1, activator of Hsp90 ATPase) that bind directly to Hsp90 and are required for the in vivo Hsp90-dependent activation of clients such as v-Src, implicating them as cochaperones of the Hsp90 system. In vitro, Aha1 and its shorter homolog, Hch1, stimulate the inherent ATPase activity of yeast and human Hsp90. The identification of these Hsp90 cochaperone activators adds to the complex roles of cochaperones in regulating the ATPase-coupled conformational changes of the Hsp90 chaperone cycle.

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Aquaporins and aquaglyceroporins form the membrane channels that mediate fluxes of water and small solute molecules into and out of cells. Eukaryotes often use mitogen-activated protein kinase (MAPK) cascades for the intracellular signaling of stress. This study reveals an aquaglyceroporin being destabilized by direct MAPK phosphorylation and also a stress resistance being acquired through this channel loss. Hog1 MAPK is transiently activated in yeast exposed to high, toxic levels of acetic acid. This Hog1 then phosphorylates the plasma membrane aquaglyceroporin, Fps1, a phosphorylation that results in Fps1 becoming ubiquitinated and endocytosed and then degraded in the vacuole. As Fps1 is the membrane channel that facilitates passive diffusional flux of undissociated acetic acid into the cell, this loss downregulates such influx in low-pH cultures, where acetic acid (pK a , 4.75) is substantially undissociated. Consistent with this downregulation of the acid entry generating resistance, sensitivity to acetic acid is seen with diverse mutational defects that abolish endocytic removal of Fps1 from the plasma membrane (loss of Hog1, loss of the soluble domains of Fps1, a T231A S537A double mutation of Fps1 that prevents its in vivo phosphorylation, or mutations generating a general loss of endocytosis of cell surface proteins [doa4⌬ and end3⌬]). Remarkably, targetting of Fps1 for degradation may be the major requirement for an active Hog1 in acetic acid resistance, since Hog1 is largely dispensable for such resistance when the cells lack Fps1. Evidence is presented that in unstressed cells, Hog1 exists in physical association with the N-terminal cytosolic domain of Fps1.Baker's yeast (Saccharomyces cerevisiae) is extensively used as a model for studying how cells adapt to and survive different forms of stress. Its responses to hyperosmotic stress have been the subject of extensive investigations (11,12,21,26). Important for an adaptation to hyperosmotic conditions is counteracting the water loss from the cell, which is achieved in yeast by accumulating a high intracellular pool of glycerol. This glycerol acts as a compatible solute, ensuring that the proteins in the intracellular environment remain hydrated and protected. Osmostress adaptation also involves the activation of the highosmolarity glycerol (HOG) mitogen-activated protein kinase (MAPK) signaling cascades, which generate an activation of a multifunctional Hog1 MAPK. This activated Hog1 then translocates to the nucleus, where, by the phosphorylation of at least three separate transcription factors (Sko1, Hot1, and Smp1), it can generate an altered regulation of Ͼ10% of the total yeast genome (21). Active Hog1 has recently been found to exert important actions, much more instant than its effects on transcription, at the plasma membrane, where it directly phosphorylates certain of the membrane ion transporters in osmostressed cells in order to rapidly readjust the transmembrane fluxes of Na ϩ and K ϩ (26). In this work, we show that activated Hog1 can also...