Heat shock transcription factors (HSFs) maintain protein homeostasis through regulating expression of heat shock proteins, especially in stressed conditions. In addition, HSFs are involved in cellular differentiation and development by regulating development-related genes, as well as heat shock genes. Here, we showed chronic sinusitis and mild hydrocephalus in postnatal HSF1-null mice, which are associated with impaired mucociliary clearance and cerebrospinal flow, respectively. Analysis of ciliary beating revealed that the amplitude of the beating was significantly reduced, and ciliary beat frequencies were lower in the respiratory epithelium, ependymal cells, oviduct, and trachea of HSF1-null mice than those of wild-type mice. Cilia possess a common axonema structure composed of microtubules of ␣-and -tubulin. We found a marked reduction in ␣-and ciliary  iv -tubulin in the HSF1-null cilia, which is developmentally associated with reduced Hsp90 expression in HSF1-null mice. Treatment of the respiratory epithelium with geldanamycin resulted in rapid reduction of ciliary beating in a dose-dependent manner. Furthermore, Hsp90 was physically associated with ciliary  iv -tubulin, and Hsp90 stabilizes tubulin polymerization in vitro. These results indicate that HSF1 is required to maintain ciliary beating in postnatal mice, probably by regulating constitutive expression of Hsp90 that is important for tubulin polymerization.Heat shock response is characterized by induction of a set of heat shock proteins (Hsps) 2 and is a fundamental adoptive response in all organisms from bacteria to humans. This response is regulated mostly at the level of transcription by heat shock transcription factors that bind to the heat shock element in eukaryotes (1, 2). Among four HSF family members (HSF1-4), HSF1 plays a key role in heat shock response in mammals, whereas HSF3 does so in avians (3, 4). This HSF-mediated induction of Hsps is required for acquisition of thermotolerance (5-7) and protection of cells from various pathophysiological conditions (8 -11). Inversely, HSF1 also regulates proapoptotic genes to decide on cell death or life in response to stress (12)(13)(14). In addition to the role in heat shock response, HSFs play critical functions in developmental processes such as gamatogenesis and neurogenesis (15)(16)(17)(18)(19)(20), in maintenance of the sensory organs (21-24), and in immune response (25,26). Although the precise mechanisms of how HSFs act in these physiological processes are still unclear, genetic evidence shows that HSFs regulate constitutive gene expression in unstressed cells and tissues (27, 28). Furthermore, it was revealed in sensory and immune cells that HSFs not only maintain protein homeostasis by regulating constitutive expression of Hsps, but are also involved in cell growth and differentiation by regulating expression of cytokines such as interleukin-6, fibroblast growth factors, and LIF (22,24,25). However, we do not know any client protein stabilized by Hsps that is under the control of HSFs...