Huntington disease and its related autosomal-dominant polyglutamine (pQ) neurodegenerative diseases are characterized by intraneuronal accumulation of protein aggregates. Studies on protein aggregates have revealed the importance of the ubiquitin-proteasome system as the front line of protein quality control (PQC) machinery against aberrant proteins. Recently, we have shown that the autophagy-lysosomal system is also involved in cytoplasmic aggregate degradation, but the nucleus lacked this activity. Consequently, the nucleus relies entirely on the ubiquitin-proteasome system for PQC. According to previous studies, nuclear aggregates possess a higher cellular toxicity than do their cytoplasmic counterparts, however degradation kinetics of nuclear aggregates have been poorly understood. Here we show that nuclear ubiquitin ligases San1p and UHRF-2 each enhance nuclear pQ aggregate degradation and rescued pQ-induced cytotoxicity in cultured cells and primary neurons. Moreover, UHRF-2 is associated with nuclear inclusion bodies in vitro and in vivo. Our data suggest that UHRF-2 is an essential molecule for nuclear pQ degradation as a component of nuclear PQC machinery in mammalian cells. Huntington disease (HD)2 and related polyglutamine (pQ) diseases are caused by the expansion of trinucleotide repeats encoding pQ within the mutant gene product (1). The pQ length dependence of disease onset and severity in pQ diseases correlates strongly with the tendency of expanded pQ proteins to aggregate in disease models (2, 3). Because of this pQ chain, the gene product assembles into oligomers, further aggregates to form microscopically visible inclusion bodies (IBs), and shows pathological extensity in diseased brains (4).Aggregated forms of pQ-expanded huntingtin (Htt) can disrupt cellular function in a variety of ways, including inactivation of transcription factors (5, 6) and impairment of the ubiquitin proteasome system (UPS) (7). The precise mechanism of UPS inhibition remains to be solved, but could be of particular interest, because it has been shown to occur in in vivo disease models (8). UPS impairment triggers aggresome formation (9), an active cellular mechanism to enrich cytoplasmic aggregates and autophagy-lysosomal components to the microtubule-organizing center by retrograde transport, which in turn enhances the efficiency and selectivity of autophagic degradation of cytoplasmic aggregates supporting the UPS as an alternative protein quality control (PQC) system (10, 11). However, autophagy is ineffective in clearing nuclear aggregates (12), and there is no known nuclear PQC mechanism other than the UPS. Therefore, the nucleus is a relatively protected environment for aggregates than the cytoplasm.The majority of the pQ proteins are functional in the nucleus, and the strong correlation between aggregate toxicity and nuclear translocation of pQ proteins has led to the hypothesis that the nucleus is the primary center for action of these proteotoxins (13). This was shown by redirecting nuclear ataxin-1 or andro...

Class III chitin synthases play important roles in tip growth and conidiation in many filamentous fungi. However, little is known about their functions in those processes. To address these issues, we characterized the deletion mutant of a class III chitin synthase-encoding gene of Aspergillus nidulans, chsB, and investigated ChsB localization in the hyphae and conidiophores. Multilayered cell walls and intrahyphal hyphae were observed in the hyphae of the chsB deletion mutant, and wavy septa were also occasionally observed. ChsB tagged with FLAG or enhanced green fluorescent protein (EGFP) localized mainly at the tips of germ tubes, hyphal tips, and forming septa during hyphal growth. EGFP-ChsB predominantly localized at polarized growth sites and between vesicles and metulae, between metulae and phialides, and between phalides and conidia in asexual development. These results strongly suggest that ChsB functions in the formation of normal cell walls of hyphae, as well as in conidiophore and conidia development in A. nidulans.Chitin, a polymer of ␤-1,4-linked N-acetylglucosmine, is one of the major structural components of the fungal cell wall. Its metabolism, including synthesis, degradation, assembly, and cross-linking to other cell wall components, is thought to be very important for many fungi (5,22,24,36,45). Fungal chitin synthases have been classified into seven groups, classes I to VII, depending on the structures of their conserved regions (6). The genes encoding the synthases belonging to classes III, V, VI, and VII are only found in fungi with high chitin contents in their cell walls. We have identified six chitin synthase genes from Aspergillus nidulans and designated them chsA, chsB, chsC, chsD, csmA, and csmB; these gene products belong to classes II, III, I, IV, V, and VI, respectively (9,13,30,31,44,52). The chsB deletion mutant grew very slowly and formed small colonies with highly branched hyphae, suggesting its important role in hyphal tip growth (3, 52). Repression of chsB expression in the deletion mutant of chsA, chsC, or chsD exaggerated the defects in the formation of aerial hyphae, the production of cell mass, or the growth under high-osmolarity conditions, respectively, compared to each single mutant. These results indicate that chsB functions at various stages of development (15, 16).The deletion of class III chitin synthase-encoding genes leads to severe defects in most of the filamentous fungi thus far investigated. However, their detailed functions are currently unknown. In Neurospora crassa, inactivation of the gene encoding Chs-1, a class III chitin synthase with 63% identity to A. nidulans ChsB, leads to slow growth, aberrant hyphal morphology, and a decrease in chitin synthase activity. The mutant of chs-1 became sensitive to Nikkomycin Z, a chitin synthase inhibitor (53). In Aspergillus fumigatus, two genes encoding class III chitin synthases, chsC and chsG, have been identified. Their gene products showed 66 and 89% identity, respectively, to A. nidulans ChsB. The chsG deletio...