E129,A131 , which causes a morphological abnormality, was expressed. These results provide a direct molecular basis for the action of Rho3 on exocytosis and the actin cytoskeleton.

We identified a novel Rho gene rho3+ and studied its interaction with diaphanous/formin for3+ in the fission yeast Schizosaccharomyces pombe. Both rho3 null cells and for3 null cells showed defects in organization of not only actin cytoskeleton but also cytoplasmic microtubules (MTs). rho3 for3double null cells had defects that were more severe than each single null cell: polarized growth was deficient in the double null cells. Function of For3 needed the highly conserved FH1 and FH2 domains, an N-terminal region containing a Rho-binding domain, and the C-terminal region. For3 bound to active forms of both Rho3 and Cdc42 but not to that of Rho1. For3 was localized as dots to the ends of interphase cells and to the mid-region in dividing cells. This localization was probably dependent on its interaction with Rho proteins. Overexpression of For3 produced huge swollen cells containing depolarized F-actin patches and thick cytoplasmic MT bundles. In addition, overexpression of a constitutively active Rho3Q71L induced a strong defect in cytokinesis. In conclusion, we propose that the Rho3-For3 signaling system functions in the polarized cell growth of fission yeast by controlling both actin cytoskeleton and MTs.

Constitutive nuclear factor (NF)-kB activation is thought to be involved in survival, invasion, and metastasis in various types of cancers. However, neither the subtypes of breast cancer cells with constitutive NF-kB activation nor the molecular mechanisms leading to its constitutive activation have been clearly defined. Here, we quantitatively analyzed basal NF-kB activity in 35 human breast cancer cell lines and found that most of the cell lines with high constitutive NF-kB activation were categorized in the estrogen receptor negative, progesterone receptor negative, ERBB2 negative basal-like subtype, which is the most malignant form of breast cancer. B reast cancer is a disease of the mammary epithelium, which is composed of two major types of differentiated cells: luminal epithelial cells and basal or myoepithelial cells.(1) Recent studies have identified self-renewing pluripotent stem cells in mammary epithelium and suggest a model in which these stem cells could differentiate into the luminal-or basal-restricted lineages. Molecular taxonomic analyses of breast cancers by gene expression profiling have identified five breast cancer subtypes: luminal A, luminal B, basal-like, ERBB2-positive, and normal breast-like.(2) This classification is closely associated with the differentiation model of mammary epithelium. Luminal-and basallike breast cancer subtypes express genes characteristic of the two distinct types of epithelial cells. These subtypes show different clinical courses and responses to therapeutic agents. The basallike subtype has been associated with aggressive behavior and poor prognosis and typically does not express estrogen receptor (ER), progesterone receptor (PR), or ERBB2 ("triple-negative" phenotype).(3) Therefore, patients with basal-like subtype are unlikely to benefit from currently available targeted therapeutic strategies, such as hormone therapy and Herceptin (Roche, Basel, Switzerland). It is thus crucial to identify effective molecular targets for this subtype of breast cancer.Nuclear factor (NF)-κB transcription factors are important regulators of the genes necessary for innate and adaptive immune responses and for the survival and proliferation of certain cell types. The NF-κB family is composed of five different proteins, including RelA, RelB, c-Rel, and the precursor and processed products of the NFKB1 (p105/p50) and NFKB2 (p100/p52) genes.These proteins homodimerize and/or heterodimerize to form active transcription factors. Two distinct NF-κB pathways have been proposed: the classical pathway, which activates the RelA-p50 complex, and the alternative pathway, which activates the RelBp52 complex.(4) In normal cells, activation of the classical and alternative pathways is tightly regulated by inhibitor of NF-κB (IκB) family proteins and a p100 protein, respectively. Both NF-κB pathways are aberrantly activated and involved in tumor development in various cancers, including breast cancer.(5,6) Previous studies have revealed that hormone-independent breast cancer cells exhibit cons...

Hsp90 has a diverse array of cellular roles including protein folding, stress response and signal transduction. Herein we report a novel function for Hsp90 in the ATP-dependent assembly of the 26S proteasome. Functional loss of Hsp90 using a temperature-sensitive mutant in yeast caused dissociation of the 26S proteasome. Conversely, these dissociated constituents reassembled in Hsp90-dependent fashion both in vivo and in vitro; the process required ATP-hydrolysis and was suppressed by the Hsp90 inhibitor geldanamycin. We also found genetic interactions between Hsp90 and several proteasomal Rpn (Regulatory particle nonATPase subunit) genes, emphasizing the importance of Hsp90 to the integrity of the 26S proteasome. Our results indicate that Hsp90 interacts with the 26S proteasome and plays a principal role in the assembly and maintenance of the 26S proteasome.