SummaryThe actin cytoskeleton of higher plants plays an essential role in plant morphogenesis and in maintaining various cellular activities. In this study we established a tobacco BY-2 cell line, stably transformed with a GFPfimbrin actin-binding domain (ABD) 2 construct, that allows visualization of actin microfilaments (MFs) in living cells. Using this cell line, designated BY-GF11, we performed time-sequential observations of MF dynamics during cell-cycle progression. Detailed analyses revealed the appearance of a broad MF band in the late G 2 phase that separated to form a structure corresponding to the so-called actin-depleted zone (ADZ) in mitosis. In BY-GF11, the MF structure at the cell cortex in mitosis appeared to form two bands rather than the ADZ. Measurements of fluorescent intensities of the cell cortex indicated an MF distribution that resembled two peaks, and we therefore named the structure MF 'twin peaks' (MFTP). The cell plate formed exactly within the valley between the MFTP at cytokinesis, and this cell-plate guidance was distorted by disruption of the MFTP by an inhibitor of actin polymerization. These results suggest that the MFTP originates from the broad MF band in the G 2 phase and functions as a marker of cytokinesis.

A key question in developmental biology is how cellular patterns are created and maintained. During the formation of the Arabidopsis root, the endodermis, middle cortex (MC), and cortex are produced by periclinal cell divisions that occur at different positions and at different times in root development. The endodermis and cortex arise continuously from the periclinal divisions of cells that surround the quiescent center (QC) at the tip of the root. The MC arises between days 7 and 14 from periclinal divisions of the endodermis. The divisions that produce the middle cortex begin in the basal region of the root meristem away from the QC and then spread apically and circumferentially around the root. Although the transcription factor SHORT-ROOT (SHR) is required for both of these divisions, the mechanism that determines where and when SHR acts to promote cell division along the longitudinal axis of the root is unknown; SHR is present along the entire length of the root tip, but only promotes periclinal divisions at specific sites. Here we show that the abundance of the SHR protein changes dynamically as the root develops, and that the pattern of cell division within the endodermis is sensitive to the dose of this protein: high levels of SHR prevent the formation of the MC, whereas intermediate levels of SHR promote MC formation. These results provide a mechanism for the longitudinal patterning of the endodermis, and represent the first example in plants of a mobile transcription factor whose function (activator or repressor) depends upon concentration.pon germination, the Arabidopsis root is composed of concentric layers of epidermis, cortex, and endodermis that surround the stele (the vascular tissues and pericycle; Fig. S1) (1). The endodermis and cortex are clonally related cell types that collectively comprise the ground tissue (2, 3). The formation of separate endodermal and cortical cell layers is mediated by the activity of two related GRAS family transcription factors, SHORT-ROOT and SCARECROW (SCR) (4-6). SHR is expressed in the stele cells of the Arabidopsis root and moves into the neighboring cells, which include the quiescent center (QC) cells, the cortical endodermal initials (CEIs), the cortical endodermal daughters (CEDs), and the endodermis. In all of these cells, SHR up-regulates the expression of the SCR transcription factor (4-7). In the CED both SHR and SCR activate the expression of a D-type cyclin, CYCD6;1, which promotes the periclinal divisions that maintain the separate endodermis and cortex cell layers (8). Although SHR and SCR are present in all cells that immediately surround the stele, during the first week of root growth, CYCD6;1 expression is largely confined to the CEI and the CED cells. Later in root development, CYCD6;1 is upregulated in the endodermis where it promotes the periclinal cell divisions that generate middle cortex (MC) "M" in Fig. S1). MC formation is significantly reduced in cycd6;1 mutants and is lost entirely in shr-2 (nulls) (8, 9). In contrast, SCR, which promote...

The nonobese diabetic (NOD) mouse is an animal model of human type I diabetes with a strong genetic component that maps to the major histocompatibility complex (MHC) of the genome. We have identified in NOD lymphocytes a specific proteasome defect that results from the lack of the LMP2 subunit. The pronounced proteasome defect results in defective production and activation of the transcription factor NF-B, which plays an important role in immune and inflammatory responses as well as in preventing apoptosis induced by tumor necrosis factor alpha. The defect in proteasome function in NOD mouse splenocytes was evident from impaired NF-B subunit p50 and p52 generation by proteolytic processing and impaired degradation of the NF-Binhibitory protein IB␣. An obligatory role of MHC-linked proteasome subunits in transcription factor processing and activation has been established in a spontaneous-disease model and mutant cells similarly lacking the MHC-encoded subunit. These data suggest that NOD proteasome dysfunction is due to a tissueand developmental-stage-specific defect in expression of the MHC-linked Lmp2 gene, resulting in altered transcription factor NF-B activity, and that this defect contributes to pathogenesis in NOD mice. These observations are consistent with the diverse symptomatology of type I diabetes and demonstrate clear sex-, tissue-, and age-specific differences in the expression of this error which parallel the initiation and disease course of insulin-dependent (type I) diabetes mellitus.

Although the majority of smooth muscle neoplasms found in the uterus are benign, uterine leiomyosarcoma (LMS) is extremely malignant, with high rates of recurrence and metastasis. We earlier reported that mice with a homozygous deficiency for LMP2, an interferon (IFN)-γ-inducible factor, spontaneously develop uterine LMS. The IFN-γ pathway is important for control of tumor growth and invasion and has been implicated in several cancers. In this study, experiments with human and mouse uterine tissues revealed a defective LMP2 expression in human uterine LMS that was traced to the IFN-γ pathway and the specific effect of JAK-1 somatic mutations on the LMP2 transcriptional activation. Furthermore, analysis of a human uterine LMS cell line clarified the biological significance of LMP2 in malignant myometrium transformation and cell cycle, thus implicating LMP2 as an anti-tumorigenic candidate. This role of LMP2 as a tumor suppressor may lead to new therapeutic targets in human uterine LMS.