Previous work had identified a corepressor, NAB1, which represses transcriptional activation mediated by NGFI-A (also known as Egr-1, zif268, and Krox24) and Krox20. These zinc finger transcription factors are encoded by immediate-early genes and have been implicated in a wide variety of proliferative and differentiative processes. We have isolated and characterized another corepressor, NAB2, which is highly related to NAB1 within two discrete domains. The first conserved domain of NAB2 mediates an interaction with the R1 domain of NGFI-A. NAB2 represses the activity of both NGFI-A and Krox20, and its expression is regulated by some of the same stimuli that induce NGFI-A expression, including serum stimulation of fibroblasts and nerve growth factor stimulation of PC12 cells. The human NAB2 gene has been localized to chromosome 12q13.3-14.1, a region that is rearranged in several solid tumors, lipomas, uterine leiomyomata, and liposarcomas. Sequencing of the Caenorhabditis elegans genome has identified a gene that bears high homology to both NAB1 and NAB2, suggesting that NAB molecules fulfill an evolutionarily conserved role.Transcriptional control plays a vital role in regulating fundamental cellular processes such as proliferation, differentiation, and cell death. Changes in gene expression are often effected by altering the expression level, activity, and/or nuclear localization of transcription factors that bind to promoter and enhancer regions. In addition, many transcription factors are regulated by direct interactions with other proteins which modulate the level of transcriptional activation.

Agrin-induced clustering of acetylcholine receptors (AChRs) in the postsynaptic membrane is a key step in synaptogenesis at the neuromuscular junction. The receptor tyrosine kinase MuSK is a component of the agrin receptor, while the cytoplasmic protein rapsyn is necessary for the clustering of AChRs and all other postsynaptic membrane components studied to date. We show here that MuSK remains concentrated at synaptic sites in rapsyn-deficient mutant mice, suggesting that MuSK forms a primary structural scaffold to which rapsyn attaches other synaptic components. Using nonmuscle cells, we show that rapsyn-MuSK interactions are mediated by the ectodomain of MuSK, suggesting the existence of a transmembrane intermediate. In addition to rapsyn's structural role, we demonstrate that it is required for an early step in MuSK signaling, AChR phosphorylation. This signaling requires the kinase domain of MuSK, but not its ectodomain. Thus, MuSK may interact with rapsyn in multiple ways to play both structural and signaling roles in agrin-induced differentiation.

We describe a novel protein, Syne-1, that is associated with nuclear envelopes in skeletal, cardiac, and smooth muscle cells. Syne-1 contains multiple spectrin repeats similar to those found in dystrophin and utrophin, as well as a domain homologous to the carboxyl-terminal of Klarsicht, a protein associated with nuclei and required for a subset of nuclear migrations in Drosophila. In adult skeletal muscle fibers, levels of Syne-1 are highest in the nuclei that lie beneath the postsynaptic membrane at the neuromuscular junction. These nuclei are transcriptionally specialized, expressing genes for synaptic components at higher levels than extrasynaptic nuclei in the same cytoplasm. Syne-1 is the first protein found to be selectively associated with synaptic nuclei. Syne-1 becomes concentrated in synaptic nuclei postnatally. It remains synaptically enriched following denervation or degeneration/regeneration, and is also present at high levels in the central nuclei of dystrophic myotubes. The location and structure of Syne-1 suggest that it may participate in the migration of myonuclei in myotubes and/or their anchoring at the postsynaptic apparatus. Finally, we identify a homologous gene, syne-2, that is expressed in an overlapping but distinct pattern.Skeletal muscle fibers are syncytial; in most mammalian skeletal muscles, each fiber contains several hundred myonuclei. Of these, a few are located beneath the postsynaptic membrane at the neuromuscular junction (NMJ).1 Synaptic nuclei are specialized in several respects. First, multiple nuclei (generally 3-8) are invariably associated with synaptic sites. Because Ͻ1% of the muscle fiber surface is synaptic, one would expect only a minority of synaptic sites to be associated with even a single nucleus if nuclear distribution were random. Second, most nuclei are well separated from their neighbors, but synaptic nuclei occur in tight clusters. Third, synaptic nuclei are larger and rounder than extrasynaptic nuclei (1-3).Finally, synaptic nuclei are transcriptionally specialized; they express genes for several synaptic proteins, including subunits of the acetylcholine receptor (AChR), at levels far higher than those of extrasynaptic nuclei in the same cytoplasm (4 -6). As a result, mRNAs for synaptic proteins are concentrated in synaptic areas, allowing local synthesis of synaptic constituents. This local synthesis has been of considerable interest to neurobiologists, because it contributes to postsynaptic differentiation, and because it may serve as a model for central synapses, in which some components of dendritic spines are thought to be synthesized within the spine itself (7).The formation of the postsynaptic apparatus, including the accumulation and specialization of synaptic nuclei, is controlled by the nerve. One critical nerve-derived signal is the proteoglycan agrin which is required for all aspects of postsynaptic differentiation, including transcriptional specialization of synaptic nuclei (8 -10). A critical component of the agrin receptor is the muscle-sp...

We have characterized ADAMTS7B, the authentic fulllength protein product of the ADAMTS7 gene. ADAMTS7B has a domain organization similar to that of ADAMTS12, with a total of eight thrombospondin type 1 repeats in its ancillary domain. Of these, seven are arranged in two distinct clusters that are separated by a mucin domain. Unique to the ADAMTS family, ADAMTS7B is modified by attachment of the glycosaminoglycan chondroitin sulfate within the mucin domain, thus rendering it a proteoglycan. Glycosaminoglycan addition has potentially important implications for ADAMTS7B cellular localization and for substrate recognition. Although not an integral membrane protein, ADAMTS7B is retained near the cell surface of HEK293F cells via interactions involving both the ancillary domain and the prodomain. ADAMTS7B undergoes removal of the prodomain by a multistep furin-dependent mechanism. At least part of the final processing event, i.e. cleavage following Arg 220 (mouse sequence annotation), occurs at the cell surface. ADAMTS7B is an active metalloproteinase as shown by its ability to cleave ␣ 2 -macroglobulin, but it does not cleave specific peptide bonds in versican and aggrecan attacked by ADAMTS proteases. Together with ADAMTS12, whose primary structure also predicts a mucin domain, ADAMTS7B constitutes a unique subgroup of the ADAMTS family. The extracellular matrix (ECM)1 is an information-rich assembly influencing cell proliferation, apoptosis, and cell migration. Proteases have an essential role in modulating the environmental cues that ECM provides for tissue morphogenesis, homeostasis, and disease progression. Metalloproteases, especially matrix metalloproteases, have a conspicuous role in ECM degradation as well as in proteolysis of cell-surface and soluble proteins (1, 2). Another metalloprotease family, ADAM (a disintegrin and metalloprotease domain), contains transmembrane enzymes with a major role in ectodomain shedding of cell-surface molecules, but a negligible function in ECM proteolysis (3). The active site of ADAM proteases, unlike that of the matrix metalloproteases, is of the reprolysin (snake venom zinc metalloprotease) type (3). The recent discovery of the ADAMTS (a disintegrin-like and metalloprotease domain with thrombospondin type 1 motif) family brought to light metalloproteases that contain a reprolysintype catalytic site, but, unlike the ADAM proteases, are secreted enzymes with a prominent role in ECM proteolysis.ADAMTS proteases have a characteristic modular structure whose hallmark is the presence of one or more thrombospondin type 1 repeats (TSRs) (4). In the short period of time since the discovery of ADAMTS1 in 1997 (4), important functions have been attributed to a number of family members, and mutations of two of these enzymes have been shown to cause human genetic disorders. Inactivating mutations of ADAMTS13 cause inherited thrombocytopenic purpura due to a failure to process von Willebrand factor (5). Mutations of ADAMTS2, a procollagen aminopropeptidase, cause skin fragility in a var...

The 43 kDa AChR-associated protein rapsyn is required for the clustering of nicotinic acetylcholine receptors (AChRs) at the developing neuromuscular junction, but the functions of other postsynaptic proteins colocalized with the AChR are less clear. Here we use a fibroblast expression system to investigate the role of the dystrophin-glycoprotein complex (DGC) in AChR clustering. The agrin-binding component of the DGC, dystroglycan, is found evenly distributed across the cell surface when expressed in fibroblasts. However, dystroglycan colocalizes with AChR-rapsyn clusters when these proteins are coexpressed. Furthermore, dystroglycan colocalizes with rapsyn clusters even in the absence of AChR, indicating that rapsyn can cluster dystroglycan and AChR independently. Immunofluorescence staining using a polyclonal antibody to utrophin reveals a lack of staining of clusters, suggesting that the immunoreactive species, like the AChR, does not mediate the observed rapsyndystroglycan interaction. Rapsyn may therefore be a molecular link connecting the AChR to the DGC. At the neuromuscular synapse, rapsyn-mediated linkage of the AChR to the cytoskeleton-anchored DGC may underlie AChR cluster stabilization.