Interleukin-1 (IL-1), a proinflammatory cytokine produced mainly by macrophages and monocytes in response to inflammation, infection, and other challenges, stimulates a wide spectrum of responses, including fever, lymphocyte activation, and leukocyte infusion to the site of injury or infection (16). IL-1 stimulates the expression of several genes by activating the transcription factors NF-B, ATF, and AP-1 (6, 51, 52).The activation of NF-B has been studied extensively (4, 6, 16). NF-B is kept in the cytoplasm through interaction with B inhibitory proteins. Following stimulation with cytokines (e.g., IL-1 and tumor necrosis factor alpha [TNF-␣]) or other agents (e.g., lipopolysaccharide, phorbol ester, and doublestranded RNA), IB undergoes phosphorylation on specific serine residues and is rapidly ubiquitinated and degraded. The liberated NF-B translocates to the nucleus, where it activates transcription (5,63,66,69). Recent studies have provided a model for how NF-B is activated in response to IL-1 (Fig. 1). First, a complex is formed between the type 1 receptor (IL-1R1) and the receptor accessory protein (IL-1RAcP) (21, 24, 29, 70). The cytosolic myeloid differentiation protein (MyD88) (36) is then recruited to the complex, where it functions as an adaptor, recruiting IL-1R-associated kinase (IRAK) in turn (10,48,71,75). IRAK is phosphorylated and then leaves the receptor complex to interact with TRAF6 (11). IRAK2, an IRAK homolog, was shown to interact with the IL-1R complex, MyD88, and TRAF6 in transfected cells, but how IRAK and IRAK2 function in IL-1 signaling is not understood (48). Six TRAFs (TNF receptor-associated factors) have been described so far (2,17,22,23,25,31,49,58). TRAF2 and TRAF5 have been implicated in activating NF-B in response to the activation of members of the TNF-␣ receptor superfamily (2,17,22,23,25,31,49,58). The TRAFs interact with NF-Binducing kinase (NIK), another serine-threonine kinase believed to be a common downstream component in activating NF-B in response to IL-1, TNF-␣, and other stimuli (41). TRAFs might also activate mitogen-activated protein kinase/ ERK kinase kinase 1 (MEKK1) (30,32,35,64,76). Recently, two IB kinases (IKK␣ and IKK) have been implicated in signal-induced phosphorylation of the IB proteins (15,44,57,73,78). Both NIK and MEKK1 activate the IKKs by serine phosphorylation (34, 50). The activated IKKs then phosphorylate IBs on specific serine residues, resulting in the degradation of IB and activation of NF-B. The IKKs are components of a large complex (15,44,78). Two additional components, NEMO (NF-B essential modulator or IKK␥) and IKAP are also part of the IKK complex and are required for its formation (12,59,74).Recent studies provide evidence for a second signaling pathway parallel to the cascade leading to IB degradation and specifically required for NF-B-dependent transcriptional competency (Fig.
Abstract. Several members of the rho/rac family of small GTP-binding proteins are known to regulate the distribution of the actin cytoskeleton in various subcellular processes. We describe here a novel rac protein, racE, which is specifically required for cytokinesis, an actomyosin-mediated process. The racE gene was isolated in a molecular genetic screen devised to isolate genes required for cytokinesis in Dictyostelium. Phenotypic characterization of racE mutants revealed that racE is not essential for any other cell motility event, including phagocytosis, chemotaxis, capping, or development. Our data provide the first genetic evidence for the essential requirement of a rho-like protein, specifically in cytokinesis, and suggest a role for these proteins in coordinating cytokinesis with the mitotic events of the cell cycle.T HE intimate association between mitosis and cytokinesis requires a means of coordination between these two processes to insure that the newly duplicated nuclei segregate properly with half of the cytoplasm into the daughter cells. Although much is known about these processes, the mechanism(s) by which they are coordinated remains unknown. The regulation of the mitotic cell cycle has been intensively studied over the last several years. Biochemical and genetic approaches have combined to identify many of the key proteins that control different aspects of the cell cycle. In addition, many of the structural proteins that compose the mitotic apparatus have been characterized. Similarly, much is understood about how cells achieve proper cytoplasmic division. In animal cells, this involves the formation of an equatorial contractile ring that consists largely of actin and myosin and constricts to divide the cell into two . However, it is not understood how these proteins localize to the equator of the cell at the appropriate time and in the correct orientation. From the work of Rappaport (1990), it is clear that the astral microtubules of the mitotic apparatus are intimately involved in determining the placement of the contractile ring. What is not clear is what kind of signals may be involved or how they may be transmitted by the mitotic apparatus to the cell cortex.The rho family of ras-related small GTP-binding proteins (including rho, cdc42, and rac proteins) are known to have profound effects on the actin cytoskeleton (Hall, 1994). Rho proteins have been implicated in the regulation of cytokinesis in both sand dollar (Mabuchi et al., 1993)
The small GTPase racE is essential for cytokinesis in Dictyostelium but its precise role in cell division is not known. To determine the molecular mechanism of racE function, we undertook a mutational analysis of racE. The exogenous expression of either wild-type racE or a constitutively active V20racE mutant effectively rescues the cytokinesis deficiency of racE null cells. In contrast, a constitutively inactive N25racE mutant fails to rescue the cytokinesis deficiency. Thus, cytokinesis requires only the activation of racE by GTP and not the inactivation of racE by hydrolysis of GTP. To determine the spatial distribution of racE, we created a fusion protein with GFP at the amino terminus of racE. Remarkably, GFP-racE fusion protein was fully competent to rescue the phenotype of racE null cells and, therefore, must reside in the same location as native racE. We found that GFP-racE localized to the plasma membrane of the cell throughout the entire cell cycle. Furthermore, constitutively active and inactive GFP-racE fusion proteins also localized to the plasma membrane. We mapped the domain required for plasma membrane localization to the carboxyl-terminal 40 amino acids of racE. This domain, however, is not sufficient to confer racE function onto a closely related GTPase. Taken together, these results suggest that racE functions at the cell cortex but it is not involved in determining the timing or placement of the contractile ring.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.