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Members of the Siah (seven in absentia homolog) family of RING domain proteins are components of E3 ubiquitin ligase complexes that catalyze ubiquitination of proteins. We have determined the crystal structure of the substrate-binding domain (SBD) of murine Siah1a to 2.6 A resolution. The structure reveals that Siah is a dimeric protein and that the SBD adopts an eight-stranded beta-sandwich fold that is highly similar to the TRAF-C region of TRAF (TNF-receptor associated factor) proteins. The TRAF-C region interacts with TNF-alpha receptors and TNF-receptor associated death-domain (TRADD) proteins; however, our findings indicate that these interactions are unlikely to be mimicked by Siah. The Siah structure also reveals two novel zinc fingers in a region with sequence similarity to TRAF. We find that the Siah1a SBD potentiates TNF-alpha-mediated NF-kappa B activation. Therefore, Siah proteins share important similarities with the TRAF family of proteins, including their overall domain architecture, three-dimensional structure and functional activity.

Proteins exist in one of two generally incompatible states: either membrane associated or soluble. Pore-forming proteins are exceptional because they are synthesized as a water-soluble molecule but end up being located in the membrane — that is, they are nonconstitutive membrane proteins. Here we report the pronounced effect of the single point mutation Y221G of the pore-forming toxin aerolysin. This mutation blocks the hemolytic activity of the toxin but does not affect its initial structure, its ability to bind to cell-surface receptors or its capacity to form heptamers, which constitute the channel-forming unit. The overall structure of the Y221G protein as analyzed by cryo-negative staining EM and three-dimensional reconstruction is remarkably similar to that of the wild type heptamer. The mutant protein forms a mushroom-shaped complex whose stem domain is thought to be within the membrane in the wild type toxin. In contrast to the wild type heptamer, which is a hydrophobic complex, the Y221G heptamer is fully hydrophilic. This point mutation has, therefore, converted a normally membrane-embedded toxin into a soluble complex. [ABSTRACT FROM AUTHOR]

Members of the Siah (seven in absentia homolog) family of RING domain proteins are components of E3 ubiquitin ligase complexes that catalyze ubiquitination of proteins. We have determined the crystal structure of the substrate-binding domain (SBD) of murine Siah1a to 2.6 Å resolution. The structure reveals that Siah is a dimeric protein and that the SBD adopts an eight-stranded β-sandwich fold that is highly similar to the TRAF-C region of TRAF (TNF-receptor associated factor) proteins. The TRAF-C region interacts with TNF-α receptors and TNF-receptor associated death-domain (TRADD) proteins; however, our findings indicate that these interactions are unlikely to be mimicked by Siah. The Siah structure also reveals two novel zinc fingers in a region with sequence similarity to TRAF. We find that the Siah1a SBD potentiates TNF-α-mediated NF-κB activation. Therefore, Siah proteins share important similarities with the TRAF family of proteins, including their overall domain architecture, three-dimensional structure and functional activity. [ABSTRACT FROM AUTHOR]

Amyloid precursor protein (APP) plays a central role in Alzheimer disease. A proteolytic-breakdown product of APP, called β-amyloid, is a major component of the diffuse and fibrillar deposits found in Alzheimer diseased brains. The normal physiological role of APP remains largely unknown despite much work. A knowledge of its function will not only provide insights into the genesis of the disease but may also prove vital in the development of an effective therapy. Here we describe the 1.8 Å resolution crystal structure of the N-terminal, heparin-binding domain of APP (residues 28?123), which is responsible, among other things, for stimulation of neurite outgrowth. The structure reveals a highly charged basic surface that may interact with glycosaminoglycans in the brain and an abutting hydrophobic surface that is proposed to play an important functional role such as dimerization or ligand binding. Structural similarities with cysteine-rich growth factors, taken together with its known growth-promoting properties, suggests the APP N-terminal domain could function as a growth factor in vivo. [ABSTRACT FROM AUTHOR]