Neuronal differentiation and axonal growth are controlled by a variety of factors including neurotrophic factors, extracellular matrix components, and cell adhesion molecules. Here we describe a novel and very efficient neuritogenic factor, the metastasis-related Mts1 protein, belonging to the S100 protein family. The oligomeric but not the dimeric form of Mts1 strongly induces differentiation of cultured hippocampal neurons. A mutant with a single Y75F amino acid substitution, which stabilizes the dimeric form of Mts1, is unable to promote neurite extension. Disulfide bonds do not play an essential role in the Mts1 neuritogenic activity. Mts1-stimulated neurite outgrowth involves activation of phospholipase C and protein kinase C, depends on the intracellular level of Ca 2؉ , and requires activation of the extracellular signal-regulated kinases (ERKs) 1 and 2.

A growing body of evidence indicates that small, soluble oligomeric species generated from a variety of proteins and peptides rather than mature amyloid fibrils are inherently highly cytotoxic. Here, we show for the first time that mature amyloid fibrils produced from full-length recombinant mammalian prion protein (rPrP) were highly toxic to cultured cells and primary hippocampal and cerebella neurons. Fibrils induced apoptotic cell death in a time-and dose-dependent manner. The toxic effect of fibrils was comparable with that exhibited by soluble small ␤-oligomers generated from the same protein. Fibrils prepared from insulin were not toxic, suggesting that the toxic effect was not solely due to the highly polymeric nature of the fibrillar form. The cell death caused by rPrP fibrils or ␤-oligomers was substantially reduced when expression of endogenous PrP C was down-regulated by small interfering RNAs. In opposition to the ␤-oligomer and amyloid fibrils of rPrP, the monomeric ␣-helical form of rPrP stimulated neurite outgrowth and survival of neurons. These studies illustrated that both soluble ␤-oligomer and amyloid fibrils of the prion protein are intrinsically toxic and confirmed that endogenously expressed PrP C is required for mediating the toxicity of abnormally folded external PrP aggregates.Several neurodegenerative maladies including Alzheimer, Parkinson, Huntington, and prion diseases have been related to the age-dependent accumulation of amyloid deposits in the brain (1, 2). A common feature among these and other "conformational" diseases is the conversion of specific proteins or peptides into polymeric forms that are characterized by cross-␤-sheet structures and referred to as amyloid (3). Even though the amyloidogenic proteins have no obvious sequence similarity, they share similar conformational features within the amyloid form (4, 5).In addition to the amyloid fibrils/deposits, the formation of nonfibrillar soluble oligomers has been observed for a number of proteins associated with conformational diseases including ␣-synuclein (6, 7), A␤ peptides (8, 9), transthyretin (10), lysozyme (11), and prion protein (12, 13). Soluble oligomers were found either as a prefibrillar intermediate that formed on the kinetic pathway to the mature amyloid fibrils (14 -17) or as off-pathway products produced via alternative aggregation mechanisms (13,18,19). For the past several years, a substantial body of evidence has accumulated indicating that soluble oligomers are toxic species that are actively involved in the impairment of cellular functions in neurodegenerative diseases. Furthermore, non-fibrillar oligomers have been shown to be intrinsically toxic to cells even when formed from proteins that are not related to any known conformational disease (20,21). This implies that oligomeric species share a common mechanism of cytotoxicity regardless of the specific protein from which they are generated (22,23). On the other hand, there is considerable debate as to whether mature amyloid fibrils also exhibit toxici...

Calgranulin C (S100A12) is a member of the S100 family of proteins that undergoes a conformational change upon calcium binding allowing them to interact with target molecules and initiate biological responses; one such target is the receptor for advanced glycation products (RAGE). The RAGE-calgranulin C interaction mediates a pro-inflammatory response to cellular stress and can contribute to the pathogenesis of inflammatory lesions. The soluble extracellular part of RAGE (sRAGE) was shown to decrease the inflammation response possibly by scavenging RAGE-activating ligands. Here, by using high resolution NMR spectroscopy, we identified the sRAGE-calgranulin C interaction surface. Ca 2؉ binding creates two symmetric hydrophobic surfaces on Ca 2؉ -calgranulin C that allow calgranulin C to bind to the C-type immunoglobulin domain of RAGE. Apocalgranulin C also binds to sRAGE using a completely different surface and with substantially lower affinity, thus underscoring the role of Ca 2؉ binding to S100 proteins as a molecular switch. By using native gel electrophoresis, chromatography, and fluorescence spectroscopy, we established that sRAGE forms tetramers that bind to hexamers of Ca 2؉ -calgranulin C. This arrangement creates a large platform for effectively transmitting RAGE-dependent signals from extracellular S100 proteins to the cytoplasmic signaling complexes.

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Papillomaviruses cause warts and proliferative lesions in skin and other epithelia. In a minority of papillomavirus types ('high risk, including human papillomaviruses 16, 18, 31, 33, 45 and 56), further transformation of the wart lesions can produce tumours. The papillomavirus E2 protein controls primary transcription and replication of the viral genome. Both activities are governed by a approximately 200 amino-acid amino-terminal module (E2NT) which is connected to a DNA-binding carboxy-terminal module by a flexible linker. Here we describe the crystal structure of the complete E2NT module from human papillomavirus 16. The E2NT module forms a dimer both in the crystal and in solution. Amino acids that are necessary for transactivation are located at the dimer interface, indicating that the dimer structure may be important in the interactions of E2NT with viral and cellular transcription factors. We propose that dimer formation may contribute to the stabilization of DNA loops which may serve to relocate distal DNA-binding transcription factors to the site of human papillomavirus transcription initiation.