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ISG15 (interferon-stimulated gene 15) is a novel ubiquitinlike (UbL) modifier with two UbL domains in its architecture.We investigated different roles for the two UbL domains in protein modification by ISG15 (ISGylation) and the impact of Influenza B virus NS1 protein (NS1B) on regulation of the pathway. The results show that, although the C-terminal domain is sufficient to link ISG15 to UBE1L and UbcH8, the N-terminal domain is dispensable in the activation and transthiolation steps but required for efficient E3-mediated transfer of ISG15 from UbcH8 to its substrates. NS1B specifically binds to the N-terminal domain of ISG15 but does not affect ISG15 linkage via a thioester bond to its activating and conjugating enzymes. However, it does inhibit the formation of cellular ISG15 conjugates upon interferon treatment. We propose that the N-terminal UbL domain of ISG15 mainly functions in the ligation step and NS1B inhibits ISGylation by competing with E3 ligases for binding to the N-terminal domain.ISG15 (interferon-stimulated gene 15) is highly induced and conjugated to a large number of cellular proteins upon Type I interferon (IFN) 3 treatment (1, 2). ISG15 protein belongs to the family of ubiquitin-like (UbL) modifiers whose members are capable of forming conjugates to cellular proteins (3). Similar to the ubiquitination pathway, the ISG15 modification or ISGylation process involves the concerted action of a set of enzymes: the activating (UBE1L), conjugating (UbcH8), and ligating enzymes (4 -8). However, ISG15 is distinct from other members of the UbLs, such as SUMO and NEDD8 that contain one single UbL domain, in that it possesses two tandem UbL domains. Another two-UbL domain-containing protein, FAT10, can modify a very limited number of as-yet-unidentified proteins and has been associated with apoptosis (9, 10). In contrast, ISG15 in response to Type I IFN stimulation is potent in modifying a wide array of cellular proteins that have such diverse roles as RNA splicing, antiviral ability, cytoskeleton regulation, and signal transduction (7,11,12). Recently, ISG15 has been shown to be a critical component in IFN-mediated inhibition of human immunodeficiency virus, type 1 release (13). Furthermore, it has also been demonstrated to be a critical antiviral molecule against influenza, herpes, and Sindbis viruses (14, 15). Given that ISG15 possesses the antiviral property, it is not surprising that the Influenza B virus has developed a strategy to block the ISG15 modification of cellular proteins through its nonstructural protein NS1 (NS1B) (4).To explore the respective roles of the two UbL domains of ISG15, we carried out a series of biochemical experiments to study their specific functions in ISGylation pathway as well as to elucidate the molecular mechanism by which NS1B inhibits ISG15 conjugation to cellular proteins. EXPERIMENTAL PROCEDURES Generation of Expression Constructs and Protein Purification-The encoding sequences for UBE1L, NS1B-1-103, NS1B-1-90, NS1B-91-103, ISG15, and its two separate domains, I...

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The interactions of huntingtin (Htt) with the SH3 domain- or WW domain-containing proteins have been implicated in the pathogenesis of Huntington's disease (HD). We report the specific interactions of Htt proline-rich region (PRR) with the SH3GL3-SH3 domain and HYPA-WW1-2 domain pair by NMR. The results show that Htt PRR binds with the SH3 domain through nearly its entire chain, and that the binding region on the domain includes the canonical PxxP-binding site and the specificity pocket. The C terminus of PRR orients to the specificity pocket, whereas the N terminus orients to the PxxP-binding site. Htt PRR can also specifically bind to WW1-2; the N-terminal portion preferentially binds to WW1, while the C-terminal portion binds to WW2. This study provides structural insights into the specific interactions between Htt PRR and its binding partners as well as the alteration of these interactions that involve PRR, which may have implications for the understanding of HD.

The 19 F nuclear magnetic resonance (NMR) spectra of 4-£uorotryptophan (4-F-Trp)-labeled Escherichia coli arginyl^tRNA synthetase (ArgRS) show that there are distinct conformational changes in the catalytic core and tRNA anticodon stem and loop-binding domain of the enzyme, when arginine and tRNA Arg are added to the unliganded enzyme. We have assigned ¢ve £uorine resonances of 4-F-Trp residues (162, 172, 228, 349 and 446) in the spectrum of the £uorinated enzyme by sitedirected mutagenesis. The local conformational changes of E. coli ArgRS induced by its substrates observed herein by 19 F NMR are similar to those of crystalline yeast homologous enzyme. ß