Tetherin/BST2 is a type-II membrane protein that inhibits the release of a range of enveloped viruses, including HIV-1. Here we report three crystal structures of human tetherin, including the fulllength ectodomain, a triple cysteine mutant and an ectodomain truncation. These structures show that tetherin forms a continuous alpha helix encompassing almost the entire ectodomain. Tetherin helices dimerize into parallel coiled coils via interactions throughout the C-terminal portion of the ectodomain. A comparison of the multiple structures of the tetherin dimer reveals inherent constrained flexibility at two hinges positioned at residues A88 and G109. In the crystals, two tetherin ectodomain dimers associate into a tetramer by forming an antiparallel four-helix bundle at their N termini. However, mutagenesis studies suggest that the tetrametric form of tetherin, although potentially contributing to, is not essential for its antiviral activity. Nonetheless, the structural and chemical properties of the N terminus of the ectodomain are important for optimal tethering function. This study provides detailed insight into the mechanisms by which this broad-spectrum antiviral restriction factor can function.transmembrane | disulfide bonds | stability | budding

MAPK-activated protein kinase 2 (MAPKAPK2), one of several kinases directly phosphorylated and activated by p38 MAPK, plays a central role in the inflammatory response. The activated MAPKAPK2 phosphorylates its nuclear targets CREB/ATF1, serum response factor, and E2A protein E47 and its cytoplasmic targets HSP25/27, LSP-1, 5-lipoxygenase, glycogen synthase, and tyrosine hydroxylase. The crystal structure of unphosphorylated MAPKAPK2, determined at 2.8 Å resolution, includes the kinase domain and the C-terminal regulatory domain. Although the protein is inactive, the kinase domain adopts an active conformation with aspartate 366 mimicking the missing phosphorylated threonine 222 in the activation loop. The C-terminal regulatory domain forms a helix-turn-helix plus a long strand. Phosphorylation of threonine 334, which is located between the kinase domain and the C-terminal regulatory domain, may serve as a switch for MAPKAPK2 nuclear import and export. Phosphorylated MAPKAPK2 masks the nuclear localization signal at its C terminus by binding to p38. It unmasks the nuclear export signal, which is part of the second C-terminal helix packed along the surface of kinase domain C-lobe, and thereby carries p38 to the cytoplasm.

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