Vaccinia virus immunomodulator A46: A lipid and protein-binding scaffold for sequestering host TIR-domain proteins

Vaccinia virus interferes with early events of the activation pathway of the transcriptional factor nf-kb by binding to numerous host tir-domain containing adaptor proteins. we have previously determined the x-ray structure of the a46 c-terminal domain; however,the structure and function of the a46 n-terminal domain and its relationship to the c-terminal domain have remained unclear. here,we biophysically characterize residues 1-83 of the n-terminal domain of a46 and present the x-ray structure at 1.55 å. crystallographic phases were obtained by a recently developed ab initio method entitled arcimboldo_borges that employs tertiary structure libraries extracted from the protein data bank; data analysis revealed an all β-sheet structure. this is the first such structure solved by this method which should be applicable to any protein composed entirely of β-sheets. the a46(1-83) structure itself is a β-sandwich containing a co-purified molecule of myristic acid inside a hydrophobic pocket and represents a previously unknown lipid-binding fold. mass spectrometry analysis confirmed the presence of long-chain fatty acids in both n-terminal and full-length a46; mutation of the hydrophobic pocket reduced the lipid content. using a combination of high resolution x-ray structures of the n-and c-terminal domains and saxs analysis of full-length protein a46(1-240),we present here a structural model of a46 in a tetrameric assembly. integrating affinity measurements and structural data,we propose how a46 simultaneously interferes with several tir-domain containing proteins to inhibit nf-κb activation and postulate that a46 employs a bipartite binding arrangement to sequester the host immune adaptors tram and myd88. © 2016 fedosyuk et al.