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Structure. 2013 Aug 6;21(8):1361-73. doi: 10.1016/j.str.2013.05.017. Epub 2013 Jul 11.

IcmQ in the Type 4b secretion system contains an NAD+ binding domain.

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  • 1Department of Physiology and Biophysics, Boston University School of Medicine, 700 Albany Street, Boston, MA 02118-2526, USA.


A Type 4b secretion system (T4bSS) is required for Legionella growth in alveolar macrophages. IcmQ associates with IcmR, binds to membranes, and has a critical role in the T4bSS. We have now solved a crystal structure of IcmR-IcmQ to further our understanding of this complex. This structure revealed an amphipathic four-helix bundle, formed by IcmR and the N-terminal domain of IcmQ, which is linked to a novel C-terminal domain of IcmQ (Qc) by a linker helix. The Qc domain has structural homology with ADP ribosyltransferase domains in certain bacterial toxins and binds NAD(+) with a dissociation constant in the physiological range. Structural homology and molecular dynamics were used to identify an extended NAD(+) binding site on Qc, and the resulting model was tested by mutagenesis and binding assays. Based on the data, we suggest that IcmR-IcmQ binds to membranes, where it may interact with, or perhaps modify, a protein in the T4bSS when NAD(+) is bound.

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