The high-resolution crystal structure of S. marcescens Lip reveals a new member of the transthyretin family of proteins. Lip, a core component of the type VI secretion apparatus, is localized to the outer membrane and is positioned to interact with other proteins forming this complex system

Coulthurst, Sarah J.

English, Grant

Hunter, William N.

Rao, Vincenzo A.

Shepherd, Sharon M.

English

PubMed

Vincenzo A. Rao

Sharon M. Shepherd

Grant English

Sarah J. Coulthurst

William N. Hunter

Crossref

Acta Crystallographica Section D Biological Crystallography

The structure of<i>Serratia marcescens</i>Lip, a membrane-bound component of the type VI secretion system

Lip is a membrane-bound lipoprotein and a core component of the type VI secretion system found in Gram-negative bacteria. The structure of a Lip construct (residues 29-176) from Serratia marcescens (SmLip) has been determined at 1.92 angstrom resolution. Experimental phases were derived using a single-wavelength anomalous dispersion approach on a sample cocrystallized with iodide. The membrane localization of the native protein was confirmed. The structure is that of the globular domain lacking only the lipoprotein signal peptide and the lipidated N-terminus of the mature protein. The protein fold is dominated by an eight-stranded beta-sandwich and identifies SmLip as a new member of the transthyretin family of proteins. Transthyretin and the only other member of the family fold, 5-hydroxyisourate hydrolase, form homotetramers important for their function. The asymmetric unit of SmLip is a tetramer with 222 symmetry, but the assembly is distinct from that previously noted for the transthyretin protein family. However, structural comparisons and bacterial two-hybrid data suggest that the SmLip tetramer is not relevant to its role as a core component of the type VI secretion system, but rather reflects a propensity for SmLip to participate in protein-protein interactions. A relatively low level of sequence conservation amongst Lip homologues is noted and is restricted to parts of the structure that might be involved in interactions with physiological partners.</p

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Supplementary material. The structure of Serratia marcescens Lip; a membrane bound component of the Type VI secretion system. Vincenzo A. Rao, Sharon M. Shepherd, Grant English, Sarah J. Coulthurst and William N. Hunter*   College of Life Sciences, University of Dundee, DD1 5EH, United Kingdom. *Correspondence e-mail: w.n.hunter@dundee.ac.uk  Figure S1. The secondary structure and fold of SmLip. -strands are shown as arrows, -helices and 310 turns as ribbons respectively. The N and C terminal residues are labelled and the polypeptide is color ramped from blue at the N-terminus, to red at the C-terminus.   Figure S2. Sequence alignment of SmLip with eight orthologues. S. marcescens Lip is aligned with proteins from Escherichia coli (GenBank: CBG37366.1), Vibrio cholerae (NCBI Reference Sequence: ZP_01680158.1), Pseudomonas aeruginosa (NCBI Reference Sequence: NP_248770.1, PA0080 and NP_250357.1, PA1666), Burkholderia cenocepacia (NCBI Reference Sequence: YP_834112.1), Burkholderia pseudomallei (NCBI Reference Sequence: YP_112106.1), Aeromonas hydrophila (NCBI Reference Sequence: YP_856372.1) and Edwardsiella tarda (GenBank: ABW69084.1). Residues conserved in all sequences are enclosed in a cyan block, conserved in all but one sequence in blue, conserved in all but two sequences magenta and conserved in all but three sequences yellow. The start and finish of the lipobox motif are marked, as well as the signal peptide sequence. The alignment was generated the same way as for Figure 2.    Figure S3. An overlay of SmLip and a transthyretin subunit complexed with thyroxine. The polypeptide trace for SmLip is black, transthyretin red. Thyroxine is shown as sticks colored purple for I, grey C, red O and blue N.    