title: Structural basis for type VI secreted peptidoglycan DL-endopeptidase function, specificity and neutralization in Serratia marcescens.
abstract: Some Gram-negative bacteria target their competitors by exploiting the type VI secretion system to extrude toxic effector proteins. To prevent self-harm, these bacteria also produce highly specific immunity proteins that neutralize these antagonistic effectors. Here, the peptidoglycan endopeptidase specificity of two type VI secretion-system-associated effectors from Serratia marcescens is characterized. These small secreted proteins, Ssp1 and Ssp2, cleave between γ-D-glutamic acid and L-meso-diaminopimelic acid with different specificities. Ssp2 degrades the acceptor part of cross-linked tetratetrapeptides. Ssp1 displays greater promiscuity and cleaves monomeric tripeptides, tetrapeptides and pentapeptides and dimeric tetratetra and tetrapenta muropeptides on both the acceptor and donor strands. Functional assays confirm the identity of a catalytic cysteine in these endopeptidases and crystal structures provide information on the structure-activity relationships of Ssp1 and, by comparison, of related effectors. Functional assays also reveal that neutralization of these effectors by their cognate immunity proteins, which are called resistance-associated proteins (Raps), contributes an essential role to cell fitness. The structures of two immunity proteins, Rap1a and Rap2a, responsible for the neutralization of Ssp1 and Ssp2-like endopeptidases, respectively, revealed two distinct folds, with that of Rap1a not having previously been observed. The structure of the Ssp1-Rap1a complex revealed a tightly bound heteromeric assembly with two effector molecules flanking a Rap1a dimer. A highly effective steric block of the Ssp1 active site forms the basis of effector neutralization. Comparisons with Ssp2-Rap2a orthologues suggest that the specificity of these immunity proteins for neutralizing effectors is fold-dependent and that in cases where the fold is conserved sequence differences contribute to the specificity of effector-immunity protein interactions.
authors: Division of Biological Chemistry and Drug Discovery, College of Life Sciences, University of Dundee, Dundee DD1 5EH, Scotland.
authors: Velupillai
authors: V
authors: Srikannathasan
authors: Grant
authors: G
authors: English
authors: Nhat Khai
authors: NK
authors: Bui
authors: Katharina
authors: K
authors: Trunk
authors: Patrick E F
authors: PE
authors: O'Rourke
authors: Vincenzo A
authors: VA
authors: Rao
authors: Waldemar
authors: W
authors: Vollmer
authors: Sarah J
authors: SJ
authors: Coulthurst
authors: William N
authors: WN
authors: Hunter
mesh_terms: D000595:Amino Acid Sequence; D058947:Bacterial Secretion Systems; D018360:Crystallography, X-Ray; D010450:Endopeptidases / Q000737:chemistry* / Q000378:metabolism*; D008958:Models, Molecular; D008969:Molecular Sequence Data; D010457:Peptidoglycan / Q000378:metabolism*; D011487:Protein Conformation; D012706:Serratia marcescens / Q000737:chemistry / Q000201:enzymology* / Q000502:physiology*; D013379:Substrate Specificity
chemical_list: D058947:Bacterial Secretion Systems; D010457:Peptidoglycan; C051467:peptidoglycan endopeptidase; D010450:Endopeptidases
keywords: Gram-negative; Serratia marcescens; amidases; cysteine proteases; disulfide linkage; effector; endopeptidases; immunity protein; peptidoglycan; type VI secretion system