Open AccessStructural and functional dissection reveals distinct roles of Ca2+-binding sites in the giant adhesin SiiE of Salmonella enterica
Author(s) -
Britta Peters,
Johanna Stein,
Stefan Klingl,
Nathalie Sander,
Achim Sandmann,
Nicola Taccardi,
Heinrich Sticht,
Roman G. Gerlach,
Yves A. Muller,
Michael Hensel
Publication year - 2017
Publication title -
plos pathogens
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 3.719
H-Index - 206
eISSN - 1553-7374
pISSN - 1553-7366
DOI - 10.1371/journal.ppat.1006418
Subject(s) - secretion , bacterial adhesin , binding site , salmonella enterica , biology , microbiology and biotechnology , biochemistry , chemistry , gene , virulence , escherichia coli
The giant non-fimbrial adhesin SiiE of Salmonella enterica mediates the first contact to the apical site of epithelial cells and enables subsequent invasion. SiiE is a 595 kDa protein composed of 53 repetitive bacterial immunoglobulin (BIg) domains and the only known substrate of the SPI4-encoded type 1 secretion system (T1SS). The crystal structure of BIg50-52 of SiiE revealed two distinct Ca 2+ -binding sites per BIg domain formed by conserved aspartate or glutamate residues. In a mutational analysis Ca 2+ -binding sites were disrupted by aspartate to serine exchange at various positions in the BIg domains of SiiE. Amounts of secreted SiiE diminish with a decreasing number of intact Ca 2+ -binding sites. BIg domains of SiiE contain distinct Ca 2+ -binding sites, with type I sites being similar to other T1SS-secreted proteins and type II sites newly identified in SiiE. We functionally and structurally dissected the roles of type I and type II Ca 2+ -binding sites in SiiE, as well as the importance of Ca 2+ -binding sites in various positions of SiiE. Type I Ca 2+ -binding sites were critical for efficient secretion of SiiE and a decreasing number of type I sites correlated with reduced secretion. Type II sites were less important for secretion, stability and surface expression of SiiE, however integrity of type II sites in the C-terminal portion was required for the function of SiiE in mediating adhesion and invasion.