Importance of conserved residues of the serine protease autotransporter beta-domain in passenger domain processing and beta-barrel assembly.
Serine protease autotransporters of the family Enterobacteriaceae (SPATE) comprise a family of virulence proteins secreted by enteric Gram-negative bacteria via the autotransporter secretion pathway. A SPATE polypeptide contains a C-terminal translocator domain that inserts into the bacterial outer membrane as a beta-barrel structure and mediates secretion of the passenger domain to the extracellular environment. In the present study, we examined the role of conserved residues located in the SPATE beta-barrel-forming region in passenger domain secretion. Thirty-nine fully conserved residues in Tsh were mutated by single-residue substitution, and defects in their secretion phenotypes were assessed by cell fractionation and immunochemistry. A total of 22 single mutants exhibited abnormal phenotypes in different cellular compartments. Most mutants affecting secretion are charged residues with side chains pointing into the beta-barrel interior. Seven mutants showed notable abnormalities in processing (constructs with the E1231A, E1249A, and R1374A mutations) and beta-barrel assembly or insertion into the outer membrane (constructs with the G1158Y, F1360A, Y1375A, and F1377A mutations). The phenotypes of the beta-barrel assembly/insertion mutants and the presence of a processed Tsh passenger domain in the periplasm support the possibility that the translocator domain must undergo extensive folding prior to insertion into the outer membrane. Results from double-mutation experiments further demonstrate that F1360 and F1377 affect beta-barrel insertion/assembly at different times. In light of these new data, a more refined model for the mechanism of SPATE secretion is presented.
Department of Biological Sciences, California State Polytechnic University, Pomona, CA 91709, USA.
Yihfen T
YT
Yen
Casey
C
Tsang
Todd A
TA
Cameron
Dennis O
DO
Ankrah
Athina
A
Rodou
Christos
C
Stathopoulos
D019943:Amino Acid Substitution / Q000235:genetics; D017124:Conserved Sequence; D004926:Escherichia coli / Q000201:enzymology*; D029968:Escherichia coli Proteins / Q000235:genetics / Q000378:metabolism*; D007120:Immunochemistry; D026901:Membrane Transport Proteins / Q000235:genetics / Q000378:metabolism*; D008958:Models, Molecular; D016297:Mutagenesis, Site-Directed; D050505:Mutant Proteins / Q000235:genetics / Q000378:metabolism; D017510:Protein Folding; D012697:Serine Endopeptidases / Q000235:genetics / Q000378:metabolism*; D037521:Virulence Factors / Q000235:genetics / Q000378:metabolism*
D029968:Escherichia coli Proteins; D026901:Membrane Transport Proteins; D050505:Mutant Proteins; D037521:Virulence Factors; C107189:EspP protein, E coli; D012697:Serine Endopeptidases