C. Stathopoulos et al., Characterization of the avian pathogenic Escherichia coli hemagglutinin Tsh, a member of the immunoglobulin a protease type family of autotransporters, INFEC IMMUN, 67(2), 1999, pp. 772-781
We reported earlier that a single gene, tsh, isolated from a strain of avia
n pathogenic Escherichia coli (APEC) was sufficient to confer on E. coil K-
12 a hemagglutinin-positive phenotype and that the deduced sequence of the
Tsh protein shared homology to the serine-type immunoglobulin A (IgA) prote
ases of Neisseria gonorrhoeae and Haemophilus influenzae, In this report we
show that E. coil K-12 containing the recombinant tsh gene produced two pr
oteins, a 106-kDa extracellular protein and a 33-kDa outer membrane protein
, and was also able to agglutinate chicken erythrocytes. N-terminal sequenc
e data indicated that the 106-kDa protein, designated Tsh(s), was derived f
rom the N-terminal end of Tsh after the removal of a 52-amino-acid N-termin
al signal peptide, while the 33-kDa protein, designated Tsh(beta), was deri
ved from the C-terminal end of Tsh starting at residue N-1101. The Tsh(s) d
omain contains the 7-amino-acid serine protease motif that includes the act
ive-site serine (S-259), found also in the secreted domains of the IgA prot
eases. However, site-directed mutagenesis of S-259 did not abolish the hema
gglutinin activity or the extracellular secretion of Tsh(s) indicating that
host-directed proteolysis was mediating the release of Tsh(s). Studies wit
h an E. coli K-12 ompT mutant strain showed that the surface protease OmpT
was not needed for the secretion of Tsh(s). Tsh belongs to a subclass of th
e IgA protease family, which also includes EspC of enteropathogenic E. coil
, EspP of enterohemorragic E. coli, and SepA and VirG of Shigella flexneri,
which seem to involve a host endopeptidase to achieve extracellular releas
e of their N-terminal domains. In proteolytic studies conducted in vitro, T
sh(s) did not cleave the substrate of the IgA proteases, human IgA1 or chic
ken IgA, and did not show proteolytic activity in a casein-based assay, Cor
relation of Tsh expression and hemagglutination activity appears to be a ve
ry complex phenomenon, influenced by strain and environmental conditions. N
evertheless, for both APEC and recombinant E. coil K-12 strains containing
the tsh gene, it was only the whole bacterial cells and not the cell-free s
upernatants that could confer hemagglutinin activity. Our results provide i
nsights into the expression, secretion, and proteolytic features of the Tsh
protein, which belongs to the growing family of gram-negative bacterial ex
tracellular virulence factors, named autotransporters, which utilize a self
-mediated mechanism to achieve export across the bacterial cell envelope.