Molecular and immunological characterization of OprL, the 18 kDa outer-membrane peptidoglycan-associated lipoprotein (PAL) of Pseudomonas aeruginosa.

Immunological screening of a Pseudomonas aeruginosa cosmid library led to the identification of clones producing an 18 kDa outer-membrane protein. This protein reacted in Western blots with a polyclonal antiserum against outer-membrane proteins of P. aeruginosa and with a monoclonal antibody (MA1-6) specific for OprL, the peptidoglycan-associated outer-membrane lipoprotein (PAL). Sequencing of pOML7, a subclone expressing oprL, revealed an ORF of 504 bp encoding a polypeptide with a typical lipoprotein signal recognition sequence. Another ORF was found upstream of oprL, with homology to the TolB protein of Escherichia coli and Haemophilus influenzae. Downstream of oprL, a second ORF, of 321 bp, was found (orf2), encoding a protein with a signal peptide and with no homology with proteins of known biological function. After the stop codon of orf2, a rho-independent terminator sequence was detected which is part of the P. aeruginosa PAO1 insertion element IS222. OprL showed homologies with all known PALs from Gram-negative bacteria, especially in the C-terminal part. mAb MA1-6 reacted with P. aeruginosa cells in immunofluorescence, and with E. coli cells expressing oprL, which had an abnormal, elongated morphology, an indication that production of the protein perturbed the division process.