Lytic transglycosylase Slt of Pseudomonas aeruginosa as a periplasmic hub protein

Peptidoglycan is a major constituent of the bacterial cell wall. Its integrity as a polymeric edifice is critical for bacterial survival and, as such, it is a preeminent target for antibiotics. The peptidoglycan is a dynamic crosslinked polymer that undergoes constant biosynthesis and turnover. The soluble lytic transglycosylase (Slt) of Pseudomonas aeruginosa is a periplasmic enzyme involved in this dynamic turnover. Using amber-codon-suppression methodology in live bacteria, we incorporated a fluorescent chromophore into the structure of Slt. Fluorescent microscopy shows that Slt populates the length of the periplasmic space and concentrates at the sites of septation in daughter cells. This concentration persists after separation of the cells. Amber-codon-suppression methodology was also used to incorporate a photoaffinity amino acid for the capture of partner proteins. Mass-spectrometry-based proteomics identified 12 partners for Slt in vivo. These proteomics experiments were complemented with in vitro pulldown analyses. Twenty additional partners were identified. We cloned the genes and purified to homogeneity 22 identified partners. Biophysical characterization confirmed all as bona fide Slt binders. The identities of the protein partners of Slt span disparate periplasmic protein families, inclusive of several proteins known to be present in the divisome. Notable periplasmic partners (K_D < 0.5 μM) include PBPs (PBP1a, K_D = 0.07 μM; PBP5 = 0.4 μM); other lytic transglycosylases (SltB2, K_D = 0.09 μM; RlpA, K_D = 0.4 μM); a type VI secretion system effector (Tse5, K_D = 0.3 μM); and a regulatory protease for alginate biosynthesis (AlgO, K_D < 0.4 μM). In light of the functional breadth of its interactome, Slt is conceptualized as a hub protein within the periplasm.