Caulobacter crescentus synthesizes an S-layer-editing metalloprotease possessing a domain sharing sequence similarity with its paracrystalline S-layer protein.
Strains of Caulobacter crescentus elaborate an S-layer, a two-dimensional protein latticework which covers the cell surface. The S-layer protein (RsaA) is secreted by a type I mechanism (relying on a C-terminal signal) and is unusual among type I secreted proteins because high levels of protein are produced continuously. In efforts to adapt the S-layer for display of foreign peptides and proteins, we noted a proteolytic activity that affected S-layer monomers with foreign inserts. The cleavage was precise, resulting in fragments with an unambiguous N-terminal sequence. We developed an assay to screen for loss of this activity (i.e., presentation of foreign peptides without degradation), using transposon and traditional mutagenesis. A metalloprotease gene designated sap (S-layer-associated protease) was identified which could complement the protease-negative mutants. The N-terminal half of Sap possessed significant similarity to other type I secreted proteases (e.g., alkaline protease of Pseudomonas aeruginosa), including the characteristic RTX repeat sequences, but the C-terminal half which normally includes the type I secretion signal exhibited no such similarity. Instead, there was a region of significant similarity to the N-terminal region of RsaA. We hypothesize that Sap evolved by combining the catalytic portion of a type I secreted protease with an S-layer-like protein, perhaps to associate with nascent S-layer monomers to "scan" for modifications.
Umelo-Njaka
Elizabeth
E
Department of Microbiology and Immunology, University of British Columbia, Vancouver, British Columbia V6T 1Z3, Canada.
Bingle
Wade H
WH
Borchani
Faten
F
Le
Khai D
KD
Awram
Peter
P
Blake
Theo
T
Nomellini
John F
JF
Smit
John
J
D000595:Amino Acid Sequence; D001425:Bacterial Outer Membrane Proteins / Q000096:biosynthesis / Q000737:chemistry* / Q000235:genetics; D001426:Bacterial Proteins; D020134:Catalytic Domain; D016935:Caulobacter crescentus / Q000378:metabolism*; D004251:DNA Transposable Elements; D004269:DNA, Bacterial / Q000737:chemistry; D015723:Gene Library; D008562:Membrane Glycoproteins; D008666:Metalloendopeptidases / Q000096:biosynthesis / Q000737:chemistry* / Q000235:genetics; D008969:Molecular Sequence Data; D009154:Mutation
D001425:Bacterial Outer Membrane Proteins; D001426:Bacterial Proteins; D004251:DNA Transposable Elements; D004269:DNA, Bacterial; D008562:Membrane Glycoproteins; C055764:S-layer proteins; D008666:Metalloendopeptidases