Evaluating secretion and surface attachment of SapA, an S-layer-associated metalloprotease of Caulobacter crescentus

Caulobacter crescentus is used to display foreign peptides at high density as insertions into the surface (S)-layer protein (RsaA). Many recombinant RsaA proteins, however, are cleaved by SapA, a 71-kDa metalloprotease, suggesting a role in maintaining S-layer integrity. When overexpressed on a multicopy plasmid SapA was detected on the surface by fluorescent antibody only if RsaA and the O-side chain of LPS that mediates S-layer attachment were removed by mutation, indicating an outer membrane location beneath the S-layer. Secretion was mediated by the RsaA type 1 transporter since secretion was eliminated in transporter deficient strains or by C-terminal deletions in SapA (the presumed location of type 1 secretion signals). Secretion was required to become an active protease; mass spectrometry suggested this might be due to N-terminal processing during secretion, a feature shared with other type 1-secreted proteases. Overexpression leads to additional processing C-terminal to the protease domain, producing a 45-kDa protein. This was demonstrated to be self-processing. Deletion analysis revealed the C-terminal 100 amino acids were sufficient for anchoring and secretion. When protein G was fused to the last 238 amino acids of SapA it was secreted, surface attached and bound immunoglobulin, indicating potential for foreign protein display.