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SPusp45

Signal peptide for direct extracellular localization by Lactococcus Lactis

Usage and Biology

USP45 is a secreted extracellular protein of L. Lactis with no known function and an N-terminal 22 amino acid signal peptide. Use of the signal peptide allows for direct extracellular secretion of any protein of interest.

Sequence and Features

Background

To get the protein of interest anchored to the surface of L. lactis, it should be secreted out of the cell first. The solution is a signal peptide from the Usp45 protein which is secreted out of L. lactis very efficiently. Fusion of the signal peptide of Usp45 to a couple of proteins resulted in an efficient way of secreting the coded proteins into the media. The fusion protein will meet a chaperone protein, SecB, which bring the fusion protein to a protein secretion apparatus. This apparatus cleaves the signal peptide and frees the proteins from the cell.

Design

The protein of interest is fused to the cA domain with the USP45 signal peptide, driven by the PnisZ promoter and followed by the nisin resistant gene nsr. To demonstrate the utility of the protein surface display, here we took the β-galactosidase protein as a proof of concept.

Results

To demonstrate whether cA could indeed anchor the protein of interest at the surface of L. lactis, the pLacZ-Cytosol and pLacZ-Surface plasmids were introduced into NZ9000 respectively. As stated above, the cA domain is derived from the AcmA protein, which is an autolysin of L. latis that cleaves the peptidoglycan to release the duplicated bacteria. Since the substrate peptidoglycan is now occupied by the cA-β-galactosidase fusion proteins, the AcmA autolysin activity is hindered, thus cell separation will be interfered. Indeed, we found that under microscopic, the NZ-Surface cells were poorly separated compared to the NZ-Cytosol strain. Further more, using a polyclonal antibody against β-galactosidase, we found that about 70% of the β-galactosidase protein is present at the cell wall, while the other 30% protein is present in the cytoplasm, perhaps due to the inefficient secretion process. In contrast, all theβ-galactosidase proteins were present in the cytoplasm in the NZ-cytosol strain.

Figure 3.3 The protein surface display system for L. lactis. A). Microscopic picture of the NZ-Cytosol and NZ-surface. The LysM domains of cA can compete with the autolysins of L. lactis, thus the NZ-surface strain is poorly separated after nisin induction. B). Western blot confirmation of cell wall anchored proteins using LacZ as an example. Lane1, NZ9000 whole proteins. Lane 2, NZ-Surface cytosolic proteins. Lane 3, NZ-Surface cell wall proteins. Lane 4, NZ-Surface membrane proteins. Lane 5, cytosolic proteins of NZ-cytosol. Lane 6, cell wall proteins of NZ-cytosol. Lane 7, membrane proteins of NZ-cytosol.