HtrA is the unique surface housekeeping protease in Lactococcus lactis and is required for natural protein processing

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Abstract
We identified an exported protease in Lactococcus lactis ssp. lactis strain IL1403 belonging to the HtrA/DegP family. Inactivation of the chromosomal gene (htrALl) encoding this protease (HtrALl) results in growth thermo‐sensitivity at very high temperatures (above 37°C for L. lactis). The role of htrALl in extracellular proteolysis under normal growth conditions was examined by testing the stability of different exported proteins (i.e. fusions, a heterologous pre‐pro‐protein or a native protein containing repeats), having different locations. In the wild‐type (wt) strain, degradation products, including the C‐terminal protein ends, were present in the medium, indicating that proteolysis occurs during or after export to the cell surface; in one case, degradation was nearly total. In contrast, proteolysis was totally abolished in the htrA strain for all five proteins tested, and the yield of full‐length products was significantly increased. These results suggest that HtrALl is the sole extracellular protease that degrades abnormal exported proteins. In addition, our results reveal that HtrALl is needed for the pro‐peptide processing of a natural pro‐protein and for maturation of a native protein. We propose that in lactococci, and possibly in other Gram‐positive organisms with small sized‐genomes, a single surface protease, HtrA, is totally responsible for the housekeeping of exported proteins.