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

We identified an exported protease in Lactococcus lactis ssp. lactis strain IL1403 belonging to the HtrA/DegP family. Inactivation of the chromosomal gene (htrA(Ll)) encoding this protease (HtrA(Ll)) results in growth thermo- sensitivity at very high temperatures (above 37 degrees C for L. lactis). T he role of htrA(Ll) in extracellular proteolysis under normal growth condit ions 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, degrad ation products, including the C-terminal protein ends, were present in the medium, indicating that proteolysis occurs during or after export to the ce ll surface; in one case, degradation was nearly total. In contrast, proteol ysis was totally abolished in the htrA strain for all five proteins tested, and the yield of full-length products was significantly increased. These r esults suggest that HtrA(Ll) is the sole extracellular protease that degrad es abnormal exported proteins. In addition, our results reveal that HtrA(Ll ) 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 possibl y in other Gram-positive organisms with small sized-genomes, a single surfa ce protease, HtrA, is totally responsible for the housekeeping of exported proteins.