DIVERSITY OF CELL-ENVELOPE PROTEINASE SPECIFICITY AMONG STRAINS OF LACTOCOCCUS-LACTIS AND ITS RELATIONSHIP TO CHARGE CHARACTERISTICS OF THE SUBSTRATE-BINDING REGION

The biochemical and genetical diversity of the subtilisin-like cell envelope proteinase (CEP) among Lactococcus lactis strains was investigated. The specificities of the proteinases of 16 strains toward the important cheese peptide alpha(s1)-casein fragment 1 to 23 and toward two differently charged chromophoric peptides have been determined. On the basis of the results, these strains could be classified into seven groups. The contribution to the specificity of specific residues in the large C-terminal segment, which differentiates this proteinase from most other members of the subtilisin family, was established with hybrid proteinases, even in the case of the small substrates. These remote residues and the subtilisin-like substrate-binding region are therefore assumed to be spatially close to each other and together constitute most of the binding region of CEP. DNA sequence analysis of fragments of the gene (prtP) encoding segments of the proteinase which contain the relevant residues of the substrate-binding region shows that among the strains studied, this-binding region is the most negatively charged in the CEP group represented by strain HP and the most positively charged in the CEP group represented by strains AM1 and SK11. Consequently, these two proteinase groups show the most divergent specificities. Each of the proteinases of the other groups shows a different intermediate specificity which in part is the reflection of an intermediate charge in the binding region. However, the results suggest that amino acid residues outside the segments known to be part of the CEP-binding region also contribute to specificity. We propose a new nomenclature of CEPs which is based on their defined specificities toward the alpha(s1)-casein fragment 1 to 23 and on their origins.