Abstract
Lactobacillus casei and Lactobacillus rhamnosus proteins P40 and P75 belong to a large family of secreted cell wall proteins that contain a carboxy(C)-terminal CHAP or NlpC/P60 superfamily domains. In addition to their peptidoglycan hydrolases activity, proteins in this family are specific antigens of pathogens, frequently responsible of interactions with the host. L. rhamnosus GG and L. casei BL23 purified P40 and P75 proteins have antiapoptotic activity by inducing the EGF/Akt pathway. The aim of this work was to study the genetics, phylogeny and dissemination of this family of proteins in the genus Lactobacillus as well as their characteristics and likely function. The scrutiny of their DNA encoding sequences revealed the presence of minisatellite DNA in the P75 encoding gene of L. casei/paracasei strains (cmuB) with intraspecific indels that gave raise to four different alleles (cmuB1–4), which are exclusive of this species. Phylogenic analyses suggest that both proteins are present mainly in the L. casei and Lactobacillus sakei phylogenomic groups. A P40 ancestral gene was possibly present in the common ancestor of Enterococcaceae, Lactobacillaceae and Streptococcaceae. P75 is also present in L. casei and L. sakei groups, but its evolution is difficult to explain only by vertical transmission. Antibodies raised against the N-terminal regions of P40 and P75 improved their immunological detection in culture supernatants as they recognized almost exclusively proteins of L. casei/paracasei/rhamnosus strains, highlighting their structural similarity, that allowed to detect them in different fermented dairy products that contained probiotic L. casei strains. Purified P40 and P75 proteins showed no evident lytic activity but they complemented L. casei BL23 cmuA and cmuB defective mutants, respectively, thus proving that they actively participate in cell division.
Highlights
Proteins carrying an NlpC/P60 domain constitute a large and widely distributed superfamily of bacterial proteins
P40 and P75 proteins belong to a family of proteins carrying a C-terminal CHAP (NlpC/P60 superfamily) domain
The analysis indicates that P75 proteins of L. casei and L. sakei groups share a common origin but it cannot be ascertained whether vertical inheritance or horizontal gene transfer would explain the current distribution of these genes
Summary
Proteins carrying an NlpC/P60 domain constitute a large and widely distributed superfamily of bacterial proteins. An important subgroup contains the NlpC/P60 domain at the C-terminal region, and it includes phage and bacterial surface bound/secreted peptidoglycan (PGN) hydrolases, muramidases or lysins (Anantharaman and Aravind, 2003) They are typical antigens or pathogenicity factors in bacterial pathogens (Hess et al, 1995; Park et al, 2000; Teng et al, 2003; Oechslin et al, 2013). A PGN hydrolase from Streptococcus thermophilus, has a LysM N-terminal domain and it constitutes an interesting example of the complex events driving the likely evolution of these proteins In this protein, there is a central peptide sequence (Var-Cse) encoded by consecutive tandem nucleotide repeats (TR) that are highly variable within the species. This Var-Cse region is necessary for normal cell division (Borges et al, 2006)
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