Structural and Functional Characterization of the Signaling Molecule Autoinducer-2 Synthesis Enzyme S-Ribosylhomocysteinase in Limosilactobacillus fermentum

S-ribosylhomocysteinase (also referred to as LuxS protein) is an important enzyme involved in the biosynthesis of autoinducer-2 (AI-2) as a signaling molecule. The physicochemical properties, hydrophilicity, hydrophobicity, signaling peptides, transmembrane structure, phosphorylation sites, structural domains and spatial structure of the LuxS protein in Limosilactobacillus fermentum A51 were studied by bioinformatics. In addition, its structural and functional properties were explored by protein-protein interaction (PPI) network analysis. The results showed that the LuxS protein was encoded by 158 amino acids, with a molecular mass of 17 718.91 Da, a theoretical isoelectric point of 5.29, an instability coefficient of 32.16, and it was an acidic, stable and hydrophilic protein. The LuxS protein, without signal peptide or transmembrane domain, was presumed to mainly play an intracellular role as an endocrine protein, and it had 16 phosphorylation sites, belonging to the LuxS superfamily, and contained one PRK02260 structural domain. The LuxS protein was a relatively stable protein with a secondary structure consisting mainly of 39.87% random coils and 31.65% α-helices, and its three-dimensional structure was folded into a baseball-like structure. The results of PPI network showed that the LuxS protein mainly interacted with pfs, metE, metC, metC-2, yhcE, mmuM, patB and cysK, and were involved in the metabolism and synthesis of cysteine, methionine, and sulfur-containing amino acids, suggesting that it is involved in the activation of the activated methyl cycle and thus regulates the quorum sensing system and the metabolism and synthesis of functional substances in the strain. This study provides a theoretical basis for an in-depth study of LuxS protein regulation of the AI-2 quorum sensing system in L. fermentum.