Enzyme from an Uncultivated Sponge Bacterium Catalyzes S-Methylation in a Ribosomal Peptide.

Amino acid modifications are essential for the structural diversity and bioactivity of ribosomally synthesized and post-translationally modified peptide natural products (RiPPs). A particularly large and virtually untapped pool of unusual RiPPs and associated modifying enzymes is provided by uncultivated bacteria. An example is the chemically rich sponge symbiont "Candidatus Entotheonella factor", which produces the hypermodified polytheonamides of the poorly studied proteusin RiPP family. In addition to the polytheonamide genes, "E. factor" contains several further additional RiPP clusters of unknown function. Here we provide insights into one of these cryptic proteusin pathways by identifying an enzyme (PtyS) that catalyzes the S-methylation of cysteine residues. S-methylcysteine is rare in natural peptides and proteins, and the enzymatic activity was previously unknown for RiPPs, thus adding a new modification to the ribosomal peptide toolbox.

Tonsillar CD56brightNKG2A+ NK cells restrict primary Epstein-Barr virus infection in B cells via IFN-γ.

Natural killer (NK) cells constitute the first line of defense against viruses and cancers cells. Epstein-Barr virus (EBV) was the first human virus to be directly implicated in carcinogenesis, and EBV infection is associated with a broad spectrum of B cell lymphomas. How NK cells restrict EBV-associated oncogenesis is not understood. Here, we investigated the efficacies and mechanisms of distinct NK cell subsets from tonsils, the portal of entry of EBV, in limiting EBV infection in naïve, germinal center-associated and memory B cells. We found that CD56bright and NKG2A expression sufficiently characterizes the potent anti-EBV capacity of tonsillar NK cells. We observed restriction of EBV infection in B cells as early as 18 hours after infection. The restriction was most efficient in naïve B cells and germinal center-associated B cells, the B cell subsets that exhibited highest susceptibility to EBV infection in vitro. IFN-γ release by and partially NKp44 engagement of CD56bright and NKG2A positive NK cells mediated the restriction that eventually inhibited B-cell transformation. Thus, harnessing CD56brightNKG2A+ NK cell function might be promising to improve treatment strategies that target EBV-associated B cell lymphomas.