Exploring protein -glycosylation in ammonia-oxidizing archaea through glycoproteomic analysis.

UNLABELLED

Ammonia-oxidizing archaea of the phylum , formerly known as , are globally distributed and play critical roles in the nitrogen and carbon cycles, particularly in environments with low ammonia concentrations. Like most archaea, cells are enveloped by S-layer proteins, implicated in concentrating ammonium ions. These proteins are typically modified post-translationally by -glycans, which often play significant roles in various biological processes, including protein function regulation, protection from phages, and environmental adaptation. Nevertheless, the glycobiological characteristics of remain largely unexplored. Here, we investigated the glycoproteome of ammonia-oxidizing , specifically focusing on the terrestrial and the marine . Both species exhibited similar protein arrays throughout their growth phases, including those associated with -glycosylation. consistently exhibited -glycosylation predominantly on an S-layer protein and multicopper oxidase domain-containing proteins throughout all growth phases, with a marked increase during and after the late exponential phase. The glycan, characterized as a novel hexasaccharide with a chitobiose core, is hypothesized to play a role in nitrogen storage due to its probable nitrogen-rich composition, modifying asparagine residues within the conserved triplet sequence (Asn-X-Ser or -Thr). In contrast, also showed a high abundance of S-layer protein but displayed no apparent -glycosylation on any protein, suggesting variability in cell surface physical properties between these archaea. Despite similarities in their proteomes and energy metabolism, these two archaea exhibited significant differences in post-translational modification of proteins, revealing previously unrecognized diversity that may have implications for understanding their adaptive transitions to diverse environments.

IMPORTANCE

Autotrophic ammonia-oxidizing archaea of the phylum , formerly known as , are notoriously difficult to culture yet play important roles in the global nitrogen and carbon cycles. Inhabiting environments with extremely low ammonia concentrations, these archaea are expected to conserve ammonia strictly for energy production. However, using advanced liquid chromatography-tandem mass spectrometry and nuclear magnetic resonance techniques, we discovered that one of these archaea decorates its cell surface proteins with the most nitrogen-rich glycan identified to date, suggesting a previously unrecognized function of protein glycosylation in nitrogen storage. This newly identified -glycan, with a chitobiose core similar to those in and eukaryotes, not only deepens our understanding of archaeal evolution but also underscores the molecular adaptations enabling these archaea to thrive in diverse environments.