Molecular diversity and functional roles of fibrillarin: Comparative insights across trypanosomatids and other model...

Fibrillarin is an evolutionarily conserved nucleolar protein and the catalytic core of the box C/D small nucleolar ribonucleoprotein (snoRNP) complex, responsible for site-specific 2'-O-methylation of ribosomal RNA. Beyond its canonical role in ribosome biogenesis, fibrillarin contributes to chromatin organization, RNA surveillance, and stress adaptation, reflecting its multifunctional nature. In trypanosomatids, fibrillarin shows remarkable diversification. These organisms encode up to three paralogs with conserved catalytic domains but divergent regulatory features, suggesting specialized functions. They also possess an unusually large repertoire of box C/D snoRNAs and lineage-specific rRNA methylation patterns, pointing to regulated ribosome heterogeneity. This review summarizes current knowledge of fibrillarin's structure and functions across eukaryotes, emphasizing recent findings in trypanosomatids. By integrating molecular and evolutionary perspectives, it highlights key gaps, such as paralog specialization, snoRNP composition, and potential extraribosomal roles, and outlines research directions that may reveal new principles of nucleolar organization as well as selective translation control in early-branching eukaryotes.