The trypanosome Variant Surface Glycoprotein (VSG) mRNA is stabilized by an essential unconventional RNA-binding protein

ESTEBAN ERBEN

Congreso; MPM XXXI; 2020

Salivarian trypanosomes cause human sleeping sickness and economically important livestock diseases, transmitted by Tsetse flies and replicating extracellularly throughout the life cycle. The surface of "bloodstream forms", which live within mammals, is coated by a monolayer of a Variant Surface Glycoprotein (VSG). Switching of the expressed VSG gene enables the parasites to evade adaptive immunity. Adequate levels of VSG expression - 10% of total protein and 7% of mRNA - are partially attained through very active RNA polymerase I transcription and efficient mRNA processing, but a long mRNA half-life is also essential. Up until now, the mechanism by which VSG mRNA stability is maintained was unknown. Here, we apply an RNA-protein crosslink, RNA pulldown, and shotgun proteomics approach to identify proteins bound to VSG mRNA in Trypanosoma brucei. We find that an F-box protein, CFB2, binds specifically to VSG mRNAs and is absolutely required to maintain VSG mRNA levels. CFB2 in turn binds to a stabilizing complex that recruits poly(A) binding protein. VSG expression is essential not only for antigenic variation but also for cell division. Correspondingly, depletion of CFB2 causes loss of VSG mRNA, cell cycle arrest, dramatic morphological abnormalities and trypanosome death. Since in these organisms the fate of mRNAs is largely influenced by RNA-binding proteins, we envision that this kind of approach will provide a platform for investigating RNA in vivo regulation.