Proteomics Uncovers Novel Components of an Interactive Protein Network Supporting RNA Export in Trypanosomes

A lineage-specific protein network at the trypanosome nuclear envelope

The nuclear envelope (NE) separates translation and transcription and is the location of multiple functions, including chromatin organization and nucleocytoplasmic transport. The molecular basis for many of these functions have diverged between eukaryotic lineages. Trypanosoma brucei, a member of the early branching eukaryotic lineage Discoba, highlights many of these, including a distinct lamina and kinetochore composition. Here, we describe a cohort of proteins interacting with both the lamina and NPC, which we term lamina-associated proteins (LAPs). LAPs represent a diverse group of proteins, including two candidate NPC-anchoring pore membrane proteins (POMs) with architecture conserved with S. cerevisiae and H. sapiens, and additional peripheral components of the NPC. While many of the LAPs are Kinetoplastid specific, we also identified broadly conserved proteins, indicating an amalgam of divergence and conservation within the trypanosome NE proteome, highlighting the diversity of nuclear biology across the eukaryotes, increasing our understanding of eukaryotic and NPC evolution.

Proteomic data of the Trypanosoma cruzi insect-dwelling epimastigotes overexpressing the RNA-binding protein UBP1

We present data on the proteome of the Trypanosoma cruzi epimastigote cells overexpressing the U-rich RNA-binding protein 1 (UBP1). The role of this regulatory protein during the epimastigote-to-metacyclic trypomastigote stage transition was clearly established by our group at the transcriptome level; nevertheless, the impact of UBP1 overexpression on protein synthesis is not known. To address this question, we performed shotgun label-free quantification proteomics using an in vitro system based on the tetracycline-inducible expression of TcUBP1 and epimastigote wildtype cells. Using tryptic peptide digestion and LC-MS/MS analysis with Orbitrap technology, this data file describes the proteome of three biological samples per condition and yields 1637 correctly quantified proteins. The statistical comparisons of the two analyzed groups within the Proteome Discoverer platform identified 379 differentially expressed proteins, with 207 being up-regulated and 172 being down-regulated. In addition, profile plots and heatmap analysis to visualize the distribution of protein abundances within replicates are also presented. Data are available via ProteomeXchange with identifier PXD047761.

Deviating from the norm: Nuclear organisation in trypanosomes

At first glance the nucleus is a highly conserved organelle. Overall nuclear morphology, the octagonal nuclear pore complex, the presence of peripheral heterochromatin and the nuclear envelope appear near constant features right down to the ultrastructural level. New work is revealing significant compositional divergence within these nuclear structures and their associated functions, likely reflecting adaptations and distinct mechanisms between eukaryotic lineages and especially the trypanosomatids. While many examples of mechanistic divergence currently lack obvious functional interpretations, these studies underscore the malleability of nuclear architecture. I will discuss some recent findings highlighting these facets within trypanosomes, together with the underlying evolutionary framework and make a call for the exploration of nuclear function in non-canonical experimental organisms.

Distinct mRNA and protein interactomes highlight functional differentiation of major eIF4F-like complexes from Trypanosoma brucei

Gene expression in pathogenic protozoans of the family Trypanosomatidae has several novel features, including multiple eIF4F-like complexes involved in protein synthesis. The eukaryotic eIF4F complex, formed mainly by eIF4E and eIF4G subunits, is responsible for the canonical selection of mRNAs required for the initiation of mRNA translation. The best-known complexes implicated in translation in trypanosomatids are based on two related pairs of eIF4E and eIF4G subunits (EIF4E3/EIF4G4 and EIF4E4/EIF4G3), whose functional distinctions remain to be fully described. Here, to define interactomes associated with both complexes in Trypanosoma brucei procyclic forms, we performed parallel immunoprecipitation experiments followed by identification of proteins co-precipitated with the four tagged eIF4E and eIF4G subunits. A number of different protein partners, including RNA binding proteins and helicases, specifically co-precipitate with each complex. Highlights with the EIF4E4/EIF4G3 pair include RBP23, PABP1, EIF4AI and the CRK1 kinase. Co-precipitated partners with the EIF4E3/EIF4G4 pair are more diverse and include DRBD2, PABP2 and different zinc-finger proteins and RNA helicases. EIF4E3/EIF4G4 are essential for viability and to better define their role, we further investigated their phenotypes after knockdown. Depletion of either EIF4E3/EIF4G4 mRNAs lead to aberrant morphology with a more direct impact on events associated with cytokinesis. We also sought to identify those mRNAs differentially associated with each complex through CLIP-seq with the two eIF4E subunits. Predominant among EIF4E4-bound transcripts are those encoding ribosomal proteins, absent from those found with EIF4E3, which are generally more diverse. RNAi mediated depletion of EIF4E4, which does not affect proliferation, does not lead to changes in mRNAs or proteins associated with EIF4E3, confirming a lack of redundancy and distinct roles for the two complexes.

Sending the message: specialized RNA export mechanisms in trypanosomes

Export of RNA from the nucleus is essential for all eukaryotic cells and has emerged as a major step in the control of gene expression. mRNA molecules are required to complete a complex series of processing events and pass a quality control system to protect the cytoplasm from the translation of aberrant proteins. Many of these events are highly conserved across eukaryotes, reflecting their ancient origin, but significant deviation from a canonical pathway as described from animals and fungi has emerged in the trypanosomatids. With significant implications for the mechanisms that control gene expression and hence differentiation, responses to altered environments and fitness as a parasite, these deviations may also reveal additional, previously unsuspected, mRNA export pathways.