Solution structure of Urm1 fromTrypanosoma brucei

Ubiquitin and ubiquitin-like conjugation systems in trypanosomatids

In eukaryotic cells, reversible attachment of ubiquitin and ubiquitin-like modifiers (Ubls) to specific target proteins is conducted by multicomponent systems whose collective actions control protein fate and cell behaviour in precise but complex ways. In trypanosomatids, attachment of ubiquitin and Ubls to target proteins regulates the cell cycle, endocytosis, protein sorting and degradation, autophagy and various aspects of infection and stress responses. The extent of these systems in trypanosomatids has been surveyed in recent reports, while in Leishmania mexicana, essential roles have been defined for many ubiquitin-system genes in deletion mutagenesis and life-cycle phenotyping campaigns. The first steps to elucidate the pathways of ubiquitin transfer among the ubiquitination components and to define the acceptor substrates and the downstream deubiquitinases are now being taken.

Ubiquitin-Like Modifiers: Emerging Regulators of Protozoan Parasites

Post-translational protein regulation allows for fine-tuning of cellular functions and involves a wide range of modifications, including ubiquitin and ubiquitin-like modifiers (Ubls). The dynamic balance of Ubl conjugation and removal shapes the fates of target substrates, in turn modulating various cellular processes. The mechanistic aspects of Ubl pathways and their biological roles have been largely established in yeast, plants, and mammalian cells. However, these modifiers may be utilised differently in highly specialised and divergent organisms, such as parasitic protozoa. In this review, we explore how these parasites employ Ubls, in particular SUMO, NEDD8, ATG8, ATG12, URM1, and UFM1, to regulate their unconventional cellular physiology. We discuss emerging data that provide evidence of Ubl-mediated regulation of unique parasite-specific processes, as well as the distinctive features of Ubl pathways in parasitic protozoa. We also highlight the potential to leverage these essential regulators and their cognate enzymatic machinery for development of therapeutics to protect against the diseases caused by protozoan parasites.

Solution structure of TbUfm1 from Trypanosoma brucei and its binding to TbUba5

Ubiquitin-like proteins are conserved in eukaryotes and involved in numerous cellular processes. Ufm1 is proved to play important roles in endoplasmic reticulum homeostasis, vesicle transportation and embryonic development. Enzyme cascade of Ufm1 is similar to that of ubiquitin. Mature Ufm1 is activated and conjugated to substrates by assistance of Ufm1 activating enzyme Uba5 (E1), Ufm1 conjugating enzyme Ufc1 (E2), and Ufm1 ligating enzyme Ufl1 (E3). Here, we determined the solution structure of TbUfm1 from Trypanosoma brucei (T. brucei) by NMR spectroscopy and explored the interactions between TbUfm1 and TbUba5/TbUfc1/TbUfl1. TbUfm1 adopts a typical β-grasp fold, which partially wraps a central α-helix and the other two helixes. NMR chemical shift perturbation indicated that TbUfm1 interacts with TbUba5 via a hydrophobic pocket formed by α1α2β1β2. Although the structure and Uba5-interaction mode of TbUfm1 are conserved in Ufm1 proteins, there are also some differences, which might reflect the potential diversity of Ufm1 in evolution and biological functions.

Solution structure of a ubiquitin-like protein from Trypanosoma brucei

Ubiquitin-like proteins, similar to ubiquitin, can either exist freely or be covalently attached to other proteins via an enzymatic cascade. The ubiquitin-like proteins play roles in multiple biological processes including transcription, stress responses, DNA repair and so on. In this study, a novel ubiquitin-like protein (TbUbl11) was identified in Trypanosoma brucei. The solution structure of TbUbl11 was solved by NMR spectroscopy. TbUbl11 adopts a conserved β-grasp fold composed by a five-stranded β-sheet curling around a central α-helix, similar to other ubiquitin-like proteins. Meanwhile, some differences between TbUbl11 and other ubiquitin-like proteins were also identified. Additionally, we revealed that TbUbl11 is located in the whole cell body of procyclic-form T. brucei.

1H, 13C and 15N resonance assignments of NEDD8 from Trypanosoma brucei

Neural precursor cell-expressed, developmently downregulated 8 (NEDD8) is a small ubiquitin-like modifier, which plays important roles in many cellular processes. Although it has been well studied in many eukaryotes, NEDD8 is still uncharacterized in some unicellular parasites, such as Trypanosoma brucei (T. brucei). Here we report the resonance assignments of NEDD8 from T. brucei.

The Protein Neddylation Pathway in Trypanosoma brucei: FUNCTIONAL CHARACTERIZATION AND SUBSTRATE IDENTIFICATION

Protein posttranslational modifications such as neddylation play crucial roles in regulating protein function. Only a few neddylated substrates have been validated to date, and the role of neddylation remains poorly understood. Here, using Trypanosoma brucei as the model organism, we investigated the function and substrates of TbNedd8. TbNedd8 is distributed throughout the cytosol but enriched in the nucleus and the flagellum. Depletion of TbNedd8 by RNAi abolished global protein ubiquitination, caused DNA re-replication in postmitotic cells, impaired spindle assembly, and compromised the flagellum attachment zone filament, leading to flagellum detachment. Through affinity purification and mass spectrometry, we identified 70 TbNedd8-conjugated and -associated proteins, including known Nedd8-conjugated and -associated proteins, putative TbNedd8 conjugation system enzymes, proteins of diverse biological functions, and proteins of unknown function. Finally, we validated six Cullins as bona fide TbNedd8 substrates and identified the TbNedd8 conjugation site in three Cullins. This work lays the foundation for understanding the roles of protein neddylation in this early divergent parasitic protozoan.

Solution structure of the Taf14 YEATS domain and its roles in cell growth of Saccharomyces cerevisiae

Chromatin modifications play important roles in cellular biological process. A novel conserved domain family, YEATS, has been discovered in a variety of eukaryotic species ranging from yeasts to humans. Taf14, which is involved in a few protein complexes of chromatin remodelling and gene transcription, and is essential for keeping chromosome stability, regular cell growth and transcriptional regulation, contains a YEATS domain at its N-terminus. In the present study, we determined the solution structure of the Taf14 YEATS domain using NMR spectroscopy. The Taf14 YEATS domain adopts a global fold of an elongated β-sandwich, similar to the Yaf9 YEATS domain. However, the Taf14 YEATS domain differs significantly from the Yaf9 YEATS domain in some aspects, which might indicate different structural classes of the YEATS domain family. Functional studies indicate that the YEATS domain is critical for the function of Taf14 in inhibiting cell growth under stress conditions. In addition, our results show that the C-terminus of Taf14 is responsible for its interaction with Sth1, which is an essential component of the RSC complex. Taken together, this implies that Taf14 is involved in transcriptional activation of Saccharomyces cerevisiae and the YEATS domain of Taf14 might play a negative role in cell growth.

Pup, a prokaryotic ubiquitin-like protein, is an intrinsically disordered protein

Pup (prokaryotic ubiquitin-like protein) from Mycobacterium tuberculosis is the first ubiquitin-like protein identified in non-eukaryotic cells. Although different ubiquitin-like proteins from eukaryotes share low sequence similarity, their 3D (three-dimensional) structures exhibit highly conserved typical ubiquitin-like folds. Interestingly, our studies reveal that Pup not only shares low sequence similarity, but also presents a totally distinguished structure compared with other ubiquitin-like superfamily proteins. Diverse structure predictions combined with CD and NMR spectroscopic studies all demonstrate that Pup is an intrinsically disordered protein. Moreover, 1H-15N NOE (nuclear Overhauser effect) data and CSI (chemical shift index) analyses indicate that there is a residual secondary structure at the C-terminus of Pup. In M. tuberculosis, Mpa (mycobacterium proteasomal ATPase) is the regulatory cap ATPase of the proteasome that interacts with Pup and brings the substrates to the proteasome for degradation. In the present paper, SPR (surface plasmon resonance) and NMR perturbation studies imply that the C-terminus of Pup, ranging from residues 30 to 59, binds to Mpa probably through a hydrophobic interface. In addition, phylogenetic analysis clearly shows that the Pup family belongs to a unique and divergent evolutionary branch, suggesting that it is the most ancient and deeply branched family among ubiquitin-like proteins. This might explain the structural distinction between Pup and other ubiquitin-like superfamily proteins.