The trypanosome homolog of human p32 interacts with RBP16 and stimulates its gRNA binding activity

DRBD18 acts as a transcript-specific RNA editing auxiliary factor in Trypanosoma brucei

Uridine insertion/deletion (U-indel) RNA editing of mitochondrial transcripts is a posttranscriptional modification in kinetoplastid organisms, resulting in the generation of mature mRNAs from cryptic precursors. This RNA editing process involves a multiprotein complex holoenzyme and multiple accessory factors. Recent investigations have highlighted the pivotal involvement of accessory RNA-binding proteins (RBPs) in modulating RNA editing in Trypanosoma brucei, often in a transcript-specific manner. DRBD18 is a multifunctional RBP that reportedly impacts the stability, processing, export, and translation of nuclear-encoded mRNAs. However, mass spectrometry studies report DRBD18-RESC interactions, prompting us to investigate its role in mitochondrial U-indel RNA editing. In this study, we demonstrate the specific and RNase-sensitive interaction of DRBD18 with multiple RESC factors. Depletion of DRBD18 through RNA interference in procyclic form T. brucei leads to a significant reduction in the levels of edited A6 and COIII mitochondrial transcripts, whereas its overexpression causes a notable increase in the abundance of these edited mRNAs. RNA immunoprecipitation/qRT-PCR analysis indicates a direct role for DRBD18 in A6 and COIII mRNA editing. We also examined the impact of arginine methylation of DRBD18 in the editing process, revealing that the hypomethylated form of DRBD18, rather than the arginine-methylated version, is essential for promoting these editing events. In conclusion, our findings demonstrate that DRBD18 directly affects the editing of A6 and COIII mRNAs, with its function being modulated by its arginine methylation status, marking the first report of a mitochondrial function for this protein and identifying it as a newly characterized RNA editing auxiliary factor.

A role for a Trypanosoma brucei cytosine RNA methyltransferase homolog in ribosomal RNA processing

In Trypanosoma brucei, gene expression is primarily regulated posttranscriptionally making RNA metabolism critical. T. brucei has an epitranscriptome containing modified RNA bases. Yet, the identity of the enzymes catalyzing modified RNA base addition and the functions of the enzymes and modifications remain unclear. Homology searches indicate the presence of numerous T. brucei cytosine RNA methyltransferase homologs. One such homolog, TbNop2 was studied in detail. TbNop2 contains the six highly conserved motifs found in cytosine RNA methyltransferases and is evolutionarily related to the Nop2 protein family required for rRNA modification and processing. RNAi experiments targeting TbNop2 resulted in reduced levels of TbNop2 RNA and protein, and a cessation of parasite growth. Next generation sequencing of bisulfite-treated RNA (BS-seq) detected the presence of two methylation sites in the large rRNA; yet TbNop2 RNAi did not result in a significant reduction of methylation. However, TbNop2 RNAi resulted in the retention of 28S internal transcribed spacer RNAs, indicating a role for TbNop2 in rRNA processing.

Translational control by Trypanosoma brucei DRBD18 contributes to the maintenance of the procyclic state

Trypanosoma brucei occupies distinct niches throughout its life cycle, within both the mammalian and tsetse fly hosts. The immunological and biochemical complexity and variability of each of these environments require a reshaping of the protein landscape of the parasite both to evade surveillance and face changing metabolic demands. In kinetoplastid protozoa, including T. brucei, posttranscriptional control mechanisms are the primary means of gene regulation, and these are often mediated by RNA-binding proteins. DRBD18 is a T. brucei RNA-binding protein that reportedly interacts with ribosomal proteins and translation factors. Here, we tested a role for DRBD18 in translational control. We validate the DRBD18 interaction with translating ribosomes and the translation initiation factor, eIF3a. We further show that DRBD18 depletion by RNA interference leads to altered polysomal profiles with a specific depletion of heavy polysomes. Ribosome profiling analysis reveals that 101 transcripts change in translational efficiency (TE) upon DRBD18 depletion: 41 exhibit decreased TE and 60 exhibit increased TE. A further 66 transcripts are buffered, that is, changes in transcript abundance are compensated by changes in TE such that the total translational output is expected not to change. In DRBD18-depleted cells, a set of transcripts that codes for procyclic form-specific proteins is translationally repressed while, conversely, transcripts that code for bloodstream form- and metacyclic form-specific proteins are translationally enhanced. RNA immunoprecipitation/qRT-PCR indicates that DRBD18 associates with members of both repressed and enhanced cohorts. These data suggest that DRBD18 contributes to the maintenance of the procyclic state through both positive and negative translational control of specific mRNAs.

Translational control by Trypanosoma brucei DRBD18 contributes to the maintenance of the procyclic state

Trypanosoma brucei occupies distinct niches throughout its life cycle, within both the mammalian and tsetse fly hosts. The immunological and biochemical complexity and variability of each of these environments require a reshaping of the protein landscape of the parasite both to evade surveillance and face changing metabolic demands. Whereas most well-studied organisms rely on transcriptional control as the main regulator of gene expression, post-transcriptional control mechanisms are particularly important in T. brucei , and these are often mediated by RNA-binding proteins. DRBD18 is a T. brucei RNA-binding protein that interacts with ribosomal proteins and translation factors. Here, we tested a role for DRBD18 in translational control. We show that DRBD18 depletion by RNA interference leads to altered polysomal profiles with a specific depletion of heavy polysomes. Ribosome profiling analysis reveals that 101 transcripts change in translational efficiency (TE) upon DRBD18 depletion: 41 exhibit decreased TE and 60 exhibit increased TE. A further 66 transcripts are buffered, i.e . changes in transcript abundance are compensated by changes in TE such that the total translational output is expected not to change. Proteomic analysis validates these data. In DRBD18-depleted cells, a cohort of transcripts that codes for procyclic form-specific proteins is translationally repressed while, conversely, transcripts that code for bloodstream form- and metacyclic form-specific proteins are translationally enhanced. These data suggest that DRBD18 contributes to the maintenance of the procyclic state through both positive and negative translational control of specific mRNAs.

Selective nuclear export of mRNAs is promoted by DRBD18 in Trypanosoma brucei

Kinetoplastids, including Trypanosoma brucei, control gene expression primarily at the posttranscriptional level. Nuclear mRNA export is an important, but understudied, step in this process. The general heterodimeric export factors, Mex67/Mtr2, function in the export of mRNAs and tRNAs in T. brucei, but RNA binding proteins (RBPs) that regulate export processes by controlling the dynamics of Mex67/Mtr2 ribonucleoprotein formation or transport have not been identified. Here, we report that DRBD18, an essential and abundant T. brucei RBP, associates with Mex67/Mtr2 in vivo, likely through its direct interaction with Mtr2. DRBD18 downregulation results in partial accumulation of poly(A)⁺ mRNA in the nucleus, but has no effect on the localization of intron-containing or mature tRNAs. Comprehensive analysis of transcriptomes from whole-cell and cytosol in DRBD18 knockdown parasites demonstrates that depletion of DRBD18 leads to impairment of nuclear export of a subset of mRNAs. CLIP experiments reveal the association of DRBD18 with several of these mRNAs. Moreover, DRBD18 knockdown leads to a partial accumulation of the Mex67/Mtr2 export receptors in the nucleus. Taken together, the current study supports a model in which DRBD18 regulates the selective nuclear export of mRNAs by promoting the mobilization of export competent mRNPs to the cytosol through the nuclear pore complex.

Trypanosome RNAEditing Substrate Binding Complex integrity and function depends on the upstream action of RESC10

Uridine insertion/deletion editing of mitochondrial mRNAs is a characteristic feature of kinetoplastids, including Trypanosoma brucei. Editing is directed by trans-acting gRNAs and catalyzed by related RNA Editing Core Complexes (RECCs). The non-catalytic RNA Editing Substrate Binding Complex (RESC) coordinates interactions between RECC, gRNA and mRNA. RESC is a dynamic complex comprising GRBC (Guide RNA Binding Complex) and heterogeneous REMCs (RNA Editing Mediator Complexes). Here, we show that RESC10 is an essential, low abundance, RNA binding protein that exhibits RNase-sensitive and RNase-insensitive interactions with RESC proteins, albeit its minimal in vivo interaction with RESC13. RESC10 RNAi causes extensive RESC disorganization, including disruption of intra-GRBC protein-protein interactions, as well as mRNA depletion from GRBC and accumulation on REMCs. Analysis of mitochondrial RNAs at single nucleotide resolution reveals transcript-specific effects: RESC10 dramatically impacts editing progression in pan-edited RPS12 mRNA, but is critical for editing initiation in mRNAs with internally initiating gRNAs, pointing to distinct initiation mechanisms for these RNA classes. Correlations between sites at which editing pauses in RESC10 depleted cells and those in knockdowns of previously studied RESC proteins suggest that RESC10 acts upstream of these factors and that RESC is particularly important in promoting transitions between uridine insertion and deletion RECCs.

Divergent Small Tim Homologues Are Associated with TbTim17 and Critical for the Biogenesis of TbTim17 Protein Complexes in Trypanosoma brucei

The small Tim proteins belong to a group of mitochondrial intermembrane space chaperones that aid in the import of mitochondrial inner membrane proteins with internal targeting signals. Trypanosoma brucei, the protozoan parasite that causes African trypanosomiasis, possesses multiple small Tim proteins that include homologues of T. brucei Tim9 (TbTim9) and Tim10 (TbTim10) and a unique small Tim that shares homology with both Tim8 and Tim13 (TbTim8/13). Here, we found that these three small TbTims are expressed as soluble mitochondrial intermembrane space proteins. Coimmunoprecipitation and mass spectrometry analysis showed that the small TbTims stably associated with each other and with TbTim17, the major component of the mitochondrial inner membrane translocase in T. brucei Yeast two-hybrid analysis indicated direct interactions among the small TbTims; however, their interaction patterns appeared to be different from those of their counterparts in yeast and humans. Knockdown of the small TbTims reduced cell growth and decreased the steady-state level of TbTim17 and T. brucei ADP/ATP carrier (TbAAC), two polytopic mitochondrial inner membrane proteins. Knockdown of small TbTims also reduced the matured complexes of TbTim17 in mitochondria. Depletion of any of the small TbTims reduced TbTim17 import moderately but greatly hampered the stability of the TbTim17 complexes in T. brucei Altogether, our results revealed that TbTim9, TbTim10, and TbTim8/13 interact with each other, associate with TbTim17, and play a crucial role in the integrity and maintenance of the levels of TbTim17 complexes.IMPORTANCETrypanosoma brucei is the causative agent of African sleeping sickness. The parasite's mitochondrion represents a useful source for potential chemotherapeutic targets. Similarly to yeast and humans, mitochondrial functions depend on the import of proteins that are encoded in the nucleus and made in the cytosol. Even though the machinery involved in this mitochondrial protein import process is becoming clearer in T. brucei, a comprehensive picture of protein complex composition and function is still lacking. In this study, we characterized three T. brucei small Tim proteins, TbTim9, TbTim10, and TbTim8/13. Although the parasite does not have the classical TIM22 complex that imports mitochondrial inner membrane proteins containing internal targeting signals in yeast or humans, we found that these small TbTims associate with TbTim17, the major subunit of the TbTIM complex in T. brucei, and play an essential role in the stability of the TbTim17 complexes. Therefore, these divergent proteins are critical for mitochondrial protein biogenesis in T. brucei.

Y-box protein-associated acidic protein (YBAP1/C1QBP) affects the localization and cytoplasmic functions of YB-1

The Y-box proteins are multifunctional nucleic acid-binding proteins involved in various aspects of gene regulation. The founding member of the Y-box protein family, YB-1, functions as a transcription factor as well as a principal component of messenger ribonucleoprotein particles (mRNPs) in somatic cells. The nuclear level of YB-1 is well correlated with poor prognosis in many human cancers. Previously, we showed that a Y-box protein–associated acidic protein, YBAP1, which is identical to complement component 1, q subcomponent-binding protein (C1QBP, also called gC1qR, hyaluronan-binding protein 1 [HABP1] or ASF/SF2-associated protein p32), relieves translational repression by YB-1. Here we show that the nuclear localization of YB-1 harboring a point mutation in the cold shock domain was inhibited when co-expressed with YBAP1, whereas cytoplasmic accumulation of the wild-type YB-1 was not affected. We showed that YBAP1 inhibited the interaction between YB-1 and transportin 1. In the cytoplasm, YBAP1 affected the accumulation of YB-1 to processing bodies (P-bodies) and partially abrogated the mRNA stabilization by YB-1. Our results, indicating that YBAP1/C1QBP regulates the nucleo-cytoplasmic distribution of YB-1 and its cytoplasmic functions, are consistent with a model that YBAP1/C1QBP acts as an mRNP remodeling factor.

MRB7260 is essential for productive protein-RNA interactions within the RNA editing substrate binding complex during trypanosome RNA editing

The trypanosome RNA editing substrate binding complex (RESC) acts as the platform for mitochondrial uridine insertion/deletion RNA editing and facilitates the protein-protein and protein-RNA interactions required for the editing process. RESC is broadly comprised of two subcomplexes: GRBC (guide RNA binding complex) and REMC (RNA editing mediator complex). Here, we characterize the function and position in RESC organization of a previously unstudied RESC protein, MRB7260. We show that MRB7260 forms numerous RESC-related complexes, including a novel, small complex with the guide RNA binding protein, GAP1, which is a canonical GRBC component, and REMC components MRB8170 and TbRGG2. RNA immunoprecipitations in MRB7260-depleted cells show that MRB7260 is critical for normal RNA trafficking between REMC and GRBC. Analysis of protein-protein interactions also reveals an important role for MRB7260 in promoting stable association of the two subcomplexes. High-throughput sequencing analysis of RPS12 mRNAs from MRB7260 replete and depleted cells demonstrates that MRB7260 is critical for gRNA exchange and early gRNA utilization, with the exception of the initiating gRNA. Together, these data demonstrate that MRB7260 is essential for productive protein-RNA interactions with RESC during RNA editing.

The Major Protein Arginine Methyltransferase in Trypanosoma brucei Functions as an Enzyme-Prozyme Complex

Prozymes are catalytically inactive enzyme paralogs that dramatically stimulate the function of weakly active enzymes through complex formation. The two prozymes described to date reside in the polyamine biosynthesis pathway of the human parasite Trypanosoma brucei, an early branching eukaryote that lacks transcriptional regulation and regulates its proteome through posttranscriptional and posttranslational means. Arginine methylation is a common posttranslational modification in eukaryotes catalyzed by protein arginine methyltransferases (PRMTs) that are typically thought to function as homodimers. We demonstrate that a major T. brucei PRMT, TbPRMT1, functions as a heterotetrameric enzyme-prozyme pair. The inactive PRMT paralog, TbPRMT1^PRO, is essential for catalytic activity of the TbPRMT1^ENZ subunit. Mutational analysis definitively demonstrates that TbPRMT1^ENZ is the cofactor-binding subunit and carries all catalytic activity of the complex. Our results are the first demonstration of an obligate heteromeric PRMT, and they suggest that enzyme-prozyme organization is expanded in trypanosomes as a posttranslational means of enzyme regulation.

Interactions between RNA-binding proteins and P32 homologues in trypanosomes and human cells

RNA-binding proteins (RBPs) are involved in many aspects of mRNA metabolism such as splicing, nuclear export, translation, silencing, and decay. To cope with these tasks, these proteins use specialized domains such as the RNA recognition motif (RRM), the most abundant and widely spread RNA-binding domain. Although this domain was first described as a dedicated RNA-binding moiety, current evidence indicates these motifs can also engage in direct protein-protein interactions. Here, we discuss recent evidence describing the interaction between the RRM of the trypanosomatid RBP UBP1 and P22, the homolog of the human multifunctional protein P32/C1QBP. Human P32 was also identified while performing a similar interaction screening using both RRMs of TDP-43, an RBP involved in splicing regulation and Amyotrophic Lateral Sclerosis. Furthermore, we show that this interaction is mediated by RRM1. The relevance of this interaction is discussed in the context of recent TDP-43 interactomic approaches that identified P32, and the numerous evidences supporting interactions between P32 and RBPs. Finally, we discuss the vast universe of interactions involving P32, supporting its role as a molecular chaperone regulating the function of its ligands.

Association of UBP1 to ribonucleoprotein complexes is regulated by interaction with the trypanosome ortholog of the human multifunctional P32 protein

Regulation of gene expression in trypanosomatid parasitic protozoa is mainly achieved posttranscriptionally. RNA-binding proteins (RBPs) associate to 3' untranslated regions in mRNAs through dedicated domains such as the RNA recognition motif (RRM). Trypanosoma cruzi UBP1 (TcUBP1) is an RRM-type RBP involved in stabilization/degradation of mRNAs. TcUBP1 uses its RRM to associate with cytoplasmic mRNA and to mRNA granules under starvation stress. Here, we show that under starvation stress, TcUBP1 is tightly associated with condensed cytoplasmic mRNA granules. Conversely, under high nutrient/low density-growing conditions, TcUBP1 ribonucleoprotein (RNP) complexes are lax and permeable to mRNA degradation and disassembly. After dissociating from mRNA, TcUBP1 can be phosphorylated only in unstressed parasites. We have identified TcP22, the ortholog of mammalian P32/C1QBP, as an interactor of TcUBP1 RRM. Overexpression of TcP22 decreased the number of TcUBP1 granules in starved parasites in vivo. Endogenous TcUBP1 RNP complexes could be dissociated in vitro by addition of recombinant TcP22, a condition stimulating TcUBP1 phosphorylation. Biochemical and in silico analysis revealed that TcP22 interacts with the RNA-binding surface of TcUBP1 RRM. We propose a model for the decondensation of TcUBP1 RNP complexes in T. cruzi through direct interaction with TcP22 and phosphorylation.

Arginine methylation of DRBD18 differentially impacts its opposing effects on the trypanosome transcriptome

Arginine methylation is a posttranslational modification that impacts wide-ranging cellular functions, including transcription, mRNA splicing and translation. RNA binding proteins (RBPs) represent one of the largest classes of arginine methylated proteins in both mammals and the early diverging parasitic protozoan, Trypanosoma brucei. Here, we report the effects of arginine methylation on the functions of the essential and previously uncharacterized T. brucei RBP, DRBD18. RNAseq analysis shows that DRBD18 depletion causes extensive rearrangement of the T. brucei transcriptome, with increases and decreases in hundreds of mRNAs. DRBD18 contains three methylated arginines, and we used complementation of DRBD18 knockdown cells with methylmimic or hypomethylated DRBD18 to assess the functions of these methylmarks. Methylmimic and hypomethylated DRBD18 associate with different ribonucleoprotein complexes. These altered macromolecular interactions translate into differential impacts on the T. brucei transcriptome. Methylmimic DRBD18 preferentially stabilizes target RNAs, while hypomethylated DRBD18 is more efficient at destabilizing RNA. The protein arginine methyltransferase, TbPRMT1, interacts with DRBD18 and knockdown of TbPRMT1 recapitulates the effects of hypomethylated DRBD18 on mRNA levels. Together, these data support a model in which arginine methylation acts as a switch that regulates T. brucei gene expression.

An arginine-glycine-rich RNA binding protein impacts the abundance of specific mRNAs in the mitochondria of Trypanosoma brucei

In kinetoplastid parasites, regulation of mitochondrial gene expression occurs posttranscriptionally via RNA stability and RNA editing. In addition to the 20S editosome that contains the enzymes required for RNA editing, a dynamic complex called the mitochondrial RNA binding 1 (MRB1) complex is also essential for editing. Trypanosoma brucei RGG3 (TbRGG3) was originally identified through its interaction with the guide RNA-associated proteins 1 and 2 (GAP1/2), components of the MRB1 complex. Both the arginine-glycine-rich character of TbRGG3, which suggests a function in RNA binding, and its interaction with MRB1 implicate TbRGG3 in mitochondrial gene regulation. Here, we report an in vitro and in vivo characterization of TbRGG3 function in T. brucei mitochondria. We show that in vitro TbRGG3 binds RNA with broad sequence specificity and has the capacity to modulate RNA-RNA interactions. In vivo, inducible RNA interference (RNAi) studies demonstrate that TbRGG3 is essential for proliferation of insect vector stage T. brucei. TbRGG3 ablation does not cause a defect in RNA editing but, rather, specifically affects the abundance of two preedited transcripts as well as their edited counterparts. Protein-protein interaction studies show that TbRGG3 associates with GAP1/2 apart from the remainder of the MRB1 complex, as well as with several non-MRB1 proteins that are required for mitochondrial RNA editing and/or stability. Together, these studies demonstrate that TbRGG3 is an essential mitochondrial gene regulatory factor that impacts the stabilities of specific RNAs.

Functional interplay between protein arginine methyltransferases in Trypanosoma brucei

Arginine methylation is a common posttranslational modification that has far-reaching cellular effects. Trypanosoma brucei is an early-branching eukaryote with four characterized protein arginine methyltransferases (PRMTs), one additional putative PRMT, and over 800 arginine methylated proteins, suggesting that arginine methylation has widespread impacts in this organism. While much is known about the activities of individual T. brucei PRMTs (TbPRMTs), little is known regarding how TbPRMTs function together in vivo. In this study, we analyzed single and selected double TbPRMT knockdowns for the impact on expression of TbPRMTs and global methylation status. Repression of TbPRMT1 caused a decrease in asymmetric dimethylarginine and a marked increase in monomethylarginine that was catalyzed by TbPRMT7, suggesting that TbPRMT1 and TbPRMT7 can compete for the same substrate. We also observed an unexpected and strong interdependence between TbPRMT1 and TbPRMT3 protein levels. This finding, together with the observation of similar methyl landscape profiles in TbPRMT1 and TbPRMT3 repressed cells, strongly suggests that these two enzymes form a functional complex. We show that corepression of TbPRMT6/7 synergistically impacts growth of procyclic-form T. brucei. Our findings also implicate the actions of noncanonical, and as yet unidentified, PRMTs in T. brucei. Together, our studies indicate that TbPRMTs display a functional interplay at multiple levels.

Identification of a novel lipin homologue from the parasitic protozoan Trypanosoma brucei

Background

Arginine methylation is a post-translational modification that expands the functional diversity of proteins. Kinetoplastid parasites contain a relatively large group of protein arginine methyltransferases (PRMTs) compared to other single celled eukaryotes. Several T. brucei proteins have been shown to serve as TbPRMT substrates in vitro, and a great number of proteins likely to undergo methylation are predicted by the T. brucei genome. This indicates that a large number of proteins whose functions are modulated by arginine methylation await discovery in trypanosomes. Here, we employed a yeast two-hybrid screen using as bait the major T. brucei type I PRMT, TbPRMT1, to identify potential substrates of this enzyme.

Results

We identified a protein containing N-LIP and C-LIP domains that we term TbLpn. These domains are usually present in a family of proteins known as lipins, and involved in phospholipid biosynthesis and gene regulation. Far western and co-immunoprecipitation assays confirmed the TbPRMT1-TbLpn interaction. We also demonstrated that TbLpn is localized mainly to the cytosol, and is methylated in vivo. In addition, we showed that, similar to mammalian and yeast proteins with N-LIP and C-LIP domains, recombinant TbLpn exhibits phosphatidic acid phosphatase activity, and that two conserved aspartic acid residues present in the C-LIP domain are critical for its enzymatic activity.

Conclusions

This study reports the characterization of a novel trypanosome protein and provides insight into its enzymatic activity and function in phospholipid biosynthesis. It also indicates that TbLpn functions may be modulated by arginine methylation.

MRB3010 is a core component of the MRB1 complex that facilitates an early step of the kinetoplastid RNA editing process

Gene expression in the mitochondria of the kinetoplastid parasite Trypanosoma brucei is regulated primarily post-transcriptionally at the stages of RNA processing, editing, and turnover. The mitochondrial RNA-binding complex 1 (MRB1) is a recently identified multiprotein complex containing components with distinct functions during different aspects of RNA metabolism, such as guide RNA (gRNA) and mRNA turnover, precursor transcript processing, and RNA editing. In this study we examined the function of the MRB1 protein, Tb927.5.3010, which we term MRB3010. We show that MRB3010 is essential for growth of both procyclic form and bloodstream form life-cycle stages of T. brucei. Down-regulation of MRB3010 by RNAi leads to a dramatic inhibition of RNA editing, yet its depletion does not impact total gRNA levels. Rather, it appears to affect the editing process at an early stage, as indicated by the accumulation of pre-edited and small partially edited RNAs. MRB3010 is present in large (>20S) complexes and exhibits both RNA-dependent and RNA-independent interactions with other MRB1 complex proteins. Comparison of proteins isolated with MRB3010 tagged at its endogenous locus to those reported from other MRB1 complex purifications strongly suggests the presence of an MRB1 "core" complex containing five to six proteins, including MRB3010. Together, these data further our understanding of the function and composition of the imprecisely defined MRB1 complex.

Endonuclease associations with three distinct editosomes in Trypanosoma brucei

Three distinct editosomes, typified by mutually exclusive KREN1, KREN2, or KREN3 endonucleases, are essential for mitochondrial RNA editing in Trypanosoma brucei. The three editosomes differ in substrate endoribonucleolytic cleavage specificity, which may reflect the vast number of editing sites that need insertion or deletion of uridine nucleotides (Us). Each editosome requires the single RNase III domain in each endonuclease for catalysis. Studies reported here show that the editing endonucleases do not form homodimeric domains, and may therefore function as intermolecular heterodimers, perhaps with KREPB4 and/or KREPB5. Editosomes isolated via TAP tag fused to KREPB6, KREPB7, or KREPB8 have a common set of 12 proteins. In addition, KREN3 is only found in KREPB6 editosomes, KREN2 is only found in KREPB7 editosomes, and KREN1 is only found in KREPB8 editosomes. These are the same associations previously found in editosomes isolated via the TAP-tagged endonucleases KREN1, KREN2, or KREN3. Furthermore, TAP-tagged KREPB6, KREPB7, and KREPB8 complexes isolated from cells in which expression of their respective endonuclease were knocked down were disrupted and lacked the heterotrimeric insertion subcomplex (KRET2, KREPA1, and KREL2). These results and published data suggest that KREPB6, KREPB7, and KREPB8 associate with the deletion subcomplex, whereas the KREN1, KREN2, and KREN3 endonucleases associate with the insertion subcomplex.

TbRGG2 facilitates kinetoplastid RNA editing initiation and progression past intrinsic pause sites

TbRGG2 is an essential kinetoplastid RNA editing accessory factor that acts specifically on pan-edited RNAs. To understand the mechanism of TbRGG2 action, we undertook an in-depth analysis of edited RNA populations in TbRGG2 knockdown cells and an in vitro examination of the biochemical activities of the protein. We demonstrate that TbRGG2 down-regulation more severely impacts editing at the 5' ends of pan-edited RNAs than at their 3' ends. The initiation of editing is reduced to some extent in TbRGG2 knockdown cells. In addition, TbRGG2 plays a post-initiation role as editing becomes stalled in TbRGG2-depleted cells, resulting in an overall decrease in the 3' to 5' progression of editing. Detailed analyses of edited RNAs from wild-type and TbRGG2-depleted cells reveal that TbRGG2 facilitates progression of editing past intrinsic pause sites that often correspond to the 3' ends of cognate guide RNAs (gRNAs). In addition, noncanonically edited junction regions are either absent or significantly shortened in TbRGG2-depleted cells, consistent with impaired gRNA transitions. Sequence analysis further suggests that TbRGG2 facilitates complete utilization of certain gRNAs. In vitro RNA annealing and in vivo RNA unwinding assays demonstrate that TbRGG2 can modulate RNA-RNA interactions. Collectively, these data are consistent with a model in which TbRGG2 facilitates initiation and 3' to 5' progression of editing through its ability to affect gRNA utilization, both during the transition between specific gRNAs and during usage of certain gRNAs.

Structure of the Trypanosoma brucei p22 protein, a cytochrome oxidase subunit II-specific RNA-editing accessory factor

Kinetoplastid RNA (k-RNA) editing is a complex process in the mitochondria of kinetoplastid protozoa, including Trypanosoma brucei, that involves the guide RNA-directed insertion and deletion of uridines from precursor-mRNAs to produce mature, translatable mRNAs. k-RNA editing is performed by multiprotein complexes called editosomes. Additional non-editosome components termed k-RNA-editing accessory factors affect the extent of editing of specific RNAs or classes of RNAs. The T. brucei p22 protein was identified as one such accessory factor. Here we show that p22 contributes to cell growth in the procyclic form of T. brucei and functions as a cytochrome oxidase subunit II-specific k-RNA-editing accessory factor. To gain insight into its functions, we solved the crystal structure of the T. brucei p22 protein to 2.0-A resolution. The p22 structure consists of a six-stranded, antiparallel beta-sheet flanked by five alpha-helices. Three p22 subunits combine to form a tight trimer that is primarily stabilized by interactions between helical residues. One side of the trimer is strikingly acidic, while the opposite face is more neutral. Database searches show p22 is structurally similar to human p32, which has a number of functions, including regulation of RNA splicing. p32 interacts with a number of target proteins via its alpha1 N-terminal helix, which is among the most conserved regions between p22 and p32. Co-immunoprecipitation studies showed that p22 interacts with the editosome and the k-RNA accessory protein, TbRGG2, and alpha1 of p22 was shown to be important for the p22-TbRGG2 interaction. Thus, these combined studies suggest that p22 mediates its role in k-RNA editing by acting as an adaptor protein.

The a2 mating-type-locus gene lga2 of Ustilago maydis interferes with mitochondrial dynamics and fusion, partially in dependence on a Dnm1-like fission component

The a2 mating-type-locus gene lga2 of the basidiomycete Ustilago maydis encodes a mitochondrial protein that interferes with mitochondrial morphology and integrity, and that plays a role in uniparental inheritance of mitochondrial DNA. To address the mode of action of Lga2, we investigated its Dnm1 (a dynamin-related protein)-dependent effects. Here, we demonstrate that Dnm1 functions as a mitochondrial fission component in U. maydis and mediates Lga2-induced mitochondrial fragmentation. Mitochondrial fusion occurred very inefficiently in matings of U. maydis wild-type strains, but was strongly stimulated in the absence of dnm1 and highest in either wild-type or Δdnm1 combinations when the a2 partner was deleted in lga2. This indicates that Dnm1 plays a central role in opposing mitochondrial fusion in response to endogenous lga2 expression and that Lga2 additionally inhibits fusion in a dnm1-independent manner. Our results further show that Lga2 does not stimulate increased turnover of the putative fusion protein Fzo1 and causes mitochondrial branching, loss of mitochondrial DNA and fitness reduction independently of dnm1. We conclude that Lga2 acts upstream of Dnm1, but controls mitochondrial integrity independently of Dnm1-mediated fission. In addition, we demonstrate a role of dnm1 in fungal virulence.

The cytoplasmic domain of influenza M2 protein interacts with caveolin-1

The cytoplasmic domain of influenza M2 protein (M2c) consists of 54 amino acid (aa) residues from aa44 to aa97. In this paper, M2c and its deletion mutant M2c(delta47-55) were expressed using prokaryotic expression system. First, glutaraldehyde crosslinking assay showed that M2c had multimerization potential mediated by aa47-55. Then, M2c, instead of M2c(delta47-55), directed eGFP from the whole cell localization to a predominately perinuclear region in CHO cells, which indicated that aa47-55 of M2c mediated the localization. Moreover, M2c colocalized with caveolin-1 (Cav) when CHO cells were cotransfected with Cav. A caveolin-1 binding motif phixxxxphixxphi (phi represents aromatic amino acid residues) in aa47-55 of M2c was found by sequence alignment and analysis. Further overlay ELISA result showed that M2c, but not M2c(delta47-55), bound to prokaryotically expressed cholesterol-free Cav(2-101), which illustrated the interaction could be cholesterol-independent. That was the first report of cellular protein bound to M2c.

gRNA/pre-mRNA annealing and RNA chaperone activities of RBP16

Editing in trypanosomes involves the addition or deletion of uridines at specific sites to produce translatable mitochondrial mRNAs. RBP16 is an accessory factor from Trypanosoma brucei that affects mitochondrial RNA editing in vivo and also stimulates editing in vitro. We report here experiments aimed at elucidating the biochemical activities of RBP16 involved in modulating RNA editing. In vitro RNA annealing assays demonstrate that RBP16 significantly stimulates the annealing of gRNAs to cognate pre-mRNAs. In addition, RBP16 also facilitates hybridization of partially complementary RNAs unrelated to the editing process. The RNA annealing activity of RBP16 is independent of its high-affinity binding to gRNA oligo(U) tails, consistent with the previously reported in vitro editing stimulatory properties of the protein. In vivo studies expressing recombinant RBP16 in mutant Escherichia coli strains demonstrate that RBP16 is an RNA chaperone and that in addition to RNA annealing activity, it contains RNA unwinding activity. Our data suggest that the mechanism by which RBP16 facilitates RNA editing involves its capacity to modulate RNA secondary structure and promote gRNA/pre-mRNA annealing.

Differential effects of arginine methylation on RBP16 mRNA binding, guide RNA (gRNA) binding, and gRNA-containing ribonucleoprotein complex (gRNP) formation

Mitochondrial gene expression in Trypanosoma brucei involves the coordination of multiple events including polycistronic transcript cleavage, polyadenylation, RNA stability, and RNA editing. Arg methylation of RNA binding proteins has the potential to influence many of these processes via regulation of protein-protein and protein-RNA interactions. Here we demonstrate that Arg methylation differentially regulates the RNA binding capacity and macromolecular interactions of the mitochondrial gene regulatory protein, RBP16. We show that, in T. brucei mitochondria, RBP16 forms two major stable complexes: a 5 S multiprotein complex and an 11 S complex consisting of the 5 S complex associated with guide RNA (gRNA). Expression of a non-methylatable RBP16 mutant protein demonstrates that Arg methylation of RBP16 is required to maintain the protein-protein interactions necessary for assembly and/or stability of both complexes. Down-regulation of the major trypanosome type 1 protein arginine methyltransferase, TbPRMT1, disrupts formation of both the 5 and 11 S complexes, indicating that TbPRMT1-catalyzed methylation of RBP16 Arg-78 and Arg-85 is critical for complex formation. We also show that Arg methylation decreases the capacity of RBP16 to associate with gRNA. This is not a general effect on RBP16 RNA binding, however, since methylation conversely increases the association of the protein with mRNA. Thus, TbPRMT1-catalyzed Arg methylation has distinct effects on RBP16 gRNA and mRNA association and gRNA-containing ribonucleoprotein complex (gRNP) formation.

Isolation of RNA binding proteins involved in insertion/deletion editing

RNA editing is a collective term referring to a plethora of reactions that ultimately lead to changes in RNA nucleotide sequences apart from splicing, 5' capping, or 3' end processing. In the mitochondria of trypanosomatids, insertion and deletion of uridines must occur, often on a massive scale, in order to generate functional messenger RNAs. The current state of knowledge perceives the editing machinery as a dynamic system, in which heterogeneous protein complexes undergo multiple transient RNA-protein interactions in the course of gRNA processing, gRNA-mRNA recognition, and the cascade of nucleolytic and phosphoryl transfer reactions that ultimately change the mRNA sequence. Identification of RNA binding proteins that interact with the mitochondrial RNAs, core editing complex, or contribute to mRNA stability is of critical importance to our understanding of the editing process. This chapter describes purification and characterization of three RNA binding proteins from kinetoplastid mitochondria that have been genetically demonstrated to affect RNA editing.

Genetics of morphogenesis and pathogenic development of Ustilago maydis

Ustilago maydis has emerged as an important model system for the study of fungi. Like many fungi, U. maydis undergoes remarkable morphological transitions throughout its life cycle. Fusion of compatible, budding, haploid cells leads to the production of a filamentous dikaryon that penetrates and colonizes the plant, culminating in the production of diploid teliospores within fungal-induced plant galls or tumors. These dramatic morphological transitions are controlled by components of various signaling pathways, including the pheromone-responsive MAP kinase and cAMP/PKA (cyclic AMP/protein kinase A) pathways, which coregulate the dimorphic switch and sexual development of U. maydis. These signaling pathways must somehow cooperate with the regulation of the cytoskeletal and cell cycle machinery. In this chapter, we provide an overview of these processes from pheromone perception and mating to gall production and sporulation in planta. Emphasis is placed on the genetic determinants of morphogenesis and pathogenic development of U. maydis and on the fungus-host interaction. Additionally, we review advances in the development of tools to study U. maydis, including the recently available genome sequence. We conclude with a brief assessment of current challenges and future directions for the genetic study of U. maydis.

Arginine methylation regulates mitochondrial gene expression in Trypanosoma brucei through multiple effector proteins

Arginine methylation is a post-translational modification that impacts gene expression in both the cytoplasm and nucleus. Here, we demonstrate that arginine methylation also affects mitochondrial gene expression in the protozoan parasite, Trypanosoma brucei. Down-regulation of the major trypanosome type I protein arginine methyltransferase, TbPRMT1, leads to destabilization of specific mitochondrial mRNAs. We provide evidence that some of these effects are mediated by the mitochondrial RNA-binding protein, RBP16, which we previously demonstrated affects both RNA editing and stability. TbPRMT1 catalyzes methylation of RBP16 in vitro. Further, MALDI-TOF-MS analysis of RBP16 isolated from TbPRMT1-depleted cells indicates that, in vivo, TbPRMT1 modifies two of the three known methylated arginine residues in RBP16. Expression of mutated, nonmethylatable RBP16 in T. brucei has a dominant negative effect, leading to destabilization of a subset of those mRNAs affected by TbPRMT1 depletion. Our results suggest that the specificity and multifunctional nature of RBP16 are due, at least in part, to the presence of differentially methylated forms of the protein. However, some effects of TbPRMT1 depletion on mitochondrial gene expression cannot be accounted for by RBP16 action. Thus, these data implicate additional, unknown methylproteins in mitochondrial gene regulation.

RBP16 stimulates trypanosome RNA editing in vitro at an early step in the editing reaction

RBP16 is an abundant RNA binding protein from Trypanosoma brucei mitochondria that affects both RNA editing and stability. We report here experiments aimed at elucidating the mechanism of RBP16 function in RNA editing. In in vitro RNA editing assays, recombinant RBP16 is able to significantly stimulate insertion editing of both CYb and A6 pre-mRNAs. Enhancement of in vitro editing activity occurs at, or prior to, the step of pre-mRNA cleavage, as evidenced by increased accumulation of pre-mRNA 3' cleavage products in the presence of RBP16. Mutated RBP16 that is severely compromised in cold shock domain (CSD)-mediated RNA binding was able to enhance editing to levels comparable to the wild-type protein in some assays at the highest RBP16 levels tested. However, at low RBP16 concentrations or in assays with native, oligo(U)-tail-bearing gRNAs, editing stimulation by mutant RBP16 was somewhat compromised. Together, these results indicate that both the N-terminal CSD and C-terminal RGG RNA binding domains of RBP16 are required for maximal editing stimulation. Finally, the relaxed specificity of RBP16 for stimulation of both CYb and A6 editing in vitro implicates additional specificity factors that account for the strict CYb specificity of RBP16 action in editing in vivo. Our results constitute the first report of any putative RNA editing accessory factor eliciting an effect on editing in vitro. Overall, these results support a novel accessory role for RBP16 in U insertion editing.

Unexplained complexity of the mitochondrial genome and transcriptome in kinetoplastid flagellates

Kinetoplastids are flagellated protozoans, whose members include the pathogens Trypanosoma brucei, T. cruzi and Leishmania species, that are considered among the earliest diverging eukaryotes with a mitochondrion. This organelle has become famous because of its many unusual properties, which are unique to the order Kinetoplastida, including an extensive kinetoplast DNA network and U-insertion/deletion type RNA editing of its mitochondrial transcripts. In the last decade, considerable progress has been made in elucidating the complex machinery of RNA editing. Moreover, our understanding of the structure and replication of kinetoplast DNA has also dramatically improved. Much less however, is known, about the developmental regulation of RNA editing, its integration with other RNA maturation processes, stability of mitochondrial mRNAs, or evolution of the editing process itself. Yet the profusion of genomic data recently made available by sequencing consortia, in combination with methods of reverse genetics, hold promise in understanding the complexity of this exciting organelle, knowledge of which may enable us to fight these often medically important protozoans.

In vitro and in vivo analysis of the major type I protein arginine methyltransferase from Trypanosoma brucei

In mammals and yeasts, arginine methylation, catalyzed by protein arginine methyltransferases (PRMTs), has been implicated in regulation of diverse processes such as protein-protein interaction, protein localization, signal transduction, RNA processing, and transcription. A large number of PRMT substrates are RNA binding proteins. In trypanosomes, gene regulation is controlled primarily at the levels of RNA processing, stability, and translation, and likely involves numerous RNA binding proteins. Thus, arginine methylation may be especially important in controlling gene expression in this evolutionarily ancient group of organisms. To begin to understand the role of arginine methylation in trypanosomes, we identified and characterized a type I PRMT from Trypanosoma brucei, termed TbPRMT1. TbPRMT1 displays 51% amino acid identity to human PRMT1. It possesses an S-adenosylmethionine binding site and double E and THW loops, common and absolute features associated with other PRMTs. Recombinant TbPRMT1 methylates both an artificial RG-rich peptide and the T. brucei mitochondrial RNA binding protein, TBRGG1, and it exhibits differences in substrate specificity compared to rat PRMT1. TbPRMT1 is constitutively expressed during the T. brucei life cycle. Disruption of TbPRMT1 gene expression by RNA interference did not result in a significant growth defect in procyclic form T. brucei. Finally, we observe a dramatic decrease in the cellular level of asymmetric dimethylarginine upon TbPRMT1 knock down, indicating that TbPRMT1 is the predominant type I PRMT in T. brucei. The strong conservation of PRMT1 homologs between protozoa and humans highlights the importance of arginine methylation as a regulatory mechanism in eukaryotes.

An acidic protein, YBAP1, mediates the release of YB-1 from mRNA and relieves the translational repression activity of YB-1

Eukaryotic Y-box proteins are nucleic acid-binding proteins implicated in a wide range of gene regulatory mechanisms. They contain the cold shock domain, which is a nucleic acid-binding structure also found in bacterial cold shock proteins. The Y-box protein YB-1 is known to be a core component of messenger ribonucleoprotein particles (mRNPs) in the cytoplasm. Here we disrupted the YB-1 gene in chicken DT40 cells. Through the immunoprecipitation of an epitope-tagged YB-1 protein, which complemented the slow-growth phenotype of YB-1-depleted cells, we isolated YB-1-associated complexes that likely represented general mRNPs in somatic cells. RNase treatment prior to immunoprecipitation led to the identification of a Y-box protein-associated acidic protein (YBAP1). The specific association of YB-1 with YBAP1 resulted in the release of YB-1 from reconstituted YB-1-mRNA complexes, thereby reducing the translational repression caused by YB-1 in the in vitro system. Our data suggest that YBAP1 induces the remodeling of YB-1-mRNA complexes.

TbDSS-1, an essential Trypanosoma brucei exoribonuclease homolog that has pleiotropic effects on mitochondrial RNA metabolism

Mitochondrial gene expression in trypanosomes is controlled primarily at the levels of RNA processing and RNA stability. This regulation undoubtedly involves numerous ribonucleases. Here we characterize the Trypanosoma brucei homolog of the yeast DSS-1 mitochondrial exoribonuclease, which we term TbDSS-1. Biochemical fractionation indicates that TbDSS-1 is mitochondrially localized, as predicted by its N-terminal sequence. In contrast to its yeast homolog, TbDSS-1 does not appear to be associated with mitochondrial ribosomes. Targeted downregulation of TbDSS-1 by RNA interference in procyclic-form T. brucei results in a severe growth defect. In addition, TbDSS-1 depletion leads to a decrease in the levels of never edited cytochrome oxidase subunit I (COI) mRNA and both unedited and edited COIII mRNAs, indicating this enzyme functions in the control of mitochondrial RNA abundance. We also observe a considerable reduction in the level of edited apocytochrome b (CYb) mRNA and a corresponding increase in unedited CYb mRNA, suggesting that TbDSS-1 functions, either directly or indirectly, in the control of RNA editing. The abundance of both gCYb[560] and gA6[149] guide RNAs is reduced upon TbDSS-1 depletion, although the reduction in gCYb[560] is much more dramatic. The significant reduction in gCYb levels could potentially account for the observed decrease in CYb RNA editing. Western blot analyses of mitochondrial RNA editing and stability factors indicate that the perturbations of RNA levels observed in TbDSS-1 knock-downs do not result from secondary effects on other mitochondrial proteins. In all, these data demonstrate that TbDSS-1 is an essential protein that plays a role in mitochondrial RNA stability and RNA editing.

The Ustilago maydis a2 mating-type locus genes lga2 and rga2 compromise pathogenicity in the absence of the mitochondrial p32 family protein Mrb1

The Ustilago maydis mrb1 gene specifies a mitochondrial matrix protein with significant similarity to mitochondrial p32 family proteins known from human and many other eukaryotic species. Compatible mrb1 mutant strains were able to mate and form dikaryotic hyphae; however, proliferation within infected tissue and the ability to induce tumor development of infected maize (Zea mays) plants were drastically impaired. Surprisingly, manifestation of the mrb1 mutant phenotype selectively depended on the a2 mating type locus. The a2 locus contains, in addition to pheromone signaling components, the genes lga2 and rga2 of unknown function. Deletion of lga2 in an a2Deltamrb1 strain fully restored pathogenicity, whereas pathogenicity was partially regained in an a2Deltamrb1Deltarga2 strain, implicating a concerted action between Lga2 and Rga2 in compromising pathogenicity in Deltamrb1 strains. Lga2 and Rga2 localized to mitochondria and Mrb1 interacted with Rga2 in the yeast two-hybrid system. Conditional expression of lga2 in haploid cells reduced vegetative growth, conferred mitochondrial fragmentation and mitochondrial DNA degradation, and interfered with respiratory activity. The consequences of lga2 overexpression depended on the expression strength and were greatly exacerbated in Deltamrb1 mutants. We propose that Lga2 interferes with mitochondrial fusion and that Mrb1 controls this activity, emphasizing a critical link between mitochondrial morphology and pathogenicity.

Trypanosoma brucei: functions of RBP16 cold shock and RGG domains in macromolecular interactions

The RNA binding protein RBP16 regulates mitochondrial RNA editing and stability in Trypanosoma brucei. To aid in understanding the biochemical mechanisms of RBP16 function, we analyzed the RNA and protein binding capacity of RBP16 and its individual cold shock (CSD) and RGG domains. Both recombinantly expressed domains possess RNA binding activity. However, the specificity and affinity of RBP16 for gRNA is mediated predominantly through the interaction of the CSD with poly(U). The RGG domain contributes to the association between full length RBP16 and gRNA, as it was required for maximal binding. We further demonstrate that both domains contribute to maximal binding of RBP16 to the mitochondrial p22 protein. However, p22 can interact with the CSD alone and stimulate its gRNA binding activity. Thus, the CSD is primary in RBP16 interactions, while the RGG domain enhances the capacity of the CSD to bind both RNA and protein. These results suggest a model for RBP16 molecular interactions.

LRP130, a pentatricopeptide motif protein with a noncanonical RNA-binding domain, is bound in vivo to mitochondrial and nuclear RNAs

LRP130 (also known as LRPPRC) is an RNA-binding protein that is a constituent of postsplicing nuclear RNP complexes associated with mature mRNA. It belongs to a growing family of pentatricopeptide repeat (PPR) motif-containing proteins, several of which have been implicated in organellar RNA metabolism. We show here that only a fraction of LRP130 proteins are in nuclei and are directly bound in vivo to at least some of the same RNA molecules as the nucleocytoplasmic shuttle protein hnRNP A1. The majority of LRP130 proteins are located within mitochondria, where they are directly bound to polyadenylated RNAs in vivo. In vitro, LRP130 binds preferentially to polypyrimidines. This RNA-binding activity maps to a domain in its C-terminal region that does not contain any previously described RNA-binding motifs and that contains only 2 of the 11 predicted PPR motifs. Therefore, LRP130 is a novel type of RNA-binding protein that associates with both nuclear and mitochondrial mRNAs and as such is a potential candidate for coordinating nuclear and mitochondrial gene expression. These findings provide the first identification of a mammalian protein directly bound to mitochondrial RNA in vivo and provide a possible molecular explanation for the recently described association of mutations in LRP130 with cytochrome c oxidase deficiency in humans.

RBP16 is a multifunctional gene regulatory protein involved in editing and stabilization of specific mitochondrial mRNAs in Trypanosoma brucei

RBP16 is a Trypanosoma brucei mitochondrial RNA-binding protein that associates with guide RNAs (gRNAs), mRNAs, and ribosomal RNAs. Based on its inclusion in the multifunctional Y-box protein family and its ability to bind multiple RNA classes, we hypothesized that RBP16 plays a role in diverse aspects of mitochondrial gene regulation. To gain insight into RBP16 function, we generated cells expressing less than 10% of wild-type RBP16 levels by tetracycline-regulated RNA interference (RNAi). Poisoned primer extension analyses revealed that edited, but not unedited, CYb mRNA is reduced by approximately 98% in tetracycline-induced RBP16 RNAi cells, suggesting that RBP16 is critical for CYb RNA editing. The down-regulation of CYb editing in RBP16 RNAi transfectants apparently entails a defect in gRNA utilization, as gCYb[560] abundance is similar in uninduced and induced cells. We observed a surprising degree of specificity regarding the ability of RBP16 to modulate editing, as editing of mRNAs other than CYb is not significantly affected upon RBP16 disruption. However, the abundance of the never edited mitochondrial RNAs COI and ND4 is reduced by 70%-80% in RBP16 RNAi transfectants, indicating an additional role for RBP16 in the stabilization of these mRNAs. Analysis of RNAs bound to RBP16 immunoprecipitated from wild-type cells reveals that RBP16 is associated with multiple gRNA sequence classes in vivo, including those whose abundance and usage appear unaffected by RBP16 disruption. Overall, our results indicate that RBP16 is an accessory factor that regulates the editing and stability of specific populations of mitochondrial mRNAs.