Biology subjects

Hashimi, H.

Publications and source records attributed to Hashimi, H..

7 recordsLinked to original sources

bioRxiv2026

Interconnections with and within the trypanosomal respiratory chain revealed by complexome profiling

Proteins are frequently integrated into multicomponent complexes that execute the elaborate processes supporting life. Thus, a protein's function can only be defined by the company it keeps within a complex. Proteomes provide informative protein inventories but lack information about protein quaternary structures. Complexome profiling (CP) has been transformative in capturing the comprehensive population structure of complexes at a given moment within a cell. We have employed CP to chart the multiprotein complex landscape of two life cycle stages of Trypanosoma brucei. These data have allowed the observation of previously hidden interactions with and within the mitochondrial respiratory chain. We have found (1) an exceptional case of a SLC25 solute transporter that interacts with NADH dehydrogenase, (2) two ATP synthase subunit g paralogs that are intriguingly excluded from the enzyme's dimers, and (3) refined the known composition of ubiquinol:cytochrome c oxidoreductase by addition of missing subunits and removing an incorrectly assigned subunit, which more likely acts to insert the iron-sulfur co-factor into the complex. Further investigation into ubiquinol:cytochrome c oxidoreductase assembly revealed crosstalk between incorporation of its nuclear subunits with mitochondrial translation, possibly facilitating a hitherto unknown quality control mechanism. These discoveries demonstrate the power of our CP data for generation and testing of hypotheses about the mitochondrial and other organellar multiprotein complexes of T. brucei, a protist of medical and evolutionary importance.

cell biology↗

bioRxiv2026

Trypanosomal MICOS is assembled on non-respiring mitochondrial crista precursors and associates with two integral microproteins

The mitochondrial contact site and cristae organizing system (MICOS) is a multiprotein complex that shapes crista junctions and maintains inner and outer membrane contacts. MICOS coordinates the assembly of electron transport chain complexes, a prerequisite for cellular respiration. Indeed, MICOS is lost in eukaryotes that dispensed with cellular respiration, suggesting that its assembly depends on the presence of an active respiratory chain. Trypanosoma brucei provides a unique system to test this hypothesis as its mitochondrion undergoes developmentally regulated remodeling. In the insect stage, the mitochondrion contains cristae with an active electron transport chain, whereas the mammalian bloodstream form possesses precursor cristae with stub-like morphology that lack respiratory activity. MICOS has been characterized in the insect stage but remains unexamined in the bloodstream form. Here, we demonstrate that all MICOS subunits assemble onto precursor cristae, retaining conserved interactions with both outer and inner membrane protein machineries. This is somewhat unexpected given the co-occurrence of MICOS with active cellular respiration in nature. Furthermore, we identify novel MICOS-associated proteins that are dispensable for its stability, suggesting auxiliary rather than core roles in MICOS function. Together, our findings establish that MICOS assembly precedes cellular respiratory competence and expand its interaction landscape in trypanosomatids.

cell biology↗

bioRxiv2025

The core MICOS complex subunit Mic60 has been substituted by two cryptic mitofilin-containing proteins in Euglenozoa

Cristae enclose respiratory chain complexes, making them the bioenergetic subcompartments of mitochondria. The MICOS complex is among the inducers of membrane curvature needed for crista formation. Resembling the respiratory chain complexes, MICOS is organized around a core protein, the mitofilin-domain bearing Mic60, that was inherited from the alphaproteobacterial progenitor of mitochondria. Extant alphaproteobacteria express Mic60 to form their own bioenergetic subcompartments, demonstrating the permeance of Mic60s form and function during prokaryotic and eukaryotic evolution. Yet, unlike virtually all aerobic eukaryotes, Mic60 is not encoded within the genomes of the multifarious protists that comprise the phylum Euglenozoa, including trypanosomes. Here, we show that Mic60 has been replaced in euglenozoans by two cryptic mitofilin domain-containing MICOS subunits, Mic34 and Mic40. Contrasting alphaproteobacterial and mitochondrial Mic60, these are not integral membrane proteins. Mic34 and Mic40 are as diverged from each other as both are to canonical Mic60. Reverse genetics revealed they are intertwined with the oxidative protein folding pathway required for mitochondrial-and crista-biogenesis, veiling a potential membrane remodeling role. Nevertheless, Mic34 binds phospholipid bilayers in vitro. Mic34 and Mic40 heterologous expression remodels gammaproteobacterial cytoplasmic membranes, like Mic60. Unexpectedly, Mic34 overexpression elaborates the simplified tubular mitochondrion of a Trypanosoma brucei life cycle stage with repressed oxidative phosphorylation. Furthermore, this activity was ablated by mutations to Mic34s mitofilin domain that correspond to essential motifs found in yeast Mic60s mitofilin domain. Thus, the mitofilin protein family is more diverse than originally supposed, with two of its structurally most divergent members altering the core of euglenozoan MICOS.

molecular biology↗

bioRxiv2022

The development of intracytoplasmic membranes in alphaproteobacteria involves the conserved mitochondrial crista-developing Mic60 protein

Mitochondrial cristae expand the surface area of respiratory membranes and ultimately allow for the evolutionary scaling of respiration with cell volume across eukaryotes. The discovery of Mic60 homologs among alphaproteobacteria, the closest extant relatives of mitochondria, suggested that cristae might have evolved from bacterial intracytoplasmic membranes (ICMs). Here, we investigated the predicted structure and function of alphaproteobacterial Mic60, and a protein encoded by an adjacent gene Orf52, in two distantly related purple alphaproteobacteria, Rhodobacter sphaeroides and Rhodopseudomonas palustris. In addition, we assessed the potential physical interactors of Mic60 and Orf52 in R. sphaeroides. We show that the three -helices of mitochondrial Mic60s mitofilin domain, as well as its adjacent membrane-binding amphipathic helix, are present in alphaproteobacterial Mic60. The disruption of Mic60 and Orf52 caused photoheterotrophic growth defects, which are most severe under low light conditions, and both their disruption and overexpression led to enlarged ICMs in both studied alphaproteobacteria. We also found that alphaproteobacterial Mic60 physically interacts with BamA, the homolog of Sam50, one of the main physical interactors of eukaryotic Mic60. This interaction, responsible for making contact sites at mitochondrial envelopes, has been conserved in modern alphaproteobacteria despite more than a billion years of evolutionary divergence. Our results suggest a role for Mic60 in photosynthetic ICM development and contact site formation at alphaproteobacterial envelopes. Overall, we provide support for the hypothesis that mitochondrial cristae evolved from alphaproteobacterial ICMs, and therefore have improved our understanding of the nature of the mitochondrial ancestor.

evolutionary biology↗

bioRxiv2021

Trypanosome morphogenesis involves recruitment of a pleckstrin homology domain protein by an orphan kinesin to the microtubule quartet

Kinesins are motor proteins found in all eukaryotic lineages that move along microtubules to mediate cellular processes such as mitosis and intracellular transport. In trypanosomatids, the kinesin superfamily has undergone a prominent expansion, resulting in one of the most diverse kinesin repertoires that includes the two kinetoplastid-restricted families X1 and X2. Here, we characterize in Trypanosoma brucei TbKifX2A, an orphaned X2 kinesin. TbKifX2A tightly interacts with TbPH1, a kinesin-like protein with a likely inactive motor domain, a rarely reported occurrence. Both TbKifX2A and TbPH1 localize to the microtubule quartet (MtQ), a characteristic but poorly understood cytoskeletal structure that wraps around the flagellar pocket as it extends to the cell body anterior. The proximal proteome of TbPH1 revealed two other interacting proteins, the flagellar pocket protein FP45 and intriguingly another X2 kinesin, TbKifX2C. Simultaneous ablation of TbKifX2A/TbPH1 results in the depletion of FP45 and TbKifX2C and also an expansion of the flagellar pocket, among other morphological defects. TbKifX2A is the first motor protein to be localized to the MtQ. The observation that TbKifX2C also associates with the MtQ suggests that the X2 kinesin family may have co-evolved with the MtQ, both kinetoplastid-specific traits.

molecular biology↗

bioRxiv2021

MICOS and F1FO-ATP synthase crosstalk is a fundamental property of mitochondrial cristae

Mitochondrial cristae are polymorphic invaginations of the inner membrane that are the fabric of cellular respiration. Both the Mitochondrial Contact Site and Cristae Organization System (MICOS) and the FOF1-ATP synthase are vital for sculpting cristae by opposing membrane bending forces. While MICOS promotes negative curvature at cristae junctions, dimeric FOF1- ATP synthase is crucial for positive curvature at cristae rims. Crosstalk between these two complexes has been observed in bakers yeast, the model organism of the Opisthokonta supergroup. Here, we report that this property is conserved in Trypanosoma brucei, a member of the Discoba supergroup that separated from Opisthokonta [~]2 billion years ago. Specifically, one of the paralogs of the core MICOS subunit Mic10 interacts with dimeric F1FO-ATP synthase, whereas the other core Mic60 subunit has a counteractive effect on F1FO- ATP synthase oligomerization. This is evocative of the nature of MICOS-F1FO-ATP synthase crosstalk in yeast, which is remarkable given the diversification these two complexes have undergone during almost 2 eons of independent evolution. Furthermore, we identified a highly diverged trypanosome homolog of subunit e, which is essential for the stability of F1FO-ATP synthase dimers in yeast. Just like subunit e, it is preferentially associated with dimers, interacts with Mic10 and its silencing results in severe defects to cristae and disintegration of F1FO-ATP synthase dimers. Our findings indicate that crosstalk between MICOS and dimeric F1FO-ATP synthase is a fundamental property impacting cristae shape throughout eukaryotes. ImportanceMitochondria have undergone profound diversification in separate lineages that have radiated since the last common ancestor of eukaryotes some eons ago. Most eukaryotes are unicellular protists, including etiological agents of infectious diseases like Trypanosoma brucei. Thus, the study of a broad range of protists can reveal fundamental features shared by all eukaryotes and lineage-specific innovations. Here we report that two different protein complexes, MICOS and F1FO-ATP synthase, known to affect mitochondrial architecture, undergo crosstalk in T. brucei, just as in bakers yeast. This is remarkable considering that these complexes have otherwise undergone many changes during their almost two billion years of independent evolution. Thus, this crosstalk is a fundamental property needed to maintain proper mitochondrial structure even if the constituent players considerably diverged.

cell biology↗

bioRxiv2021

Mammalian adipogenesis regulators (Aregs) exhibit robust non- and anti-adipogenic properties that arise with age and involve retinoic acid signalling

Adipose stem and precursor cells (ASPCs) give rise to adipocytes and determine the composition and plasticity of adipose tissue. Recently, several studies have demonstrated that ASPCs partition into at least three distinct cell subpopulations: Dpp4+ stem-like cells, Aoc3+ pre-adipocyte-like cells, and the enigmatic CD142+ cells. A great challenge now is to functionally characterize these distinct ASPC populations. Here, we focus on CD142+ ASPCs since discrepant properties have been assigned to this subpopulation, from adipogenic to non- and even anti-adipogenic. To address these inconsistencies, we comprehensively characterized mammalian subcutaneous CD142+ ASPCs across various sampling conditions. Our findings demonstrate that CD142+ ASPCs exhibit high molecular and phenotypic robustness, firmly supporting their non- and anti-adipogenic properties. However, these properties emerge in an age-dependent manner, revealing surprising temporal CD142+ ASPC behavioural alterations. Finally, using multi-omic and functional assays, we show that the inhibitory nature of these adipogenesis-regulatory CD142+ ASPCs (Aregs) is driven by specifically expressed secretory factors that cooperate with the retinoic acid signalling pathway to transform the adipogenic state of CD142- ASPCs into a non-adipogenic, Areg-like one.

cell biology↗