All Publications

  • A mutagenesis screen for essential plastid biogenesis genes in human malaria parasites. PLoS biology Tang, Y., Meister, T. R., Walczak, M., Pulkoski-Gross, M. J., Hari, S. B., Sauer, R. T., Amberg-Johnson, K., Yeh, E. 2019; 17 (2): e3000136


    Endosymbiosis has driven major molecular and cellular innovations. Plasmodium spp. parasites that cause malaria contain an essential, non-photosynthetic plastid-the apicoplast-which originated from a secondary (eukaryote-eukaryote) endosymbiosis. To discover organellar pathways with evolutionary and biomedical significance, we performed a mutagenesis screen for essential genes required for apicoplast biogenesis in Plasmodium falciparum. Apicoplast(-) mutants were isolated using a chemical rescue that permits conditional disruption of the apicoplast and a new fluorescent reporter for organelle loss. Five candidate genes were validated (out of 12 identified), including a triosephosphate isomerase (TIM)-barrel protein that likely derived from a core metabolic enzyme but evolved a new activity. Our results demonstrate, to our knowledge, the first forward genetic screen to assign essential cellular functions to unannotated P. falciparum genes. A putative TIM-barrel enzyme and other newly identified apicoplast biogenesis proteins open opportunities to discover new mechanisms of organelle biogenesis, molecular evolution underlying eukaryotic diversity, and drug targets against multiple parasitic diseases.

    View details for PubMedID 30726238

  • A mutagenesis screen for essential plastid biogenesis genes in human malaria parasites PLOS BIOLOGY Tang, Y., Meister, T. R., Walczak, M., Pulkoski-Gross, M. J., Hari, S. B., Sauer, R. T., Amberg-Johnson, K., Yeh, E. 2019; 17 (2)
  • ATG8 Is Essential Specifically for an Autophagy-Independent Function in Apicoplast Biogenesis in Blood-Stage Malaria Parasites. mBio Walczak, M., Ganesan, S. M., Niles, J. C., Yeh, E. 2018; 9 (1)


    Plasmodium parasites and related pathogens contain an essential nonphotosynthetic plastid organelle, the apicoplast, derived from secondary endosymbiosis. Intriguingly, a highly conserved eukaryotic protein, autophagy-related protein 8 (ATG8), has an autophagy-independent function in the apicoplast. Little is known about the novel apicoplast function of ATG8 and its importance in blood-stage Plasmodiumfalciparum Using a P.falciparum strain in which ATG8 expression was conditionally regulated, we showed that P. falciparum ATG8 (PfATG8) is essential for parasite replication. Significantly, growth inhibition caused by the loss of PfATG8 was reversed by addition of isopentenyl pyrophosphate (IPP), which was previously shown to rescue apicoplast defects in P.falciparum Parasites deficient in PfATG8, but whose growth was rescued by IPP, had lost their apicoplast. We designed a suite of functional assays, including a new fluorescence in situ hybridization (FISH) method for detection of the low-copy-number apicoplast genome, to interrogate specific steps in apicoplast biogenesis and detect apicoplast defects which preceded the block in parasite replication. Though protein import and membrane expansion of the apicoplast were unaffected, the apicoplast was not inherited by daughter parasites. Our findings demonstrate that, though multiple autophagy-dependent and independent functions have been proposed for PfATG8, only its role in apicoplast biogenesis is essential in blood-stage parasites. We propose that PfATG8 is required for fission or segregation of the apicoplast during parasite replication.IMPORTANCEPlasmodium parasites, which cause malaria, and related apicomplexan parasites are important human and veterinary pathogens. They are evolutionarily distant from traditional model organisms and possess a unique plastid organelle, the apicoplast, acquired by an unusual eukaryote-eukaryote endosymbiosis which established novel protein/lipid import and organelle inheritance pathways in the parasite cell. Though the apicoplast is essential for parasite survival in all stages of its life cycle, little is known about these novel biogenesis pathways. We show that malaria parasites have adapted a highly conserved protein required for macroautophagy in yeast and mammals to function specifically in apicoplast inheritance. Our finding elucidates a novel mechanism of organelle biogenesis, essential for pathogenesis, in this divergent branch of pathogenic eukaryotes.

    View details for DOI 10.1128/mBio.02021-17

    View details for PubMedID 29295911

  • Association of condensin with chromosomes depends on DNA binding by its HEAT-repeat subunits. Nature structural & molecular biology Piazza, I. n., Rutkowska, A. n., Ori, A. n., Walczak, M. n., Metz, J. n., Pelechano, V. n., Beck, M. n., Haering, C. H. 2014; 21 (6): 560–68


    Condensin complexes have central roles in the three-dimensional organization of chromosomes during cell divisions, but how they interact with chromatin to promote chromosome segregation is largely unknown. Previous work has suggested that condensin, in addition to encircling chromatin fibers topologically within the ring-shaped structure formed by its SMC and kleisin subunits, contacts DNA directly. Here we describe the discovery of a binding domain for double-stranded DNA formed by the two HEAT-repeat subunits of the Saccharomyces cerevisiae condensin complex. From detailed mapping data of the interfaces between the HEAT-repeat and kleisin subunits, we generated condensin complexes that lack one of the HEAT-repeat subunits and consequently fail to associate with chromosomes in yeast and human cells. The finding that DNA binding by condensin's HEAT-repeat subunits stimulates the SMC ATPase activity suggests a multistep mechanism for the loading of condensin onto chromosomes.

    View details for DOI 10.1038/nsmb.2831

    View details for PubMedID 24837193

  • Dissecting the role of the Atg12-Atg5-Atg16 complex during autophagosome formation. Autophagy Walczak, M. n., Martens, S. n. 2013; 9 (3): 424–25


    The activity of the conserved Atg12-Atg5-Atg16 complex is essential for autophagosome formation. However, little is known about its mechanism of action during this process. In our study we employed in vitro systems consisting of purified proteins and giant unilamellar vesicles (GUVs) or small liposomes to investigate membrane binding by the Atg12-Atg5-Atg16 complex and its interplay with the Atg8 conjugation system. We showed that Atg5 directly binds membranes and that this membrane binding is negatively regulated by Atg12 conjugation but activated by Atg16. Membrane binding by the Atg12-Atg5-Atg16 complex is required for efficient promotion of Atg8 lipidation. Additionally, we found that the Atg12-Atg5-Atg16 complex tethered vesicles in an Atg8-independent manner. In yeast, membrane binding by Atg5 is not required for its recruitment to the phagophore assembly site (PAS) but is essential for efficient promotion of autophagy and the cytoplasm-to-vacuole targeting (Cvt) pathway at a stage preceding Atg8 lipidation and autophagosome closure. Our findings provide new insights into the role of the Atg12-Atg5-Atg16 complex during autophagosome formation.

    View details for DOI 10.4161/auto.22931

    View details for PubMedID 23321721

    View details for PubMedCentralID PMC3590266

  • Mechanism and functions of membrane binding by the Atg5-Atg12/Atg16 complex during autophagosome formation EMBO JOURNAL Romanov, J., Walczak, M., Ibiricu, I., Schuechner, S., Ogris, E., Kraft, C., Martens, S. 2012; 31 (22): 4304–17


    Autophagy is a conserved process for the bulk degradation of cytoplasmic material. Triggering of autophagy results in the formation of double membrane-bound vesicles termed autophagosomes. The conserved Atg5-Atg12/Atg16 complex is essential for autophagosome formation. Here, we show that the yeast Atg5-Atg12/Atg16 complex directly binds membranes. Membrane binding is mediated by Atg5, inhibited by Atg12 and activated by Atg16. In a fully reconstituted system using giant unilamellar vesicles and recombinant proteins, we reveal that all components of the complex are required for efficient promotion of Atg8 conjugation to phosphatidylethanolamine and are able to assign precise functions to all of its components during this process. In addition, we report that in vitro the Atg5-Atg12/Atg16 complex is able to tether membranes independently of Atg8. Furthermore, we show that membrane binding by Atg5 is downstream of its recruitment to the pre-autophagosomal structure but is essential for autophagy and cytoplasm-to-vacuole transport at a stage preceding Atg8 conjugation and vesicle closure. Our findings provide important insights into the mechanism of action of the Atg5-Atg12/Atg16 complex during autophagosome formation.

    View details for DOI 10.1038/emboj.2012.278

    View details for Web of Science ID 000311157500007

    View details for PubMedID 23064152

    View details for PubMedCentralID PMC3501226