Biology subjects

Gerbich, T.

Publications and source records attributed to Gerbich, T..

4 recordsLinked to original sources

bioRxiv2024

The cell biology and genome of Stentor pyriformis, a giant cell that embeds symbiotic algae in a microtubule meshwork

Endosymbiotic events in which an endosymbiont is retained within a cell that remains capable of phagocytosis, a situation known as mixotrophy, provide potentially important clues about the evolutionary origins of eukaryotes, particularly regarding the relative evolutionary sequence of phagocytosis and endosymbiosis. Mixotrophy in ciliates is commonplace, but has been investigated at a cellular and molecular level almost entirely in one organism, Paramecium bursaria. Reliance on just one model system makes it difficult to know which cell biological aspects of this system represent general features of ciliate mixotrophy versus accidental features of the specific organism. Here we describe the cell biology and genome of the giant heterotrichous ciliate Stentor pyriformis. We show that this giant unicellular organism contains Chlorella variabilis as its endosymbiont, that the Chlorella can live freely outside the host, that within the host the Chlorella cells are docked near the cell surface, surrounded by microtubule "baskets", and that the photosynthetic efficiency of the Chlorella is reduced inside the Stentor cell compared to when it is free-living outside the host, with photon energy instead being shunted to non-photochemical quenching. Compared to the non-mixotrophic Stentor coeruleus, S. pyriformis has several distinct cellular features that may be related to endosymbiosis: the presence of microtubule baskets, which are absent in S. coeruleus; positive rather than negative phototaxis, which is likely an adaptation to allow the photosynthetic symbionts access to sufficient light; and a lack of pigment in the host cell, which may be an adaptation to tolerate high light levels. Compared to P. bursaria, S. pyriformis has several similar cellular features: in both organisms, the symbiont is a strain of Chlorella variabilis; the Chlorella endosymbiont retains the ability to live freely when separated from the host; and the algal symbionts contained in perialgal vesicles are docked at the cell surface. One potentially informative difference between P. bursaria and S. pyriformis is that S. pyriformis employs a standard genetic code, similar to other Stentor species but different from most other ciliates, including P. bursaria, which use a non-standard code in which one or more stop-codons are respecified to encode amino acids. This difference in genetic code could serve as a barrier to impede gene transfer from symbiont to host in other ciliates, but this would not be a factor in S. pyriformis. A second cell biological difference is that whereas P. bursaria performs phototaxis by a kinetic accumulation mechanism, in which swimming is non-directional but cells slow down in regions of higher light intensity, S. pyriformis performs directed swimming towards the direction of high light intensity. However, as in P. bursaria, phototaxis in S. pyriformis requires the presence of the Chlorella, implying a potential flow of information from the symbiont to direct the orientation and swimming of the host cell. We propose that S. pyriformis will serve as a useful model system for studying the evolution of mixotrophy and endosymbiosis, with unique advantages in terms of size and regenerative ability as well as distinct cellular and genomic features compared with other mixotrophic ciliate models.

cell biology↗

bioRxiv2021

RNA-specific condensation promotes auto-regulation of polyQ RNA-binding protein Whi3

RNA-binding proteins are frequently seen to be capable of auto-regulation by binding their own transcripts. In this work, we show that in the multinucleated fungus Ashbya gossypii, the phase-separating RNA-binding protein Whi3 binds and regulates its own transcripts in distinct condensates from its other targets. Failure of Whi3 to bind its own transcript leads to a reduction in Whi3 protein level and its inability to properly regulate its other targets, leading to defects in nuclear cycling, polarized growth, and transcription of START-regulated genes. These results present a role for auto-regulation and condensate formation triggered by an RNA-binding protein interacting with its own coding transcript. Given the propensity of RNA-binding proteins to interact with their own coding mRNAs, this may be a wide-spread mechanism of feedback that utilizes biomolecular condensates.

cell biology↗

bioRxiv2019

Phosphoregulation provides functional specificity to biomolecular condensates in the cell cycle and cell polarity

Cytoplasmic patterning is a feature of many cells from embryos to neurons and fungi. Biomolecular condensation is a way of organizing cytosol in which proteins and nucleic acids coassemble into compartments. How molecular identity of condensates is achieved is not well understood. In the multinucleate, filamentous fungus Ashbya gossypii, the RNA-binding protein Whi3 regulates the cell cycle and cell polarity through forming macromolecular structures that behave like condensates. Whi3 has distinct spatial localizations and mRNA targets making it a powerful model for how, when and where specific identities are established for condensates. Using mass-spectrometry, we identified residues on Whi3 that are differentially phosphorylated under specific conditions and generated mutants which ablate this regulation. This yielded separation of function alleles that were functional for either cell polarity or nuclear cycling but not both. This study shows that phosphorylation of individual residues on molecules in biomolecular condensates can provide specificity that give rise to distinct functional identities in the same cell.\n\nSummaryResidue specific phosphorylation of the RNA-binding protein Whi3 is used to specifically regulate subsets of functionally distinct condensates in the multinucleate fungus Ashbya gossypii.

cell biology↗

bioRxiv2017

mRNA structure determines specificity of a polyQ-driven phase separation

RNA promotes liquid-liquid phase separation (LLPS) to build membrane-less compartments in cells. How distinct molecular compositions are established and maintained in these liquid compartments is unknown. Here we report that secondary structure allows mRNAs to self-associate and determines if an mRNA is recruited to or excluded from liquid compartments. The polyQ-protein Whi3 induces conformational changes in RNA structure and generates distinct molecular fluctuations depending on the RNA sequence. These data support a model in which structure-based, RNA-RNA interactions promote assembly of distinct droplets and protein-driven, conformational dynamics of the RNA maintain this identity. Thus, the shape of RNA can promote the formation and coexistence of the diverse array of RNA-rich liquid compartments found in a single cell.\n\nOne Sentence SummaryIdentity in cellular, phase-separated compartments arises from RNA-RNA complexes encoded by mRNA secondary structures.

cell biology↗