Electron tomography reveals novel microtubule lattice and microtubule organizing centre defects in +TIP mutants

Large organellar changes occur during mild heat shock in yeast

When the temperature is increased, the heat-shock response is activated to protect the cellular environment. The transcriptomics and proteomics of this process are intensively studied, while information about how the cell responds structurally to heat stress is mostly lacking. Here, Saccharomyces cerevisiae were subjected to a mild continuous heat shock (38°C) and intermittently cryo-immobilised for electron microscopy. Through measuring changes in all distinguishable organelle numbers, sizes and morphologies in over 2100 electron micrographs, a major restructuring of the internal architecture of the cell during the progressive heat shock was revealed. The cell grew larger but most organelles within it expanded even more, shrinking the volume of the cytoplasm. Organelles responded to heat shock at different times, both in terms of size and number, and adaptations of the morphology of some organelles (such as the vacuole) were observed. Multivesicular bodies grew by almost 70%, indicating a previously unknown involvement in the heat-shock response. A previously undescribed electron-translucent structure accumulated close to the plasma membrane. This all-encompassing approach provides a detailed chronological progression of organelle adaptation throughout the cellular heat-stress response.

Molecular model of fission yeast centrosome assembly determined by superresolution imaging

Microtubule-organizing centers (MTOCs), known as centrosomes in animals and spindle pole bodies (SPBs) in fungi, are important for the faithful distribution of chromosomes between daughter cells during mitosis as well as for other cellular functions. The cytoplasmic duplication cycle and regulation of the Schizosaccharomyces pombe SPB is analogous to centrosomes, making it an ideal model to study MTOC assembly. Here, we use superresolution structured illumination microscopy with single-particle averaging to localize 14 S. pombe SPB components and regulators, determining both the relationship of proteins to each other within the SPB and how each protein is assembled into a new structure during SPB duplication. These data enabled us to build the first comprehensive molecular model of the S. pombe SPB, resulting in structural and functional insights not ascertained through investigations of individual subunits, including functional similarities between Ppc89 and the budding yeast SPB scaffold Spc42, distribution of Sad1 to a ring-like structure and multiple modes of Mto1 recruitment.

A stable microtubule array drives fission yeast polarity reestablishment upon quiescence exit

Cells perpetually face the decision to proliferate or to stay quiescent. Here we show that upon quiescence establishment, Schizosaccharomyces pombe cells drastically rearrange both their actin and microtubule (MT) cytoskeletons and lose their polarity. Indeed, while polarity markers are lost from cell extremities, actin patches and cables are reorganized into actin bodies, which are stable actin filament-containing structures. Astonishingly, MTs are also stabilized and rearranged into a novel antiparallel bundle associated with the spindle pole body, named Q-MT bundle. We have identified proteins involved in this process and propose a molecular model for Q-MT bundle formation. Finally and importantly, we reveal that Q-MT bundle elongation is involved in polarity reestablishment upon quiescence exit and thereby the efficient return to the proliferative state. Our work demonstrates that quiescent S. pombe cells assemble specific cytoskeleton structures that improve the swiftness of the transition back to proliferation.

Cell cycle control of spindle pole body duplication and splitting by Sfi1 and Cdc31 in fission yeast

Spindle pole biogenesis and segregation are tightly coordinated to produce a bipolar mitotic spindle. In yeasts, the spindle pole body (SPB) half-bridge composed of Sfi1 and Cdc31 duplicates to promote the biogenesis of a second SPB. Sfi1 accumulates at the half-bridge in two phases in Schizosaccharomyces pombe, from anaphase to early septation and throughout G2 phase. We found that the function of Sfi1-Cdc31 in SPB duplication is accomplished before septation ends and G2 accumulation starts. Thus, Sfi1 early accumulation at mitotic exit might correspond to half-bridge duplication. We further show that Cdc31 phosphorylation on serine 15 in a Cdk1 (encoded by cdc2) consensus site is required for the dissociation of a significant pool of Sfi1 from the bridge and timely segregation of SPBs at mitotic onset. This suggests that the Cdc31 N-terminus modulates the stability of Sfi1-Cdc31 arrays in fission yeast, and impacts on the timing of establishment of spindle bipolarity.

Regulation of spindle pole body assembly and cytokinesis by the centrin-binding protein Sfi1 in fission yeast

A previous model suggested doubling of Sfi1 as the first step of SPB assembly. Here it is shown that Sfi1 is gradually recruited to SPBs throughout the cell cycle. Conserved tryptophans in Sfi1 are required for its equal partitioning during mitosis, and unequal partitioning of Sfi1 underlies SPB assembly and mitotic defects in the next cell cycle.

Centrosomes play critical roles in the cell division cycle and ciliogenesis. Sfi1 is a centrin-binding protein conserved from yeast to humans. Budding yeast Sfi1 is essential for the initiation of spindle pole body (SPB; yeast centrosome) duplication. However, the recruitment and partitioning of Sfi1 to centrosomal structures have never been fully investigated in any organism, and the presumed importance of the conserved tryptophans in the internal repeats of Sfi1 remains untested. Here we report that in fission yeast, instead of doubling abruptly at the initiation of SPB duplication and remaining at a constant level thereafter, Sfi1 is gradually recruited to SPBs throughout the cell cycle. Like an sfi1Δ mutant, a Trp-to-Arg mutant (sfi1-M46) forms monopolar spindles and exhibits mitosis and cytokinesis defects. Sfi1-M46 protein associates preferentially with one of the two daughter SPBs during mitosis, resulting in a failure of new SPB assembly in the SPB receiving insufficient Sfi1. Although all five conserved tryptophans tested are involved in Sfi1 partitioning, the importance of the individual repeats in Sfi1 differs. In summary, our results reveal a link between the conserved tryptophans and Sfi1 partitioning and suggest a revision of the model for SPB assembly.

Modes of flagellar assembly in Chlamydomonas reinhardtii and Trypanosoma brucei

Defects in flagella growth are related to a number of human diseases. Central to flagellar growth is the organization of microtubules that polymerize from basal bodies to form the axoneme, which consists of hundreds of proteins. Flagella exist in all eukaryotic phyla, but neither the mechanism by which flagella grow nor the conservation of this process in evolution are known. Here, we study how protein complexes assemble onto the growing axoneme tip using (cryo) electron tomography. In Chlamydomonas reinhardtii microtubules and associated proteins are added simultaneously. However, in Trypanosoma brucei, disorganized arrays of microtubules are arranged into the axoneme structure by the later addition of preformed protein complexes. Post assembly, the T. brucei transition zone alters structure and its association with the central pair loosens. We conclude that there are multiple ways to form a flagellum and that species-specific structural knowledge is critical before evaluating flagellar defects.

DOI:http://dx.doi.org/10.7554/eLife.01479.001

Some cells have a whip-like appendage called a flagellum. This is most often used to propel the cell, notably in sperm cells, but it can also be involved in sensing cues in the surrounding environment. Flagella are found in all three domains of life—the eukaryotes (which include the animals), bacteria and ancient, single-celled organisms called Archaea—and they perform similar functions in each domain. However, they also differ significantly in their protein composition, overall structure, and mechanism of propulsion.

The core of the flagellum in eukaryotes is made up of 20 hollow filaments called ‘microtubules’ arranged so that nine pairs of microtubules form a ring around two central microtubules. The core also contains many other proteins, but it is not clear how all these components come together to make a working flagellum. Moreover, it is not known if the flagella of different groups of eukaryotes are all assembled in the same way.

Now, Höög et al. have discovered that although the core structure of the eukaryote flagellum is highly conserved, it can be assembled in markedly different ways. Some species of eukaryote—such as Chlamydomonas reinhardtii, a single-celled green alga, and Trypanosoma brucei, the protist parasite that causes African sleeping sickness—must grow new flagella when their cells divide, so that each new cell can swim. Using a form of electron microscopy called electron tomography, Höög et al. could see the detailed structure of the growing flagella in three dimensions. At first the cores of the flagella in these two distantly related species grow in the same way. However as the flagella get longer their cores grow in completely different ways. The microtubule filaments in longer flagella grow in a synchronized manner in the alga, but in a disorganized way in the protist.

The results of Höög et al. illustrate that it is not advisable to draw generalised conclusions based on studies of a few model species. However, since defects in flagella are known to cause several diseases in humans, this knowledge might inform future studies aimed at developing treatments for infertility, respiratory problems, and certain kinds of cancer.

DOI:http://dx.doi.org/10.7554/eLife.01479.002