Fusion of Protein Aggregates Facilitates Asymmetric Damage Segregation

Asymmetric segregation of damaged proteins at cell division generates a cell that retains damage and a clean cell that supports population survival. In cells that divide asymmetrically, such as Saccharomyces cerevisiae, segregation of damaged proteins is achieved by retention and active transport. We have previously shown that in the symmetrically dividing Schizosaccharomyces pombe there is a transition between symmetric and asymmetric segregation of damaged proteins. Yet how this transition and generation of damage-free cells are achieved remained unknown. Here, by combining in vivo imaging of Hsp104-associated aggregates, a form of damage, with mathematical modeling, we find that fusion of protein aggregates facilitates asymmetric segregation. Our model predicts that, after stress, the increased number of aggregates fuse into a single large unit, which is inherited asymmetrically by one daughter cell, whereas the other one is born clean. We experimentally confirmed that fusion increases segregation asymmetry, for a range of stresses, and identified Hsp16 as a fusion factor. Our work shows that fusion of protein aggregates promotes the formation of damage-free cells. Fusion of cellular factors may represent a general mechanism for their asymmetric segregation at division

NSC230378

A key aspect of life is sexual reproduction, which requires concerted movement. For successful mixing of the genetic material, molecular motors move the nucleus back and forth inside the cell. How motors work together to produce these large-scale movements, however, remains a mystery. To answer this question, we studied nuclear movement in fission yeast, which is driven by motor proteins pulling on microtubules. We show that motor proteins dynamically redistribute from one part of the cell to the other, generating asymmetric patterns of motors and, consequently, of forces that generate movement. By combining quantitative live cell imaging and laser ablation with a theoretical model, we find that this dynamic motor redistribution occurs purely as a result of changes in the mechan- ical strain sensed by the motor proteins. Our work therefore demonstrates that spatio-temporal pattern formation within a cell can occur as a result of mechani- cal cues (Vogel et al., 2009), which differs from conventional molecular signaling, as well as from self-organization based on a combination of biochemical reactions and diffusion

Nuclear and Division-Plane Positioning Revealed by Optical Micromanipulation

SummaryThe position of the division plane affects cell shape and size, as well as tissue organization. Cells of the fission yeast Schizosaccharomyces pombe have a centrally placed nucleus and divide by fission at the cell center. Microtubules (MTs) are required for the central position of the nucleus [1–4]. Genetic studies lead to the hypothesis that the position of the nucleus may determine the position of the division plane [5–10]. Alternatively, the division plane may be positioned by the spindle or by morphogen gradients or reaction diffusion mechanisms [7, 11]. Here, we investigate the role of MTs in nuclear positioning and the role of the nucleus in division-plane positioning by displacing the nucleus with optical tweezers. A displaced nucleus returned to the cell center by MT pushing against the cell tips. Nuclear displacement during interphase or early prophase resulted in asymmetric cell division, whereas displacement during prometaphase resulted in symmetric division as in unmanipulated cells. These results suggest that the division plane is specified by the predividing nucleus. Because the yeast nucleus is centered by MTs during interphase but not in mitosis, we hypothesize that the establishment of the division plane at the beginning of mitosis is an optimal mechanism for accurate symmetric division in these cells

Quantitative studies of subdiffusion in living cells and actin networks

Optical tweezers are a versatile tool in biophysics and have matured from a tool of manipulation to a tool of precise measurements. We argue here that the data analysis with advantage can be developed to a level of sophistication that matches that of the instrument. We review methods of analysis of optical tweezers data, primarily based on the power spectra of time series of positions for trapped spherical objects. The majority of precise studies in the literature are performed on in vitro systems, whereas in the present work, an example of an in vivo system is presented for which precise power spectral analysis is both useful and necessary. The biological system is the cytoplasm of ssion yeast, Schizosaccharomyces pombe in which we observe subdi usion of lipid granules. In a search for the cause of subdi usion, we chemically disrupt the actin network in the cytoplasm and further consider in vitro networks of lamenteous actin undergoing similar chemical disruption

Laser microsurgery provides evidence for merotelic kinetochore attachments in fission yeast cells lacking Pcs1 or Clr4

In order to segregate chromosomes properly, the cell must prevent merotelic kinetochore attachment, an error that occurs when a single kinetochore is attached to microtubules emanating from both spindle poles. Merotelic kinetochore orientation represents a major mechanism of aneuploidy in mitotic mammalian cells and it is the primary mechanism of chromosome instability in cancer cells. Fission yeast mutants defective in putative microtubule-site clamp Pcs1/Mde4 or Clr4/Swi6-dependent centromeric heterochromatin display high frequencies of lagging chromosomes during anaphase. Here, we developed an assay based on laser microsurgery to show that the stretched morphology of lagging kinetochores in pcs1? and clr4? mutant cells is due to merotelic attachment. We further show that Mde4 is regulated by Cdc2 and that Cdc2 activity prevents precocious localization of Mde4 to the metaphase spindle. Finally, we show that Pcs1/Mde4 complex shares similar features with the conserved kinetochore complex Spc24/Spc25 suggesting that these two complexes may occupy a similar functional niche