Project description:Cell size varies greatly between cell types, yet within a specific cell type and growth condition, cell size is narrowly distributed. Why maintenance of a cell-type specific cell size is important is not understood. Here we show that growing beyond a certain size has wide-ranging effects on cell physiology. Large cells are defective in gene induction, cell cycle progression and cell signaling. We further show that these defects are caused by the inability of large cells to scale nucleic acid and protein biosynthesis in accordance with cell volume increase, which effectively leads to cytoplasm dilution. Finally, we determine why nucleic acid and protein biosynthesis do not scale with cell volume beyond a certain critical size. DNA becomes limiting. We conclude that the correct DNA to cytoplasm ratio is vital for many perhaps all cellular functions and that the range where this ratio supports optimal cell function is remarkably narrow.
Project description:Cell size varies greatly between cell types, yet within a specific cell type and growth condition, cell size is narrowly distributed. Why maintenance of a cell-type specific cell size is important is not understood. Here we show that growing beyond a certain size has wide-ranging effects on cell physiology. Large cells are defective in gene induction, cell cycle progression and cell signaling. We further show that these defects are caused by the inability of large cells to scale nucleic acid and protein biosynthesis in accordance with cell volume increase, which effectively leads to cytoplasm dilution. Finally, we determine why nucleic acid and protein biosynthesis do not scale with cell volume beyond a certain critical size. DNA becomes limiting. We conclude that the correct DNA to cytoplasm ratio is vital for many perhaps all cellular functions and that the range where this ratio supports optimal cell function is remarkably narrow.
Project description:Compartmentalization is an essential feature of eukaryotic life and is achieved both via membrane-bound organelles, such as mitochondria, and membrane-less biomolecular condensates, such as the nucleolus. Known biomolecular condensates typically exhibit liquid-like properties and are visualized by microscopy on the scale of ~1µm. They have been studied mostly by microscopy, examining select individual proteins. So far, several dozen biomolecular condensates are known, serving a multitude of functions, for example, in the regulation of transcription, RNA processing or signalling and their malfunction can cause diseases. However, it remains unclear to what extent biomolecular condensates are utilized in cellular organization and at what length scale they typically form. Here we examine native cytoplasm from Xenopus egg extract on a global scale with quantitative proteomics, filtration, size exclusion and dilution experiments. These assays reveal that at least 18% of the proteome is organized into mesoscale biomolecular condensates at the scale of ~100nm and appear to be stabilized by RNA or gelation. We confirmed mesoscale sizes via imaging below the diffraction limit by investigating protein permeation into porous substrates with defined pore sizes. Our results show that eukaryotic cytoplasm organizes extensively via biomolecular condensates, but at surprisingly short length scales.
Project description:Proper organelle size is critical for many cell functions. However, how cells sense and control their organelle size remains elusive. Here, we develop a general model to study the size control of mitotic spindles by considering both extrinsic and intrinsic factors, such as the limited number of building blocks of the spindle, the interaction between the spindle and cell boundary, the DNA content, the forces generated by various molecular motors, and the dynamics of microtubules. We show that multiple pairs of chromatids, two centrosomes, and microtubules can self-assemble to form a mitotic spindle robustly. We also show that the boundary-sensing and volume-sensing mechanisms coexist in small cells, but both break down in large cells. Strikingly, we find that the upper limit of spindle length naturally arises from the geometric asymmetry of the spindle structure. Thus, our findings reveal, to our knowledge, a novel intrinsic mechanism that limits the organelle size.
Project description:How cells control the overall size of membrane-bound organelles is an important unanswered question of cell biology. Fission yeast cells maintain a nuclear size that is proportional to cellular size, resulting in a constant ratio between the nuclear and cellular volumes (N/C ratio). To shed light on nuclear size control we conducted a genome-wide visual screen of a fission yeast gene deletion collection for mutants altered in their N/C ratio, and found that defects in both nucleocytoplasmic mRNA transport and nuclear membrane proliferation alter the N/C ratio. Perturbing nuclear mRNA export causes general bulk accumulation of both mRNA and protein, and a N/C ratio increase which is dependent on new membrane synthesis. Dysregulation of nuclear membrane growth also results in an enlarged N/C ratio, and additionally generates an aberrant nuclear shape. We propose that both properly regulated nucleocytoplasmic transport and nuclear membrane growth are central for controlling nuclear size.
Project description:Yeast cells must grow to a critical size before committing to division. It is unknown how size is measured. We find that as cells grow, mRNAs for some cell cycle activators scale faster than size, increasing in concentration, while mRNAs for some inhibitors scale slower than size, decreasing in concentration. Size-scaled gene expression could cause an increasing ratio of activators to inhibitors with size, triggering cell cycle entry. Consistent with this, expression of the CLN2 activator from the promoter of the WHI5 inhibitor, or vice versa, interfered with cell size homeostasis, yielding a broader distribution of cell sizes. We suggest that size homeostasis comes from differential scaling of gene expression with size. Such regulation of gene expression as a function of cell size could affect many cellular processes.