Lecture 8 and 9 - Cell Polarity Flashcards
What is cell polarity?
Cell polarity is the asymmetric distribution of cellular components, such as organelles, proteins, and lipids, within a cell.
Why is cell polarity important?
Cell polarity is important because it is essential for the proper function and morphology of different cell types. It allows cells to perform specialized functions and to interact with other cells and the extracellular environment.
What are the internal and external signals that cause morphological changes in yeast?
Yeast undergoes significant morphological changes in response to both internal and external signals. Internal signals include growth and division signals such as the growth of a bud and cytokinesis, while external signals include pheromones (for mating) and nutritional signals (cells can elongate).
Where on the cell surface do budding yeast cells generate cell polarity in order to grow and divide?
Budding yeast cells must generate cell polarity in order to grow and divide, and they do so at the mother bud neck.
What is the purpose of calcofluor staining in yeast cell imaging?
Calcofluor staining is used in yeast cell imaging to bind to a component of the yeast cell wall called chitin, and to allow the birth scars which mark the sites of previous cell separations to be viewed as bright rings on the cell wall.
What is the difference between the axial and bipolar budding patterns in yeast?
Haploid a and a cells bud in an axial pattern, in which both mother and daughter cells are constrained to form buds immediately adjacent to the previous site of cell separation. In contrast, diploid cells bud in a bipolar manner, in which mother and daughter cells bud at the poles of their ellipsoidal cells.
What genes are required for the yeast axial budding pattern?
Genes required for the yeast axial budding pattern include BUD10, BUD3, BUD4, and the septins. Products from these genes are involved in marking the mother bud neck during one cycle as a site for budding in the next cycle.
What genes are required for the yeast bipolar budding pattern?
Genes required for the yeast bipolar budding pattern include BUD8, BUD9, RAX2, and components of the actin cytoskeleton. Products from these genes mark the ends of diploid cells.
What proteins are involved in establishing cell polarity in yeast?
The family of Rho-GTPases are important proteins involved in polarity establishment in yeast. In particular, Cdc42 is the most important protein for polarity establishment, and it is highly conserved across evolution.
What happens to yeast cells when Cdc42 is mutated?
When Cdc42 is mutated, yeast cells are blocked because they cannot direct their growth to form a new bud. For example, a temperature-sensitive mutant of Cdc42 will grow normally at 24°C but show isotropic growth (grow all over the surface) and cannot establish an axis of polarity at 37°C.
What is the role of Cdc42 in establishing cell polarity in yeast?
Cdc42 is a small GTPase of the Rho family that is regulated through cycles of activation and inactivation by its binding partners, including Cdc24 (a GEF) and several GAPs. Once polarity is established, cell components can be trafficked to the bud via proteins like Bem1 and Cdc43, which adds a lipid moiety. The GEF for Cdc42 (Cdc24) binds to the active form of Bud1 at sites marked for budding. Cdc24 then binds Bud1 and can then activate PAK family kinases, allowing the polarity site to become linked to the cell cycle.
What is the purpose of the exocyst complex in yeast cell polarity?
The exocyst complex in yeast cell polarity ensures polarized membrane trafficking, which is responsible for the movement of cytoplasmic organelles and other material from mother to daughter cell. Cdc42 is also involved in exocyst function.
What steps can be defined that lead to the generation of cell polarity?
1.Marking the site: The site for polarity establishment is marked through previous sites of cell separation, such as the birth scar, or through specific cues from the environment, such as pheromones or nutrients.
2.Decoding the site: Proteins encoded by genes such as BUD1, BUD2, and BUD5 decode the axial and bipolar marks and signal to the machinery involved in generating the polarity axis.
3.Recruitment of machinery: Proteins such as Rho-GTPases and the family of PAK kinases are recruited to the site to establish the polarity axis.
4.Establishing the site: Cdc42, a small GTPase of the Rho family, is involved in establishing the polarity site. Cdc42 is regulated through cycles of activation and inactivation by its binding partners Cdc24 (a GEF) and several GAPs.
5.Maintaining the site: Feedback loops ensure that the machinery remains in place at the site of polarity establishment.
How and why might a simple organism such as yeast need to polarise its cell constituents?
In summary, polarization of cell constituents in yeast is necessary for essential cellular functions such as nutrient uptake, growth, and reproduction. It enables the selective accumulation of proteins and organelles at specific sites, leading to asymmetric cell division and polarized cell growth.
What are the two main routes to cell diversity?
Polar mother cells could divide to generate daughters that have inherited different components or daughters could be equal at ‘birth’ but then become different by exposure to different environmental signals.
