6 - Cell Polarity

Question 1

What is cell polarity?

Answer 1

The organisation of proteins inside and at the surface of cells so regions of the cell have distinct protein compositions.
This cases the cell to have different capabilities morphologies and functions.

Question 2

What are the function requirements to be able to polarise a cell?

Answer 2

• Marking a site - where on the cell surface
• Decoding the site - signalling
• Establishing the site - recruitment of machinery
• Maintaining the site - remembering where the machinery is and keeping it in place

Question 3

Why is yeast used to study cell polarity growth and division?

Answer 3

• The entire genome sequence is known
• It undergoes significant morphological changes in response to both internal and external signals.
  (internal - in repose to growth and division signals eg growth of a bud and cytokinesis)
  (external - in response to pheromones and nutritional signals)

Question 4

Marking the site - Where on the cell surface?

Answer 4

The site is marked by staining cells with a fluorescent dye CALCOFLOUR, which binds to a compartment on the yeast cell wall called CHITIN.

Question 5

Marking the site - What does the position of the new bud depend on?

Answer 5

• The cell type either haploid or diploid.
• Haploid cells bud in an AXIAL pattern, both mother and daughter cells are constrained to form buds immediately.
• Diploid cells bind in a BIPOLAR manner, mother and daughter cells bud at the poles of their ellipsoidal cells.

Question 6

Marking the site - What are the genes required for the yeast axial budding pattern?

Answer 6

BUD10, BUD3 and BUD4 also the septins.

Question 7

Marking the site - What are the genes required for the biopolar budding pattern?

Answer 7

BUD8, BUD9 and RAX2 also components of the actin cytoskeleton.
Products from these genes mark the ends of diploid cells.

Question 8

Marking the site - What are the genes required for both the axial and bipolar budding patterns?

Answer 8

BUD1, BUD2 and BUD5
proteins encoded by these genes decode the axial and bipolar marks and signals to the machinery in generating polarity axis.

Question 9

Decoding the site

Answer 9

BUD1, BUD2 and BUD5 function together to signal to the polarity establishment machinery the position of bud site cortical landmarks
- These function together in a GTPase cycle.

Question 10

Establishing the site

Answer 10

• When the cell has integrated spatial cues from budding landmarks the info if fed to establishment machinery which polarises the cell cytoskeleton and other compartments.
• Proteins involved are the family of Rho-GTPases, most importantly in yeast this is Cdc42.

Question 11

How is Cdc42 regulated?

Answer 11

Through cycles of activation and inactivation by its binding partners (Cdc24 and several GAPs)

Question 12

How does Cdc24 work?

Answer 12

It binds to the active form of BUD1 at sites marked for budding, it can then activate Cdc42 to allow establishment of polarity.

Question 13

Why do budding yeast generate cell polarity?

Answer 13

In order to mate.

Question 14

What can haploid yeast cells do to generate mating?

Answer 14

they can redirect their axes, this is a chemotropic response due to secretion of mating pheromones.

Question 15

What are some yeast cell polarity diseases?

Answer 15

Candidiasis and Candidemia

Question 16

What is the fungus that causes the yeast cell polarity diseases and what group is it a part of?

Answer 16

• Candida albicans part for the mucosal flora, commonly causing mucosal diseases.
• Its homologous to saccharomyces cerevisiae.