8 - Cell Polarity

Question 1

What is Cell Polarity?

Answer 1

the organisation of proteins inside + at the surface of cells
regions have distinct protein composition
the cell can thereby have diff capabilities, morphologies and functions

Question 2

Why is cell polarity necessary for?

Answer 2

the cell to be able to generate a wide variety of forms to perform a diverse array of functions

Question 3

What are the key functional requirements to be able to polarise a cell?

Answer 3

1. Marking the site
2. Decoding the site - cell must know something has happened
3. Establishing the site - key proteins go to site - organise cytoskeleton, make other prot etc
4. maintaining the site - feedback loops
   (5. How to unpolarise)

Question 4

Why is budding yeast (Saccharomyces cerevisiae) good for studying cell polarity?

Answer 4

• yeast undergoes sig morphological changes in response to external + internal sig
• is genetically tractable - entire genome is known + annotated
• has been used to understand fundamental aspects of key cell processes inc cell cycle, secretion + cell polarity

Question 5

Internal changes of budding yeast:

Answer 5

in response to growth + division signals
e.g. growth of bud + cytokinesis

Question 6

External changes of budding yeast:

Answer 6

in response to pheromones (for mating)
and nutritional signals (cells can elongate)

Question 7

What does budding yeast need to grow and divide?

Answer 7

Cell polarity

Question 8

How do yeast cells bud?

Answer 8

bud + divide in precise spatial patterns
can be followed by staining w/ fluorescent dye - calcifluor
binds to chitin
allows birth scars (mark sites of previous sites of separation) - to be viewed as bright rings on the cell wall

Question 9

What does the position of the new cell bud depend on?

Answer 9

if the cell is haploid or diploid

Question 10

Where do new haploid cell buds form?

Answer 10

haploid a and alpha cells bud in axial pattern
- both mother + daughter cells are constrained to form buds immediately adjacent to previous site of cell separation

Question 11

Where do new diploid cell buds form?

Answer 11

bud in bipolar manner
mother + daughter constrained to form bud at the poles of ellipsoidal cells
allows to explore wider environment

Question 12

What genes are required for the yeast axial budding pattern?

Answer 12

BUD10, BUD3, BUD4 and the septins

Question 13

What do products of BUD10, BUD3, BUD4 and the septins do?

Answer 13

are involved in marking the mother bud neck during one cycle as a site for budding in the next cycle
mutations do not affect haploid cells
Haploid cells now mostly bud in bipolar manner

Question 14

What genes are required for the yeast bipolar budding pattern?

Answer 14

BUD8,BUD9,RAX2 and components of actin skeleton are involved

Question 15

What do products of BUD8,BUD9,RAX2 and components of actin skeleton are involved
do?

Answer 15

They mark the ends of the diploid cells
Haploid mutants in these genes still use the axial pattern but the bipolar pattern is disrupted

Question 16

What genes are required for both axial and bipolar budding patterning?

Answer 16

BUD1, BUD2,BUD5

Question 17

What do proteins encoded by BUD1, BUD2, BUD5 do?

Answer 17

decode the axial + bipolar marks + signal to the machinery involved in generating the polarity axis
Mutations in these genes cause a random budding pattern in both haploid + diploid cells

Question 18

How is the site decoded?

Answer 18

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

Question 19

How is the site established?

Answer 19

cell integrates spacial cues from budding landmarks
information fed to polarity establishment machinery
responsible for cell cytoskeleton + other cell components

Question 20

What are the most important proteins involved in polarity establishment?

Answer 20

the family of rho-GTPases

Question 21

What are the most important proteins involved in polarity establishment in yeast?

Answer 21

Cdc42

Question 22

What mutations in yeast stop the formation of new buds?

Answer 22

Genes - cdc24, cdc43, cdc42

Question 23

How is Cdc42 temperature sensitive?

Answer 23

is a temperature sensitive mutant
At 24C cells can polarise, form a bud, grow + divide
At 37C cells show isotropic growth (grow all over surface) + cannot establish an axis of polarity

Question 24

How is Cdc42 regulated?

Answer 24

through cycles of activation + inactivation by its binding partners Cdc24 (a GEF) and several GAPs

Question 25

How does Cdc42 function to establish cell polarity?

Answer 25

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 Cdc42 to allow the polarity site to become established Can recruit a number of proteins

Question 26

Mating in budding cell yeast and polarisation

Answer 26

Haploid yeast cells can polarise and redirect growth to facilitate mating response is chemotrophic due to mating pheromones being secreted from different cell types

Question 27

Explain MATa and MAT alpha cells?

Answer 27

Involved in mating in budding yeast MATa cells secrete a-factor MATalpha cells secrete alpha-factor MATa cells have a receptor on their cell surface (Ste2) - can bind to alpha factor MATalpha cells have a receptor (Ste3) - can bind to a factor

Question 28

What receptors detect + bind to pheromones?

Answer 28

members of conserved g-coupled receptor family They interact w/ a heterotrimeric g-protein that orchestrated the downstream cell response Cell cycle arrest

Question 29

When do budding yeast cells mate?

Answer 29

Only mate during G1 Far1 binds to heterotrimeric g protein*** Sec3 - more elongated growth

Question 30

what key proteins require asymmetric inheritance in daughter cells?

Answer 30

key proteins (including myosins Myo2 and Myo4)required asymmetric inheritance of specific factors (proteins and mRNAs)

Question 31

what does the degradation of cargo adaptors in daughter cells do?

Answer 31

prevents backwards movement of organelles

Question 32

What is the fungus candida albicans?

Answer 32

normally a beign member of mucosal flora 50% of people carry it However it commonly causes mucosal disease It becomes invasive there is substantial morbidity - in vulnerable patients causes life threatening bloodstream infections Freq seen in AIDS patients, premature babies and terminally ill patients The fungus can leave the bloodstream and penetrate cell tissues and organs

Question 33

Explain how candida albicans is dimorphic?

Answer 33

able to switch btwn yeast and hyphal forms can grow in both forms - this is central to the virulence

Question 34

Hyphal forms of candida albicans:

Answer 34

Hyphal formation - stimulated at 37C by serum or neutral pH Hyphae are more adherent to mammalian cells + imp for tissue penetration

Question 35

Yeast forms of candida albicans:

Answer 35

Yeast cells taken up my macrophages - can switch to filamentous growth + lyse the macrophage Yeast cells - carried more effectively in the bloodstream promoting fungal dissemination in the body

Question 36

What is cell polarity crucial for?

Answer 36

1. Asymmetric cell division - including cell fate decisions 2. Epithelial cells - to make an effective barrier 3.Cell migration e.g. In development

Question 37

Whitman (1878)

Answer 37

studied leeches and showed that distinct cytoplasmic domains are differently partitioned to descendants and that these differences were reflected in different cell lineages

Question 38

Conklin (1905)

Answer 38

identified 5 different cytoplasm types in the ascidina oocyte that were differently inherited to determine tissue types

Question 39

What are the two main routes of diversity in daughter cells?

Answer 39

1. Polar mother cells could divide to generate daughters that have inherited different components 2. Daughters could be equal at birth but then become different by exposure to different environmental signals

Question 40

How does asymmetric cell division take place?

Answer 40

1. Establishment of an axis of polarity (this involves marking a site, signalling and establishing - as discussed earlier) 2. Mitotic spindle is positioned along the axis 3. Differential distribution of cell fate determinants to daughter cells

Question 41

What do PAR proteins do?

Answer 41

PAR proteins form the core of a cell polarity network in many animal cells and in many developmental contexts

Question 42

When does polarisation start?

Answer 42

Polarisation starts w/ entry of sperm to oocyte Position of sperm entry defines the posterior end of the zygote

Question 43

How does the zygote divide?

Answer 43

The zygote - also called PO cell - divides asymmetrically along A-P axis Produces a larger anterior cells (AB) and a smaller posterior cell (P1) The daughters are different in size and are committed to diff cell fates

Question 44

When is symmetry broken in the oocyte?

Answer 44

at fertilization the sperm delivers a microtubule organising centre (MTOC) This site becomes posterior pole and so defined the axis of polarity

Question 45

What does the recruitment of PAR1 and PAR2 by microtubules in the oocyte lead to?

Answer 45

antagonises anterior Par proteins and they accumulate at anterior cortical domain - this results in distinct localizations of the par proteins

Question 46

Where do different PAR proteins localise?

Answer 46

Par3.Par6/aPkc localise to the anterior cortex Par1 and Par2 are at the posterior cortex and Par maintains the boundary

Question 47

where are neuroblasts found in drosophila?

Answer 47

found within a specific region of an epithelial monolayer - called the ventral neuroectoderm

Question 48

Drosophila asymmetric division:

Answer 48

polarity established when cell is still in neuroectoderm layer neuroblasts delaminate Par3 Bazooka/Par6 are found in a stalk that continues to extend into the epithelium After delamination, they continue to localise to the apical region Baz anchors another complex (insc/Pins) at the membrane in order to orient the mitotic spindle Scribble complex helps in spindle alignment

Question 49

what cell fate determinants are found in drosophila

Answer 49

transported in the basal direction to the ganglion mother cell includes factors such as prospero and staufen - regulate expression of specific genes in the GMC Following cell division the GMC has a different fate because of asymmetric inheritance of these determinants

Question 50

What are the 3 main activities required for movement

Answer 50

1. Protrusion - the pushing out of the plasma membrane in front of the cell 2. Attachment - the actin cytoskeleton inside the cell is attached via interacting proteins across the plasma membrane to the substratum (e.g. Extracellular matrix) 3. Traction - the bulk of the cell body is drawn forward through the process of contraction

Question 51

What role do actin structures play in migration?

Answer 51

diff types of protrusion - including filopodia + lamellipodia Are filled w/ filaments of the cytoskeleton protein -actin Filopodia or microspikes are a dense core of bundled actin filaments - are involved in the contractility required to move body of cell forward

Question 52

What Rho GTPases are involved in cell migration?

Answer 52

Cdc42, Rac and Rho diff GTPases responsible for generating different actin structures in cells

Question 53

What is chemotaxis?

Answer 53

This is the movement of cells towards/ away from a signal - e.g. A diffusible chemical An example is the movement of a neutrophil moving towards a site of bacterial infection