Cell Polarity in Budding Yeast and Actin Organization

Study Notes

Cell polarity often starts with local regulation of the actin cytoskeleton via external/internal signals
Polarity signals converge beneath the plasma membrane, activating Rho family GTPases (Cdc42, Rac, Rho)
These GTPases are small, monomeric proteins that switch between active (GTP-bound) and inactive (GDP-bound) states
Guanine nucleotide exchange factors (GEFs) activate GTPases by replacing GDP with GTP
GTPase activating proteins (GAPs) inactivate GTPases by promoting GTP hydrolysis
Guanine nucleotide dissociation inhibitors (GDIs) can bind to GDP-bound GTPases and inhibit GTP exchange

Actin filaments are labeled with fluorescent phalloidin
Serum-starved fibroblasts have primarily cortical actin filaments and few stress fibers
Microinjection of active Cdc42 leads to numerous long filopodia at the cell periphery
Microinjection of active Rac causes extensive lamellipodium formation at the cell's circumference
Microinjection of active Rho leads to rapid assembly of prominent stress fibers

Cdc42 is a highly conserved Rho family GTPase, crucial for cell polarity in yeast
Budding in yeast involves selecting a single bud site
Cdc42-GTP recruits its own GEF for a positive feedback loop
Local activation of formin proteins by Cdc42-GTP nucleates actin filaments, facilitating vesicle transport toward the new bud site via myosin V motors

PAR proteins establish two distinct cortical domains in C. elegans embryos
Symmetry breaking occurs before fertilization
Sperm entry decreases Rho GEF activity at the future posterior end, leading to anterior PAR protein accumulation
After fertilization, Rho GEF is depleted at sperm entry site (posterior), reducing myosin levels and contractility
Par-3 localizes to the anterior and Par-2 to the posterior, maintaining asymmetry via mutual antagonism

PAR and Crumbs proteins assemble the apical domain and junctional complexes
Scribble defines the basolateral domain
PAR and Crumbs reinforce each other, while Scribble and PAR are mutually antagonistic
Cdc42 helps recruit PAR proteins.

Rac-GTP promotes protrusive actin networks (lamellipodia/pseudopodia)
Rho-GTP leads to actin bundle formation (stress fibers) and reduced actin network formation
Activation of Rac leads to altering actin accessory proteins and branching actin network in protrusion
Rho-GTP activation leads to actin-myosin contraction and diminished actin network size

Bacterial molecules binding to neutrophil receptors stimulate directed motility
Receptors are distributed on the cell surface, but more likely to bind to ligand at the front
Rac pathway leads to outgrowth at the front
Rho pathway leads to contraction at the rear, generating directional movement.