Cell Polarity in Budding Yeast and Actin Organization

Cell Polarity in Budding Yeast

• 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

Dramatic Effects of Cdc42, Rac, and Rho on Actin Organization in Fibroblasts

• 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 Establishes Yeast Cell Polarity

• 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 and Anterior-Posterior Polarity in Embryos

• 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

Conserved Complexes Polarize Epithelial Cells

• 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.

Contrasting Effects of Rac and Rho Activation on Actin Organization

• 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

Neutrophil Polarization and Chemotaxis

• 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.