Beyond symmetry-breaking: competition and negative feedback in GTPase regulation

Abstract

Cortical domains are often specified by the local accumulation of active GTPases. Such domains can arise through spontaneous symmetry-breaking, suggesting that GTPase accumulation occurs via positive feedback. Here, we focus on recent advances in fungal and plant cell models - where new work suggests that polarity-controlling GTPases develop only one 'front' because GTPase clusters engage in a winner-takes-all competition. However, in some circumstances two or more GTPase domains can coexist, and the basis for the switch from competition to coexistence remains an open question. Polarity GTPases can undergo oscillatory clustering and dispersal, suggesting that these systems contain negative feedback. Negative feedback may prevent polarity clusters from spreading too far, regulate the balance between competition and coexistence, and provide directional flexibility for cells tracking gradients.

Keywords: Cdc42; GAP; GEF; Rac; Rop.

Figure I

Positive feedback via GEF-effector complexes

Figure 1. Competition and co-existence among GTPase clusters

A) In S. cerevisiae cells breaking symmetry, clusters of Cdc42 compete until a single winner emerges. B) In S. pombe, initial symmetry breaking is followed by stereotypical cycles of old-end growth and NETO (new end take-off) leading to bipolar growth. C) In A. gossypii, initial symmetry breaking is followed by hyphal branching (not shown) and then apical branching that involves the splitting of one Cdc42 cluster into two. D) In A. thaliana, heterologous expression of the xylem-specific GTPase Rop11 and its GEF and GAP in nonxylem plant cell types leads to formation of multiple dispersed clusters containing Rop11-GTP and its GEF (left). In leaf pavement cells, mutually inhibitory clusters of Rop2/Rop4 and Rop6 GTPases establish zones of growth versus indentation (right).

Figure 2. Oscillatory GTPase clustering and dispersal indicates presence of negative feedback

A) In S. cerevisiae cells breaking symmetry, the Cdc42 cluster that wins the competition goes on to partially disperse and reform. B) In S. pombe after NETO, Cdc42 alternately clusters and disperses in an anticorrelated manner at each tip. C) In pollen tubes, Rop1 clusters and disperses in an oscillatory manner. Clustering precedes (and probably causes) each growth spurt.

Figure 3. Potential negative feedback pathways and their effects

A) Fungal hyphae (left) and pollen tubes (right) display concentrated GTPases at the tip plasma membrane. Large vesicle clusters (spitzenkörpers or inverted cones) are not enriched for the GTPases, implying that fusion of the vesicles to the plasma membrane would dilute the GTPases. B) In pollen tubes, the Rop1-GAP, Ren1, is concentrated on vesicles. Thus, vesicle fusion would not only dilute but also inactivate Rop1, providing strong negative feedback. C) In S. cerevisiae, stochastic off-center delivery of vesicles may nudge the Cdc42 cluster away from the vesicle fusion site, promoting wandering of the polarity site in cells tracking pheromone gradients. D) In U. maydis, Rac-GTP activates the effector kinase Cla4 to phosphorylate the Rac-GEF, promoting its degradation.