Regulation of Cdc42/Rac Signaling in the Establishment of Cell Polarity and Control of Cell Motility

摘要

Cdc42p and other proteins polarize to a cap at the presumptive bud site and the tip of the bud. The observation that Cdc42p polarizes in the complete absence of F-actin has been confirmed repeatedly. These studies used LatA as a method to completely depolymerize actin, and suggested that Cdc42p polarization is actin-independent. Polarized secretion and endocytic uptake require F-actin cables and patches, respectively. When LatA is applied to yeast cells neither actin patches nor cables are detectable, disrupting all F-actin- dependent processes. Thus, Cdc42p polarization occurs by a non-secretory pathway. Is the polarized cap static. Current models of Cdc42p function propose that the cap is an organizing center for polarity. Since it is associated with the plasma membrane, the cap may be static, acting as a polymerization site for actin cables by formins, assembly of septins, signal transduction, etc. Our data indicate that the cap is, in reality, dynamic. We show that the loss of actin cables, but not actin patches, leads to Cdc42p delocalization. We propose that the initial polarization of Cdc42p is an actin-independent process; continued polarization is the result of two competing processes: endocytic removal of the plasma membrane, and a deposition process, dependent on polarized secretion.