Cooperative activation of PI3K By ras and rho family small GTPases in an isoform specific manner

Abstract

Class I Phosphoinositide 3-kinases (PI3Ks) and Ras and Rho family small GTPases are key regu-lators of processes such as cell polarization, motility and chemotaxis. Class I PI3Ks are comprised of four p110 catalytic subunits (p110α, p110β, p110γ, and p110δ). These signaling components influence each other’s activities by direct and indirect feedback processes that are only partially understood. A number of studies have shown that PI3Ks are activated by Ras and Rho family small GTPases, but little is known about isoform-specific activation of the four PI3K isoforms. In chapter 1, I identified 21 small GTPase homologs activate PI3K. K-Ras, H-Ras and five other homologs function upstream of PI3K and directly bind the PI3Ks catalytic subunit, p110. In addition, three Rac isoforms and 11 other small GTPase homologs activate PI3K by indirect feedback mecha-nisms. Furthermore, a large portion of PI3K activation was generated by positive feedback that did not result from the activity of a single small GTPase but was instead the result of cooperation between several Rho family small GTPases that together form a distributed network to reinforce PI3K activity. This model explains the past challenges in elucidating the relevance of specific small GTPases in processes such as cell polarization and chemotaxis. In chapter 2, I show that Ras and Rho family small GTPases interact with and activate PI3Ks in an isoform-specific manner. Using a recently developed protein-protein interaction assay, I visualized the interaction between 21 PI3K-activating small GTPases and the four PI3K isoforms. Surprisingly, I found that Ras family small GTPases bind to p110α, p110γ?? and p110δ, whereas Rho family small GTPases bind to p110β. These isoform-specific interactions with different subfamilies of small GTPases lead to the isoform-specific functions of PI3K, as shown by the translocation of PHAKT1. Furthermore, to induce cell migration, I discovered that Ras family small GTPases initiall...