G PROTEINS Endosomal PI3K, Effector and Gß-Like Partner for G

Mating pheromone signaling in yeast requires the G protein ß subunits; however, constitutively active G (Gpa1^Q323L) stimulates many of the mating responses. Using a screen of yeast deletion mutants, Slessareva et al. found that the catalytic and regulatory subunits of class III phosphoinositide 3-kinase (PI3K), Vps34 and Vps15, respectively, were required to mediate mating responses triggered by Gpa1^Q323L. Responses to mating pheromone were diminished, but not eliminated, in yeast deficient for either subunit of PI3K. Both Gpa1^Q323L and PI3K were localized to endosomes, whereas wild-type Gpa1 was predominantly at the plasma membrane. Gpa1 interacted directly with both subunits of PI3K (based on copurification experiments with purified proteins or proteins expressed in yeast cells). Activated Gpa1 exhibited enhanced binding to the catalytic subunit of PI3K, and the regulatory subunit of PI3K interacted with inactive Gpa1. The catalytic subunit of PI3K was a true G effector, because cells expressing the Gpa1^Q323L exhibited increased production of phosphatidylinositol 3-phosphate. Modeling of the regulatory subunit Vps15 suggests that it has a seven-bladed propeller structure similar to that of Gß subunits. The authors suggest that PI3K serves as both effector and ß subunit for the endosomally localized G. Questions remain regarding how G becomes activated and localized at endosomes and what role PI3K plays in pheromone signaling. However, these results broaden the roles for G proteins to signaling from internal membranes and suggest a novel regulatory mechanism, with G cycling between active and inactive through its interactions with the effector and ß subunit of PI3K (see Koelle for discussion).

J. E. Slessareva, S. M. Routt, B. Temple, V. A. Bankaitis, H. G. Dohlman, Activation of the phosphatidylinositol 3-kinase Vps34 by a G protein subunit at the endosome. Cell 126, 191-203 (2006). [Online Journal]

M. R. Koelle, Heterotrimeric G protein signaling: Getting inside the cell. Cell 126, 25-27 (2006). [Online Journal]