Assigning Functional Domains within the p101 Regulatory Subunit of Phosphoinositide 3-Kinase γ

Abstract Phosphoinositide 3-Kinase (PI3K) γ is a lipid kinase that is regulated by G-protein-coupled receptors. It plays a crucial role in inflammatory and allergic processes. Activation of PI3Kγ is primarily mediated by Gβγ subunits. The regulatory p101 subunit of PI3Kγ binds to Gβγ and, thereby, recruits the catalytic p110γ subunit to the plasma membrane. Despite its crucial role in the activation of PI3Kγ, the structural organization of p101 is still largely elusive. Employing fluorescence resonance energy transfer measurements, coimmunoprecipitation and colocalization studies with p101 deletion mutants, we show here that distinct regions within the p101 primary structure are responsible for interaction with p110γ and Gβγ. The p110γ binding site is confined to the N terminus, whereas binding to Gβγ is mediated by a C-terminal domain of p101. These domains appear to be highly conserved among various species ranging from Xenopus to men. In addition to establishing a domain structure for p101, our results point to the existence of a previously unknown, p101-related regulatory subunit for PI3Kγ.