Phosphoinositide 3-kinase (PI3K) gamma has been implicated in a vast array of physiological settings including the activation of different leukocyte species and the regulation of myocardial contractility. Activation of PI3Kgamma is primarily mediated by Gbetagamma subunits of heterotrimeric G proteins, which are recognized by a p101 regulatory subunit. Here, we describe the identification and characterization of a novel regulatory subunit of PI3Kgamma, which we termed p87(PIKAP) (PI3Kgamma adapter protein of 87 kDa). It is homologous to p101 in areas that we have recently shown that they mediate binding to the catalytic p110gamma subunit and to Gbetagamma. Like p101, p87(PIKAP) binds to both p110gamma and Gbetagamma and mediates activation of p110gamma downstream of G protein-coupled receptors. In contrast to p101, p87(PIKAP) is highly expressed in heart and may therefore be crucial to PI3Kgamma cardiac function. Moreover, p87(PIKAP) and p101 are both expressed in dendritic cells, macrophages, and neutrophils, raising the possibility of regulatory subunit-dependent differences in PI3Kgamma signaling within the same cell type. We further provide evidence that p87(PIKAP) physically interacts with phosphodiesterase (PDE) 3B, suggesting that p87(PIKAP) is also involved in the recently described noncatalytic scaffolding interaction of p110gamma with PDE3B. However, coexpression of PDE3B and PI3Kgamma subunits was not sufficient to reconstitute the regulatory effect of PI3Kgamma on PDE3B activity observed in heart, implying further molecules to be present in the complex regulating PDE3B in heart.