Two highly similar, PtdIns(4,5)P2-selective, G beta gamma-activated PI3Ks were purified from pig neutrophil cytosol. Both were heterodimers, were composed of a 101 kDa protein and either a 120 kDa or a 117 kDa catalytic subunit, and were activated greater than 100-fold by G beta gammas. Peptide sequence-based oligonucleotide probes were used to clone cDNAs for the p120 and p101 species. The cDNA of p120 is highly related to p110 gamma, while the cDNA of p101 is not substantially related to anything in current databases. The proteins were expressed in and purified from insect and mammalian cells. They bound tightly to one another, both in vivo and in vitro, and in so doing, p101 amplified the effect of G beta gammas on the PI3K activity of p120 from less than 2-fold to greater than 100-fold.

These results suggest that Rac activation lies downstream of PI 3-kinase activation on a PDGF-stimulated signalling pathway. Furthermore, as Rac has been implicated in at least two diverse cellular responses that are also though to require activation of PI 3-kinase--a reorganization of the actin cytoskeleton known as membrane ruffling and the neutrophil oxidative burst--these results suggest that Rac may be a major effector protein for the PI 3-kinase signalling pathway in many cell types.

We show that matrices carrying the tethered homologs of natural phosphoinositides can be used to capture and display multiple phosphoinositide binding proteins in cell and tissue extracts. We present the mass spectrometric identification of over 20 proteins isolated by this method, mostly from leukocyte extracts: they include known and novel proteins with established phosphoinositide binding domains and also known proteins with surprising and unusual phosphoinositide binding properties. One of the novel PtdIns(3,4,5)P3 binding proteins, ARAP3, has an unusual domain structure, including five predicted PH domains. We show that it is a specific PtdIns(3,4,5)P3/PtdIns(3,4)P2-stimulated Arf6 GAP both in vitro and in vivo, and both its Arf GAP and Rho GAP domains cooperate in mediating PI3K-dependent rearrangements in the cell cytoskeleton and cell shape.

Bruton's tyrosine kinase (Btk) is essential for normal B lymphocyte development and function. The activity of Btk is partially regulated by transphosphorylation within its kinase domain by Src family kinases at residue Tyr-551 and subsequent autophosphorylation at Tyr-223. Activation correlates with Btk association with cellular membranes. Based on specific loss of function mutations, the Btk pleckstrin homology (PH) domain plays an essential role in this activation process. The Btk PH domain can bind in vitro to several lipid end products of the phosphatidylinositol 3-kinase (PI 3-kinase) family including phosphatidylinositol 3,4,5-trisphosphate. Activation of Btk as monitored by elevation of phosphotyrosine content and a cellular transformation response was dramatically enhanced by coexpressing a weakly activated allele of Src (E378G) and the two subunits of PI 3-kinase-␥. This activation correlates with new sites of phosphorylation on Btk identified by two-dimensional phosphopeptide mapping. Activation of Btk was dependent on the catalytic activity of all three enzymes and an intact Btk PH domain and Src transphosphorylation site. These combined data define Btk as a downstream target of PI 3-kinase-␥ and Src family kinases.Bruton's tyrosine kinase (Btk) is a nonreceptor tyrosine kinase that contains a pleckstrin homology (PH) domain but no apparent lipid modification motif (1). Btk is critical for development and signaling. Btk mutations are associated with the genetic diseases human X-linked agammaglobulinemia (XLA) and murine X-linked immunodeficiency (Xid; refs. 2-5). XLA patients have a dramatic decrease in the number of mature B cells and circulating Ig levels (6). Xid mice or mice with a targeted disruption of Btk have diminished B cell numbers and levels of certain Ig classes (7-9).PH domains are primarily involved in protein-protein or protein-lipid interactions and regulate enzyme function by controlling interacting partners or cellular localization (10,11). The N-terminal PH domain of Btk is essential for its activation and biological activity. A mutation in the Btk PH domain causes Xid (R28C; refs. 4 and 5), and other mutations within the PH domain also result in XLA (12, 13). In contrast, a Glu-to-Lys mutation (E41K, BTK*) in the PH domain activates Btk and increases membrane association (14). These gain or loss of function mutations suggest that the PH domain is a critical regulatory domain for Btk activation but give little information regarding specific signaling mechanisms.The PH domain of Btk was recently shown to bind the phosphatidylinositol 3-kinase (PI 3-kinase) lipid product phosphatidylinositol 3,4,5-trisphosphate [PI(3,4,5)P 3 ] (15, 16) and inositol 3-phosphates in vitro (17). Computer modeling identified several residues within the Btk PH domain including Lys-12, Phe-25, and Arg-28, which are thought to be essential for binding these lipid molecules (15,16,18,19). Interestingly, mutation of these residues results in human XLA (e.g., F25S and R28H; ref. 12) or murine Xid (R28C...