No abstract

Phosphoinositide 3-kinases (PI3Ks) can be divided into three distinct classes (I, II, and III) on the basis of their domain structures and the lipid signals that they generate. Functions have been assigned to the class I and class III enzymes but have not been established for the class II PI3Ks. We have obtained the first evidence for a biological function for a class II PI3K by expressing this enzyme during Drosophila melanogaster development and by using deficiencies that remove the endogenous gene. Wild-type and catalytically inactive PI3K_68D transgenes have opposite effects on the number of sensory bristles and on wing venation phenotypes induced by modified epidermal growth factor (EGF) receptor signaling. These results indicate that the endogenous PI3K_68D may act antagonistically to the EGF receptor-stimulated Ras-mitogen-activated protein kinase pathway and downstream of, or parallel to, the Notch receptor. A class II polyproline motif in PI3K_68D can bind the Drk adaptor protein in vitro, primarily via the N-terminal SH3 domain of Drk. Drk may thus be important for the localization of PI3K_68D, allowing it to modify signaling pathways downstream of cell surface receptors. The phenotypes obtained are markedly distinct from those generated by expression of the Drosophila class I PI3K, which affects growth but not pattern formation.Phosphoinositide 3-kinases (PI3Ks) signal to activate downstream molecules through the generation, within cellular membranes, of specific lipids that are phosphorylated at the D-3 position of the inositol ring. These lipids function as second messengers and appear to be involved in processes such as mitogenesis, cell survival, growth, differentiation, adhesion, motility, and vesicle trafficking (reviewed in reference 67). Hence, PI3Ks have been the focus of intensive research. In multicellular organisms, PI3Ks occur as a family of enzymes (40, 67). These can be assigned to three separate classes (class I, class II, and class III) based on their domain structures, differences in catalytic activity towards defined substrates, and modes of regulation. For many of the downstream events mediated by PI3K activation, the exact nature of the PI3K involved has not been defined.The class I PI3Ks are heterodimers that associate with cell surface receptors via an adaptor subunit (reviewed in reference 67). Two class I PI3K subclasses have been identified. The prototypical class I A PI3Ks are composed of a catalytic p110 subunit and a regulatory subunit. The regulatory subunit contains two SH2 domains that bind phosphotyrosine residues in specific motifs on activated tyrosine kinase receptors or receptor substrates; this interaction results in translocation of the associated catalytic activity to its lipid substrates at the membrane. A number of distinct adaptor subunits (50 to 85 kDa) and p110 subunits (p110␣, p110␤, and p110␦) have been identified. These show some variation in tissue distribution. The use of isoform-specific inhibitory antibodies has identified distinct roles for differen...

The genes encoding the ribosomal P-proteins CcP0, CcP1 and CcP2 of Ceratitis capitata were expressed in the conditional P0-null strains W303dGP0 and D67dGP0 of Saccharomyces cerevisiae, the ribosomes of which contain either standard amounts or are totally deprived of the P1/P2 proteins, respectively. The presence of the CcP0 protein restored cell viability but reduced the growth rate. In the W303CcP0 strain, all four acidic yeast proteins were found on the ribosomes, but in notably less quantity, while a preferable binding of the YP1alpha/YP2betapair was established. In the absence of the endogenous P1/P2 proteins in the D67CcP0 strain, the complementation capacity of the CcP0 protein was considerably reduced. The simultaneous expression of the three medfly genes resulted in alterations of the stalk composition: both the CcP1 and CcP2 proteins were found on the particles substituting the YP1alphaand YP2alpha proteins, respectively, but their presence did not alter the growth rate, except in the case of the YP1alpha/betadefective strain, where a helping effect on the binding of the YP2alphaand YP2betaproteins on the ribo-somes was confirmed. Therefore, the medfly ribosomal P-proteins complement the yeast P-protein deficient strains forming an heterogeneous ribosomal stalk, which, however, is not functionally equivalent to the endogenous one.