Phosphoinositide 3‐kinases (PI3Ks) have been identified in an evolutionarily diverse range of organisms, including mammals, Drosophila, yeast, plants and Dictyostelium. They are activated by a multitude of extracellular signals and implicated in mitogenesis, differentiation and cell survival, as well as in the control of the cytoskeleton and cell shape. Here we describe the molecular and functional analysis of Drosophila p110 (Dp110). A full‐length Dp110 cDNA was isolated and found to encode a protein homologous throughout its length to the class I mammalian PI3Ks p110alpha and p110beta. Overexpression of Dp110 in wing or eye imaginal discs resulted in flies with enlarged wings or eyes respectively. In contrast, overexpression of Dp110 containing a mutation predicted to result in the loss of catalytic activity resulted in smaller wings and eyes. The alterations in wing size result from changes in both cell size and cell number, whereas in the eye only differences in cell size were detected. These data imply a role for Dp110 in growth control during Drosophila development and have implications for the function of class I PI3Ks in other organisms.

The major protein phosphatase that dephosphorylates smooth-muscle myosin was purified from chicken gizzard myofibrils and shown to be composed of three subunits with apparent molecular masses of 130,37 and 20 kDa, the most likely structure being a heterotrimer. The 37-kDa component was the catalytic subunit, while the 130-kDa and 20-kDa components formed a regulatory complex that enhanced catalytic subunit activity towards heavy meromyosin or the isolated myosin P light chain from smooth muscle and suppressed its activity towards phosphorylase, phosphorylase kinase and glycogen synthase. The catalytic subunit was identified as the /3 isoform of protein phosphatase-1 (PPl) and the 130-kDa subunit as the PP1-binding component. The distinctive properties of smooth and skeletal muscle myosin phosphatases are explained by interaction of PPlg with different proteins and (in conjunction with earlier analysis of the glycogen-associated phosphatase) establish that the specificity and subcellular location of PP1 is determined by its interaction with a number of specific targetting subunits.Although phosphoserine and phosphothreonine account for more than 99% of phosphorylated amino acids [I] and hundreds of phosphoproteins are present in cells, only four major classes of protein serineithreonine phosphatase (PP) catalytic subunits, termed PP1, PP2A, PP2B and PP2C have been identified (reviewed in [2]). Three of these, namely PP1 , PP2A and PP2C, have broad and overlapping substrate specificities in vitro, accounting for most, if not all, of the phosphatase activity in tissue extracts towards a variety of enzymes that control the major metabolic pathways (reviewed in [3]). Furthermore, studies with the tumour promoter okadaic acid, a potent and specific inhibitor of PP1 and PP2A (reviewed in [4]), and the phenotypes of genetic mutants lacking these enzymes in fungi [5], fission yeast [6] and fruit flies [7, 81, has implicated PP1 and PP2A in the regulation of additional processes, such as the transport of ions and nutrients into cells, gene transcription, muscle contractility and the celldivision cycle. The regulation of many physiological processes by a limited number of phosphatase catalytic subunits, has therefore raised the question of how each of these processes can be regulated independently of one another.A major clue originated from studies on thc control of glycogen metabolism in mammalian skeletal muscle, whereCorrespondence to P. Cohen, MRC Protein Phosphorylation Unit,

Phosphoinositide (PI) 3‐kinases have been characterized as enzymes involved in receptor signal transduction in mammalian cells and in a complex which mediates protein trafficking in yeast. PI 3‐kinases linked to receptors with intrinsic or associated tyrosine kinase activity are heterodimeric proteins, consisting of p85 adaptor and p110 catalytic subunits, which can generate the 3‐phosphorylated forms of phosphatidylinositol (PtdIns), PtdIns4P and PtdIns(4,5)P2 as potential second messengers. Yeast Vps34p kinase, however, has a substrate specificity restricted to PtdIns and is a PtdIns 3‐kinase. Here the molecular characterization of a new human PtdIns 3‐kinase with extensive sequence homology to Vps34p is described. PtdIns 3‐kinase does not associate with p85 and phosphorylates PtdIns, but not PtdIns4P or PtdIns(4,5)P2. In vivo PtdIns 3‐kinase is in a complex with a cellular protein of 150 kDa, as detected by immunoprecipitation from human cells. Protein sequence analysis and cDNA cloning show that this 150 kDa protein is highly homologous to Vps15p, a 160 kDa protein serine/threonine kinase associated with yeast Vps34p. These results suggest that the major components of the yeast Vps intracellular trafficking complex are conserved in humans.