title: The Drosophila Arf GEF Steppke controls MAPK activation in EGFR signaling.
abstract: Guanine nucleotide exchange factors (GEFs) of the cytohesin protein family are regulators of GDP/GTP exchange for members of the ADP ribosylation factor (Arf) of small GTPases. They have been identified as modulators of various receptor tyrosine kinase signaling pathways including the insulin, the vascular epidermal growth factor (VEGF) and the epidermal growth factor (EGF) pathways. These pathways control many cellular functions, including cell proliferation and differentiation, and their misregulation is often associated with cancerogenesis. In vivo studies on cytohesins using genetic loss of function alleles are lacking, however, since knockout mouse models are not available yet. We have recently identified mutants for the single cytohesin Steppke (Step) in Drosophila and we could demonstrate an essential role of Step in the insulin signaling cascade. In the present study, we provide in vivo evidence for a role of Step in EGFR signaling during wing and eye development. By analyzing step mutants, transgenic RNA interference (RNAi) and overexpression lines for tissue specific as well as clonal analysis, we found that Step acts downstream of the EGFR and is required for the activation of mitogen-activated protein kinase (MAPK) and the induction of EGFR target genes. We further demonstrate that step transcription is induced by EGFR signaling whereas it is negatively regulated by insulin signaling. Furthermore, genetic studies and biochemical analysis show that Step interacts with the Connector Enhancer of KSR (CNK). We propose that Step may be part of a larger signaling scaffold coordinating receptor tyrosine kinase-dependent MAPK activation.
authors.affiliation: LIMES-Institute, Program Unit Development, Genetics and Molecular Physiology, Molecular Developmental Biology, University of Bonn, Carl-Troll-Str. 31, D-53115 Bonn, Germany.
authors.forename: Ines
authors.initials: I
authors.lastname: Hahn
authors.forename: Bernhard
authors.initials: B
authors.lastname: Fuss
authors.forename: Annika
authors.initials: A
authors.lastname: Peters
authors.forename: Tamara
authors.initials: T
authors.lastname: Werner
authors.forename: Andrea
authors.initials: A
authors.lastname: Sieberg
authors.forename: Dominic
authors.initials: D
authors.lastname: Gosejacob
authors.forename: Michael
authors.initials: M
authors.lastname: Hoch
mesh_terms: D048868:Adaptor Proteins, Signal Transducing / Q000235:genetics / Q000378:metabolism; D000818:Animals; D029721:Drosophila Proteins / Q000235:genetics / Q000378:metabolism*; D004331:Drosophila melanogaster; D004789:Enzyme Activation / Q000502:physiology; D048049:Extracellular Signal-Regulated MAP Kinases / Q000235:genetics / Q000378:metabolism*; D020662:Guanine Nucleotide Exchange Factors / Q000235:genetics / Q000378:metabolism*; D020935:MAP Kinase Signaling System / Q000502:physiology*; D051379:Mice; D009154:Mutation; D017468:Receptors, Fibroblast Growth Factor / Q000235:genetics / Q000378:metabolism*
chemical_list: D048868:Adaptor Proteins, Signal Transducing; C115753:CNK protein, Drosophila; D029721:Drosophila Proteins; D020662:Guanine Nucleotide Exchange Factors; D017468:Receptors, Fibroblast Growth Factor; C516026:Step protein, Drosophila; D048049:Extracellular Signal-Regulated MAP Kinases
keywords: Arf GEF Steppke; Drosophila; EGFR signalling; MAP kinase; RAS