The drosophila Arf GEF steppke controls MAPK activation in EGFR signaling

Abstract

SummaryGuanine 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.

Dynamic changes in host–virus interactions associated with colony founding and social environment in fire ant queens (Solenopsis invicta)

“…Several of these genes have important functions in Drosophila : Phosphogluconate dehydrogenase and cytohesin‐1‐like are involved in the activation of the mitogen‐activated protein kinase (MAPK) (Hahn et al. 2013), Juvenile hormone epoxide hydrolase 2 ( Jheh2 ) mediates the catabolism of juvenile hormone (Share and Roe 1988) (but has been co‐opted in Apis mellifera to regulate dietary lipid catabolism; Mackert et al. 2010), toll like receptor 1 ( Toll ‐ 1 ) is an important player in the antimicrobial humoral response and regulation of haemocyte differentiation (Lemaitre et al.…”

ARF1–GTP regulates Asrij to provide endocytic control of Drosophila blood cell homeostasis

“…Recently insulin signaling was shown to control prohemocyte maintenance (22) and an ARF-GEF Steppke is speculated to regulate the insulin signaling pathway by activating ARF (23), thereby completing the cycle of ARF activation and inactivation. Steppke is thought to act upstream of PI3K (24). Hence it is likely that an ARF regulates hematopoiesis via its control of insulin signaling.…”

Apical Accumulation of the Sevenless Receptor Tyrosine Kinase During Drosophila Eye Development Is Promoted by the Small GTPase Rap1

“…The mechanism governing these functions of CNK has been partly solved and requires the action of both Ras and the Src family kinase Src42A (Laberge et al 2005). More recently, CNK has been shown to form a complex with Steppke, the fly cytohesin homolog, and collaborate in EGFR-induced MAPK activation by a mechanism that has yet to be characterized (Hahn et al 2013). Whether CNK plays other roles outside the Ras/MAPK pathway in flies is currently not clear, although mammalian CNKs have been found to participate in signaling events regulated by other small GTPases, such as Rho, Rac, Ral, and Arf6 (Lanigan et al 2003;Venkateswarlu 2003;Jaffe et al 2005;Lim et al 2010).…”