The localisation of PtdIns3P in Drosophila fat responds to nutrients but not insulin: a role for Class III but not Class II phosphoinositide 3-kinases

PtdIns3P and PtdIns(3,4,5)P3 are regulated differently in fat body in response to nutritional status and insulin signalling. In feeding larvae PtdIns(3,4,5)P3 is upregulated at the cell membrane where it is generated in response to insulin signalling. In contrast PtdIns3P is down regulated in the fat body of well-fed larvae but on starvation it accumulates in punctate vesicles throughout the cytoplasm and on refeeding it relocalises to vesicles at the periphery of the cell. Both responses are independent of insulin signalling and on the presence of glutamine which has previously been linked to nutritional sensing. We find that both Class II and Class III PI3Ks are capable of generating PtdIns3P in vivo but the source of PtdIns3P in the fat body and the response to nutritional status can be exclusively accounted for by Class III PI3K activity.