Mammalian angiotensin I-converting enzyme (ACE, peptidyl dipeptidase A, EC 188.8.131.52) is a zinc metallopeptidase that cleaves dipeptides from the C-terminus of oligopeptides and is best known for its pivotal role in blood homeostasis, as part of the Renin-Angiotens in and Kinin-Kininogen systems. ACE activity is also found in invertebrates from several phyla, including all insect species studied so far. Since invertebrates do not possess peptide hormones structurally related to mammalian ACE substrates, understanding the physiological roles of these invertebrate ACEs is likely to reveal novel functions of ACE-like peptidases in biological processes. Two Drosophila homologues of human ACE (ANCE and ACER) have been the subject of genetic and biochemical studies in our lab. They are both single domain proteins which, unlike mammalian ACE, are secreted and do not possess a protein membrane anchor. ANCE can remove dipeptides from a wide range of peptide substrates and, therefore, might have a general role in peptide metabolism. However, it is quite clear that selected peptides are cleaved with very high efficiency (e.g. locust tachykinin I and mammalian bradykinin) and, therefore, the enzyme can also be a specialist, regulating the activity of key signalling peptides. ACER is a more enigmatic enzyme. Despite having a full complement of active site residues, it has only weak peptidase activity against most substrates. Sequencing of the Drosophila melanogaster genome identified a further four ACE-like genes, called Ance-2, Ance-3, Ance-4, and Ance-5. Any enzymatic function for the protein products of these genes is doubtful, since the conceptual proteins lack one or more key active site residues.
Neuropeptides are a remarkably diverse group of signalling molecules with key roles in regulating physiology and behaviour in both vertebrates and invertebrates. Following the release of the Drosophila melanogaster genome sequence in 2000 we identified genes encoding the precursors of proctolin (Proct) and tachykinin (Tk). Despite the fact that proctolin (RYLPT) was the first insect peptide to be characterised and has been extensively studied at the pharmacological level, relatively little is known about its processing and function in vivo. The sequence of the proctolin precursor reveals that its N terminus is likely to be generated by signal peptidase (SP) cleavage and the C-terminus by the sequential actions of a furin-like pro-hormone convertase (PC) and carboxypeptidase D. The sequence of the proctolin precursor protein is conserved among Drosophila species and a related sequence is present in the Tribolium genome but, to date, no other proctolin precursors have been identified in insects. Over-expression of the Proct gene causes an increase in heart rate in pre-pupae. We are currently investigating further functions of proctolin in Drosophila by ectopic expression and RNAi driven by the GAL4-UAS system.
Vertebrate tachykinins represent a large family of peptides which elicit a wide range of both central and peripheral responses. Although these peptides are structurally diverse, all contain a conserved C-terminal FXGLMamide motif. Tachykinins have also been isolated from a variety of invertebrate species - almost all of these contain a conserved C-terminal FXGXRamide motif, and for this reason have been termed tachykinin-related peptides (TRPs). Analysis of the Drosophila genome allowed us to identify a putative tachykinin-related peptide prohormone gene (TkI). The protein encoded by this gene contains five putative tachykinin-related peptides with conserved C-terminal FXGXRamide motifs. The Tk gene is expressed and the prohormone processed in larval and adult midgut endocrine cells and in the central nervous system, with midgut expression starting at stage 17 of embryogenesis. The predicted TRPs have potent stimulatory effects on the contractions of insect gut. These data provide additional evidence for the conservation of both the structure and function of the tachykinin peptides in the brain and gut during the course of evolution.
with Ken Wilson (PI, Lancaster), David Grzywacz (Natural Resources Institute, Chatham), Jenny Cory (Department of Biology, Laurentian University, Canada), Wilfred Mushobozi (Eco Agri Consult Ltd., Tanzania), and Seif Madoffe (Sokoine University of Agriculture).
The African armyworm (Spodoptera exempta) is a major pest in eastern and southern Africa that can cause massive losses to crops, and livestock production. Control of armyworms using chemical insecticides is too costly and environmentally-damaging for most smallholder farmers. Consequently, an alternative method of control is highly desirable. Armyworms may be infected by a highly specific, natural virus (S. exempta nucleopolyhedrovirus, SpexNPV). Previous research has identified SpexNPV as a biopesticide with great potential but we know relatively little about the natural biology of the virus in the field or how this might best be exploited to control armyworms. We are investigating the wider role of SpexNPV in the natural population dynamics of armyworm, focusing on recent findings that the virus is widely prevalent in insects in a "latent", non-pathogenic form, which can be vertically transmitted to their offspring.
This work is supported by a BBSRC-DfID award. For more information, see the ARMYWEB site.