Abstract
The production of antimicrobial peptides (AMPs) is a major defense mechanism against pathogen infestation and of particular importance for insects relying exclusively on an innate immune system. Here, we report on the characterization of three AMPs from the carpenter ant Camponotus floridanus. Due to sequence similarities and amino acid composition these peptides can be classified into the cysteine-rich (e.g. defensin) and glycine-rich (e.g. hymenoptaecin) AMP groups, respectively. The gene and cDNA sequences of these AMPs were established and their expression was shown to be induced by microbial challenge. We characterized two different defensin genes. The defensin-2 gene has a single intron, whereas the defensin-1 gene has two introns. The deduced amino acid sequence of the C. floridanus defensins is very similar to other known ant defensins with the exception of a short C-terminal extension of defensin-1. The hymenoptaecin gene has a single intron and a very peculiar domain structure. The corresponding precursor protein consists of a signal- and a pro-sequence followed by a hymenoptaecin-like domain and six directly repeated hymenoptaecin domains. Each of the hymenoptaecin domains is flanked by an EAEP-spacer sequence and a RR-site known to be a proteolytic processing site. Thus, proteolytic processing of the multipeptide precursor may generate several mature AMPs leading to an amplification of the immune response. Bioinformatical analyses revealed the presence of hymenoptaecin genes with similar multipeptide precursor structure in genomes of other ant species suggesting an evolutionary conserved important role of this gene in ant immunity.
Highlights
Insects have evolved multiple innate defense mechanisms to respond to microbial invasion [1,2,3,4]
We show that in comparison to bee hymenoptaecins the C. floridanus hymenoptaecin gene is much longer and encodes a multipeptide precursor with structural similarities to apidaecin precursors from A. mellifera [17], the proteolytic processing of which possibly leads to a massive amplification of the antimicrobial response [13,29]
The search for immune inducible genes in C. floridanus by a suppression subtractive hybridization (SSH) approach revealed the presence of a cDNA encoding a homologue of hymenoptaecin, an antimicrobial peptides (AMPs) known from other hymenopteran species [28]
Summary
Insects have evolved multiple innate defense mechanisms to respond to microbial invasion [1,2,3,4]. Protective measure involves the ‘‘constitutive’’ immediate-acting defenses including phagocytes and reactive oxygen species. An inducible immune response is mounted which mainly involves the production of antimicrobial peptides (AMPs) [5,6]. It is believed that this late-acting humoral response is required to kill those bacteria that have survived the immediate host’s constitutive defenses [5]. In 1981, the first AMPs were described from the cecropia moth [7]. Insect AMPs can be divided into several groups, mainly a-helical peptides (e.g. cecropin), cysteine-rich peptides (e.g. defensin), proline-rich peptides (e.g. drosocin), and glycine-rich peptides (e.g. hymenoptaecin) [8]
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