Abstract
We have identified 44 serine protease (SP) and 13 serine protease homolog (SPH) genes in the genome of Apis mellifera. Most of these genes encode putative secreted proteins, but four SPs and three SPHs may associate with the plasma membrane via a transmembrane region. Clip domains represent the most abundant non-catalytic structural units in these SP-like proteins −12 SPs and six SPHs contain at least one clip domain. Some of the family members contain other modules for protein–protein interactions, including disulphide-stabilized structures (LDLrA, SRCR, frizzled, kringle, Sushi, Wonton and Pan/apple), carbohydrate-recognition domains (C-type lectin and chitin-binding), and other modules (such as zinc finger, CUB, coiled coil and Sina). Comparison of the sequences with those from Drosophila led to a proposed SP pathway for establishing the dorsoventral axis of honey bee embryos. Multiple sequence alignments revealed evolutionary relationships of honey bee SPs and SPHs with those in Drosophila melanogaster, Anopheles gambiae, and Manduca sexta. We identified homologs of D. melanogaster persephone, M. sexta HP14, PAP-1 and SPH-1. A. mellifera genome includes at least five genes for potential SP inhibitors (serpin-1 through -5) and three genes of SP putative substrates (prophenoloxidase, spätzle-1 and spätzle-2). Quantitative RT-PCR analyses showed an elevation in the mRNA levels of SP2, SP3, SP9, SP10, SPH41, SPH42, SP49, serpin-2, serpin-4, serpin-5, and spätzle-2 in adults after a microbial challenge. The SP41 and SP6 transcripts significantly increased after an injection of Paenibacillus larva, but there was no such increase after injection of saline or Escherichia coli. mRNA levels of most SPs and serpins significantly increased by 48 h after the pathogen infection in 1st instar larvae. On the contrary, SP1, SP3, SP19 and serpin-5 transcript levels reduced. These results, taken together, provide a framework for designing experimental studies of the roles of SPs and related proteins in embryonic development and immune responses of A. mellifera.
Highlights
Serine proteases in the S1 family are involved in various physiological processes, such as digestion, development, and defense responses (Rawlings & Barrett, 1993; Krem & Di Cera, 2002)
Based on the presence or absence of residues essential for the catalytic activity of serine protease (SP), we classify them as SPs or serine protease homolog (SPH)
A. mellifera SP11, SP29, SPH50 and SPH51 are clustered in Group 9.19–20; SP4, SP5, SP8, SP13 and SP27 in Group 15.3–8; SP25, SP33 and SPH56 in Group 13.1–3
Summary
Serine proteases in the S1 family (e.g. chymotrypsin) are involved in various physiological processes, such as digestion, development, and defense responses (Rawlings & Barrett, 1993; Krem & Di Cera, 2002) They are typically synthesized as zymogens, which require proteolysis at a specific site for activation. In Drosophila adults, another set of SPs leads to spätzle activation and drosomycin production (Lemaitre et al, 1996) Another insect defense mechanism involving a SP cascade is the proteolytic activation of prophenoloxidase (proPO) (Ashida & Brey, 1998; Ligoxygakis et al, 2002b; Kanost et al, 2004). We hope that these results could provide evolutionary perspectives of the S1 family of protease genes in insects and stimulate interest for in-depth analyses of SP-related proteins (i.e. SPs, SPHs, serpins and SP substrates) in the honey bee
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