Biological functions and synthesis of Bursicon
Discovery
Initially bursicon was isolated as a 30- to 40-kDa protein in several insects, based on cuticle tanning assays. The partial aa sequences of a protein with bursicon-like bioactivity were identified in the cockroach central nervous system (CNS).A BLAST search with these sequences was used to identify a gene encoding bursicon (burs) in the Drosophila melanogaster (fruit fly) genome. Later, a partner of burs (pburs) along with the bursicon receptor (rickets) was identified and characterized. Descriptions of both bursicon subunits, named either burs and pburs or burs-α and burs-β, and its receptor were independently published in the same year.Bursicon genes are highly conserved in many arthropods.
Structure
Structural features
Bursicon is a cystine-knot heterodimer glycoprotein composed of two subunits, burs (burs-α) and pburs (burs-β). However, in several neurons it is also produced as a monomer or homodimer.
Primary structure
The homologs of burs and pburs have been identified or deduced in many insects, crustaceans, a tick, and a sea urchin. The positions of cysteines are conserved in all animals.
Properties
burs, Mr 15,678; pburs, Mr 13,059.
Regulation of synthesis and release
In the larvae of D. melanogaster, bursicon appears to be released in two temporal waves, the first prior to the onset of ecdysis and the second immediately after ecdysis onset. In adults, bursicon is also released in two consecutive waves. Initial bursicon secretion from two neurons in the SG subsequently elicits its secondary release from a cluster of abdominal neurons, which controls the expansion, hardening, and pigmentation of the cuticle and wings.In Neocaridina heteropoda, the abdominal ganglion is the main neurohemal release site of bursicon.
Biological functions
Target cells and function In D. melanogaster, bursicon controls cuticle sclerotization and melanization.In D. melanogaster, B. mori, and the beetle Tribolium castaneum, it is necessary for the expansion and hardening of the wings.The expansion of wings requires bursicon-induced extensibility of the wing cuticle, as demonstrated in the moth M. sexta. In D. melanogaster, burs-β is an important signal for controlling ecdysis behavior.In the crab C. sapidus, bursicon controls the deposition and thickening of the new cuticle as well as the granulation of hemocytes. In both Procambarus clarkii and D. melanogaster, bursicon homodimers induce innate immunity by altering antimicrobial peptides via Relish. Besides, burs-α in D. melanogaster binds to its neuronal receptor DLgr2, eventually regulating organismal resistance to metabolic challenges through Adipokinetic hormone signaling.
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