Theta defensin
Theta-defensins are a family of mammalian antimicrobial peptides. They are found in non-human 'Old World' primates, but not in human, gorilla, bonobo, and chimpanzee.
Structure
θ-defensins are cyclic peptides of 18 amino acids, possessing antimicrobial activity against a range of Gram-positive and Gram-negative bacteria, fungi, and some retroviruses. They consist of a pair of antiparallel β-sheets linked by three disulfide bonds arranged as a ladder along the sheets to form an extremely stable structure. Additionally, the peptides may self-associate into trimers.Biosynthesis
In rhesus macaque and olive baboon, θ-defensins are produced from precursor proteins with 76 amino acids each. A single nine amino acid peptide is derived from each precursor. Two of these nine amino acid peptides are spliced together to form the circular 18 amino acid defensin. Since there are two precursor genes they can form 3 different mature θ-defensins: the homodimer of processed RTD-1, The homodimer of processed RTD-2 or the heterodimer composed of both precursors. The heterodimeric form is the most abundant.In the olive baboon, four θ-defensin precursor genes have been isolated: BTD-a, BTD-b, BTD-c and BTD-d, which encode subunits A, B, C and D. These four subunits could theoretically combine to produce 10 different processed defensins. However, only five have been observed: consisting of subunits A+A, A+B, A+C, A+D and B+B. Finally, orangutan genomes encode 4 θ-defensin precursor genes and gibbon genomes encode 2.