A potent antidiuretic factor (ADF) that inhibits Malpighian tubule secretion was isolated from pupal heads of the beetle Tenebrio molitor (Eigenherr et al., 2002; Schooley et al., 2012). This Tenmo-ADFa is non-amidated and has the sequence VVNTPGHAVSYHVY-OH. A second less potent ADF (Tenmo-ADFb) was later identified with the sequence YDDGSYKPHIYGF-OH (Eigenherr et al., 2003). Both peptides were found to be possible cleavage products of larger proteins in T. molitor. ADFa is near identical to the carboxy terminus of the protein CAA03880 and ADFb is identical to the carboxyterminal 13 residues of cuticle protein 9.2 (Schooley et al., 2012). ADFa furthermore displays 57% identity with mammalian big endothelin I (however, a portion that is cleaved off and posseses no known biological function). No clearcut precursor-encoding gene has been identified in T. molitor where ADFa or ADFb could be liberated via conventional dibasic cleavage sites. In a related beetle, Tribolium castaneum, five clustered genes have been identified that encode ADFb-type peptide sequences at their C-terminus, but without canonical cleavage sites (Li et al., 2008). No ADFa precursor was discovered and furthermore, no ADFa or ADFb peptides could be identified by mass spectrometry (Li et al., 2008). ADFa and ADFb peptides have not been detected outside coleoptera. Thus, these peptides are somewhat enigmatic, and the possibility remains that the chemically isolated peptides are fragments of larger proteins that are not normally liberated and part of regulatory functions. Perhaps the isolation of a peptide fragment with potent antidiuretic activity is a serendipitous discovery that is still useful as a lead for biologically active reagents in pest control. No receptor has been identified for ADFa or ADFb.
ADFb has been localized by immunocytochemistry to two pairs of neuroendocrine cells in the brain of T. molitor, but no labeled axons were detected in the corpora cardiaca, a common release site for peptide hormones (Eigenherr et al., 2003). Immunolabeled cells were also detected in the median nerve between the last thoracic and first abdominal ganglion.
Both ADFa and ADFb inhibit fluid secretion in Malpighian tubules of T. molitor. These peptides increase cyclic GMP in the tubules, but have no effect on transepithelial voltage or other electrophysiological properties (Eigenherr et al., 2003; Massaro et al., 2004).
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