Sulfakinin neuropeptides in a crustacean - Isolation, identification and tissue localization in the tiger prawn Penaeus monodon

The sulfakinin (SK) family of neuropeptides are characterized by a C-termin al octapeptide sequence that begins with two acidic residues (most commonly DD), and ends with YGHMRF-NH2, usually with the tyrosyl residue sulfated. So far, sulfakinins have only been identified in insects and the present st udy was initiated to investigate if the family is mole widely distributed w ithin the arthropods. Purification of an extract of the central nervous sys tem of the giant tiger prawn Penaeus monodon has revealed three novel membe rs of the sulfakinin peptide family. One of the peptides, Pem SKI, has the sequence <QFDEY(SO3H)GHMRF-NH2, where <Q denotes a pyroglutamic acid residu e, and is for all criteria typical of insect sulfakinins, several of which also have an N-terminal pyroglutamic acid. Tyrosyl O-sulfation was verified by mass spectrometry. The two other peptides have a hitherto unknown L/M v ariation at position three from the C-terminus. One of these, Pem SKII, has a particularly glycine-ri ch N-terminus, AGGSGGVGGEYDDYGHLRF-NH2. The othe r, Pem SKIII, is a truncated form of Pem SKII, with the sequence VGGEYDDYGH LRF-NH2. Mass spectrometry of the latter two peptides indicated that only o ne of the two tyrosyl residues is sulfated. By analogy, it is suggested tha t the sulfation is located at the residue in position six from the C-termin us. A small amount of a nonsulfated variant of Pem SKII was also present in the extract. Immunocytochemical studies with sulfakinin antisera show a sp arse neuronal distribution pattern, similar to that of insects. A prominent pair of large (approximate to 25 mu m) cells and 6-8 pairs of smaller (app roximate to 10 mu m) cells are present in the protocerebrum. The larger cel ls have prominent neurites that give rise to varicosities in the centre of the brain. Their axons exit the brain via the circumoesophageal connectives and continue along the intersegmental connectives. Each of the thoracic an d abdominal ganglia has sulfakinin-immunoreactive arborizations as a result of branching from the intersegmental nerves. This distribution pattern str ongly suggests a role in neurotransmission or neuromodulation, although it remains to be elucidated what the exact role(s) is. However, on account of the conservation of peptide structure during the evolutionary period spanni ng the insect/crustacean lineage, especially between Pem SKI and insect sul fakinins, it may be assumed that the sulfakinins have a significant physiol ogical role.