Mono- and dibasic proteolytic cleavage sites in insect neuroendocrine peptide precursors

Regulatory peptides are synthesized as part of larger precursors that are s ubsequently processed into the active substances. After cleavage of the sig nal peptide, further proteolytic processing occurs predominantly at basic a mino acid residues. Rules have been proposed in order to predict which puta tive proteolytic processing sites are actually used, but these rules have b een established for vertebrate peptide precursors and it is unclear whether they are also valid for insects. The aim of this paper is to establish the validity of these rules to predict proteolytic cleavage sites at basic ami no acids in insect neuropeptide precursors. Rules describing the cleavage o f mono-and dibasic potential processing sites in insect neuropeptide precur sors are summarized below. Lys-Arg pairs not followed by an aliphatic or ba sic amino acid residue are virtually always cleaved in insect regulatory pe ptide precursors, but cleavages of Lys-Arg pairs followed by either an alip hatic or a basic amino acid residue are ambiguous, as is processing at Arg- Arg pairs. Processing at Arg-Lys pairs has so far not been demonstrated in insects and processing at Lys-Lys pairs appears very rare. Processing at si ngle Arg residues occurs only when there is a basic amino acid residue in p osition -4, -6, or -8, usually an Arg, but Lys or His residues work also. A lthough the current number of such sites is too limited to draw definitive conclusions, it seems plausible that cleavage at these sites is inhibited b y the presence of aliphatic residues in the +1 position. However, cleavage at single Arg residues is ambiguous. When several potential cleavage sites overlap the one most easily cleaved appears to be processed. It cannot be e xcluded that some of the rules formulated here will prove less than univers al, as only a limited number of cleavage sites have so far been identified. It is likely that, as in vertebrates, ambiguous processing sites exist to allow differential cleavage of the same precursor by different convertases and it seems possible that the precursors of allatostatins and PBAN are dif ferentially cleaved in different cell types. (C) 2000 Wiley-Liss, Inc.