Reinvestigation of the proposed folding and self-association of the Neuropeptide Head Activator

The Neuropeptide Head Activator (HA), pGlu-Pro-Pro-Gly-Gly-Ser-Lys-Val-Ile-Leu-Phe (pGlu is pyroglutamic acid), is involved in head-specific growth and differentiation processes in the freshwater coelenterate Hydra attenuata. Peptides of identical sequence have also been isolated from higher-organism tissues such as human and bovine hypothalamus. Early studies by molecular sieve chromatography suggested that HA dimerizes with high affinity (K_d ≈ 1 nM). This dimerization was proposed to occur via antiparallel β-sheet formation between the Lys₇-Phe₁₁ segments in each HA molecule. We conducted biophysical studies on synthetic HA in order to gain insight into its structure and aggregation tendencies. We found by analytical ultracentrifugation that HA is monomeric at low millimolar concentrations. Studies by ¹H-NMR revealed that HA did not adopt any significant secondary structure in solution. We found no NOEs that would support the proposed dimer structure. We probed the propensity of the Lys₇-Phe₁₁ fragment to form antiparallel β-sheet by designing peptides in which two such fragments are joined by a two-residue linker. These peptides were intended to form stable β-hairpin structures with cross-strand interactions that mimic those of the proposed HA dimer interface. We found that the HA-derived fragments may be induced to form intramolecular β-sheet, albeit only weakly, when linked by the highly β-hairpin-promoting D-Pro-Gly turn, but not when linked by the more flexible Gly-Gly unit. These findings suggest that the postulated mode of HA dimerization and the proposed propensity of the molecule to form discrete aggregates with high affinity are incorrect.