Purification and characterization of the head-activator receptor from a multi-headed mutant of Chlorohydra viridissima.

In hydra the neuropeptide head activator (HA) is responsible for head-specific growth and differentiation processes. The effects of HA are mediated by high and low affinity receptors on hydra cells. In the current study HA receptors from a multi-headed mutant of Chlorohydra viridissima were solubilized from the membrane fraction using 1% Triton X-100 and 2.5 M urea. Scatchard analysis showed that the solubilized receptor had a Kd of 1.55 x 10(-9) M, indicating the low affinity subtype of the HA receptor. The solubilized receptor was purified by DMAE chromatography and subsequent affinity chromatography to homogeneity. SDS-PAGE revealed a single protein band with a molecular mass of 96 +/- 4 kDa. The native receptor eluted during gel filtration as a 113 kDa protein, and focussed with an isoelectric point of 4.8.

Characterisation of two types of head activator receptor on hydra cells.

A head activator (HA) analogue is described which even at high concentrations does not lose its biological activity. By cross-linking two HA molecules over a C8 spacer, the conformation was sufficiently altered, such that self-inactivation of HA by dimerisation was prevented. In addition, the introduction of a tyrosine instead of phenylalanine in one of the two HA molecules allowed radioactive labelling with iodine. This HA bipeptide was used to investigate the effect of HA at different concentrations and as ligand for HA receptor characterisation. We found that low concentrations (0.1-10 pM) sufficed to stimulate interstitial cell mitosis, and that higher concentrations (10-1000 pM) were required for the determination of interstitial cells to nerve cells. Binding of the radioactive HA ligand to living hydra and to purified membrane fractions was saturable and specific. Binding was compatible with HA analogues with a stable monomeric conformation, but less well with dimerising HA and HA analogues. Scatchard and kinetic analyses revealed the presence of at least two types of binding site in the membrane fraction, one with a 'lower' affinity (Kd = 10(-9) M) and one with a 100-fold higher affinity (Kd = 10(-11) M). Autoradiography showed that interstitial cells were differentially labelled, suggesting that the number or types of HA receptors may vary depending on cell cycle status. A mutant of hydra with a multiheaded morphology contained 6-20-times more HA receptors per mg protein than other hydra species or mutants.