A head-activator binding protein is present in hydra in a soluble and a membrane-anchored form

The neuropeptide head activator plays an important role for proliferation a nd determination of stem cells in hydra, By affinity chromatography a 200 k Da head-activator binding protein, HAB, was isolated from the multiheaded m utant of Chlorohydra viridissima, Partial amino acid sequences were used to clone the HAB cDNA which coded for a receptor with a unique alignment of e xtracellular modules, a transmembrane domain, and a short carboxyterminal c ytoplasmic tail. A mammalian HAB homologue with identical alignment of thes e modules is expressed early in brain development. Specific antibodies reve aled the presence of HAB in hydra as a transmembrane receptor, but also as secreted protein, both capable of binding head activator. Secretion of HAB during regeneration and expression in regions of high determination potenti al hint at a role for HAB in regulating the concentration and range of acti on of head activator.