Isolation and partial characterization of the insulin binding sites of Tetrahymena pyriformis

The free-living protozoan, Tetrahymena pyriformis, has been shown to specifically bind insulin. Insulin causes alterations in metabolic function and an increase in insulin binding capacity (hormonal imprinting) in this organism. Previously, an insulin-like material was found in both Tetrahymena cell isolates and in their growth media. The purpose of this study was to isolate and partially characterize the ciliary membrane binding sites for insulin. The methods employed, DEAE and affinity chromatography, were published previously for the mammalian insulin receptor. Here we report, not the isolation of a mammalian-like receptor protein, but a ciliary membrane protein (62-67 kDa) that is immunologically similar to insulin. This insulin-like protein may function as both precursor to a soluble form and membrane-bound binding site/receptor as has been suggested for Euplotes pheromones and mammalian growth factors such as transforming growth factor a and epidermal growth factor.