Characterization of the receptor protein-tyrosine kinase gene from the marine sponge Geodia cydonium.

Abstract

Cells are provided with well-defined receptor structures (signal receivers) which interact with their corresponding ligands (signal molecules) and initiate a signal transduction pathway resulting in a change of cellular behavior or metabolism (Stoddard et al. 1992). It is well established that cells from both eukaryotic protists (single-cell organisms) and from Metazoa (multicellular organisms) respond to signals emanating from the extracellular environment. The extracellular signals to which protists respond are mainly nutrients which diffuse to their surfaces, and in most cases cross the cell membrane. In addition, they are able to bind peptide hormones, e.g., insulin or adrenocorticotropic hormone, as in the unicellular Tetrahymena, by receptor-like structures (Kohidai et al. 1994). Based on experimental data obtained with Tetrahymena, it has been proposed that the survival of protists presupposes the operation of highly dynamic membrane structures capable of recognizing a variety of environmental signals, interactions which are stored in a form of “memory” and transmitted to the progeny generation (Csaba 1987, 1994). Hence, in unicellular eukaryotes, the membrane-bound receptor(s) have a nondetermined ability to recognize ligands and are initially not genetically programmed. In contrast, in Metazoa the receptors are genetically preprogrammed.