Einleitung

Life and death of uni- and multicellular organisms crucially depends on the ability of cells to communicate with each other and their environment. In order to execute important physiological tasks, to exploit their environment for growth and reproduction and to avoid predators and other unfavorable environmental conditions cells must readily respond to important signals. Signal reception and transduction is mainly carried out by membrane-bound receptors devised to recognize messages from the environment and intercellular messages such as neurotransmitters, hormones as well as growth and developmental factors. They can be classified on the basis of their structure and function into (i) ion channel-linked receptors, (ii) enzyme-linked receptors e.g. receptor tyrosine kinases or receptor guanylyl cyclases and (iii) G-protein-coupled receptors (GPCRs) (Prabhu and Eichinger, 2006).

GPCRs are a superfamily of diverse integral membrane proteins that enable detection and transduction of a large variety of extracellular signals.
In Dictyostelium there are no fewer than 55 such receptors in the genome corresponding to a significant fraction of the Dictyostelium proteome (~0.5%), a proportion lying between that of fungi (e.g. N. crassa: 0.1%) and animals (e.g. Drosophila: ~2%; vertebrates: 2–4%). Detailed analysis reveals that Dictyostelium has members of all major GPCR families except family 1 (which in mammals includes light and odorant receptors) and family 4, which contains pheromone receptors. In addition to members of the secretin (family 2) and metabotropic glutamate GABAB (family 3) families, the Dictyostelium genome also encodes 25 potential Frizzled or Smoothened (family 5) receptors. The presence of family 2, 3 and 5 receptors in Dictyostelium is particularly interesting because they had been thought to be animal-specific (Williams et al., 2005). We are particularly interested in RpkA.

Prabhu, Y., and Eichinger, L. (2006). The Dictyostelium repertoire of seven transmembrane domain receptors. European journal of cell biology 85, 937-946.

Williams, J. G., Noegel, A. A., and Eichinger, L. (2005). Manifestations of multicellularity: Dictyostelium reports in. Trends in genetics : TIG 21, 392-398.