Chemosensory signal transduction in Paramecium

Paramecia are ciliated single-cell eukaryotic organisms that can respond to chemical cues in their environment. Glutamate is among those cues, which a ttract cells. We describe briefly here the following attributes of glutamat e chemoresponse: 1) Cells are attracted to L-glutamate relative to KCl at h igh concentrations of glutamate. 2) There are at least two specific, relati vely low affinity glutamate binding sites on the cell surface. Glutamate ca n be displaced from only one of the binding sites by inosine monophosphate (IMP), and quisqualate displaces glutamate from the second site, which is l ikely to be the glutamate receptor involved in attraction to glutamate. 3) IMP is a repellent and does not act synergistically with glutamate, whereas guanosine monophosphate (GMP) does, 4) Similarly, glutathione is an attrac tant, but glutamate and glutathione appear to use different transduction pa thways. 5) Glutamate hyperpolarizes the cell. The ionic mechanism is not ye t verified, but is likely to involve a K conductance. 6) Glutamate induces a rapid and robust increase in cAMP in the cell. Protein kinase A (PKA) is possibly involved In the transduction pathway because kinase inhibitors suc h as H7 and H8 inhibit glutamate response, but do not affect responses to o ther attractants, such as acetate and ammonium, Activation of PKA by the ra pid rise in cAMP may sustain the hyperpolarization phosphorylation and acti vation of the plasma membrane calcium pump. 7) Candidate glutamate binding proteins are being identified among the cell surface proteins with the use of affinity chromatography.