Relationships between the ligand specificity of cell surface folate binding sites, folate degrading enzymes and cellular responses in Dictyostelium discoideum

Abstract

The affinity of four distinct types of folate binding sites and of two membrane-bound folate degrading enzymes for 15 folic acid derivatives was monitored. Apart from two types (A H and A L of binding sites, all binding classes or enzymes show different affinity patterns. This strongly suggests the observed binding sites to be nonidentical to the membrane-bound folate deaminase or folate C 9-N 10 cleaving enzyme. The analog specificity of the chemotactic response towards folates shows a strong resemblance to the specificity of binding to one of the binding classes: ‘B-sites’ ( p < 0.01%). Less or no correlation was observed between the other classes or the enzymes and the chemotactic response. This may indicate that the B-sites are involved in the transduction of an extracellular signal to chemotactic cell movement. Folates elicit secretion of cAMP. Recently, the activity of several folate derivatives to evoke a cAMP response was studied (Devreotes, P.N. (1983) Dev. Biol. 95, 154–162). Comparison of this activity and the specificity of the binding sites in this study, suggests that the ‘A-sites’ are involved in the cAMP response.