The cAMP Receptor Subtype cAR2 Is Restricted to a Subset of Prestalk Cells duringDictyosteliumDevelopment and Displays Unexpected DIF-1 Responsiveness

Abstract

Dictyostelium discoidiumcells express a family of cell surface cAMP receptors, and these G-protein-coupled receptors are each expressed with unique spatial and temporal patterns. One of these receptors, cAR2, is present during the postaggregative stages of development and our previous work suggests that it is preferentially expressed in prestalk cells. We report here the isolation of the promoter forcarB,the gene which encodes cAR2. Using this fragment to generate acarB::lacZgene fusion construct, we investigatedcarBexpression in detail. Expression is first detected at the tight aggregate stage and subsequently in a pattern reminiscent of the prestalk-specific geneecmA.There are subtle differences, however, withecmAbeing expressed significantly in the anterior-like cells of the migrating pseudoplasmodium and in the basal disc and lower cup supporting the sorus during terminal development.carBis not expressed in any of these places. The presence of these different prestalk cell subtypes was confirmed by double indirect immunofluorescence using anti-cAR2 and anti-β-galactosidase antibodies. While virtually all cAR2-expressing cells also expressecmA::lacZ,a substantial fraction ofecmA::lacZ-positive cells do not express cAR2. We also found the regulation ofcarBgene expression to differ from that ofecmA. carBexpression is inducedin vitroby extracellular cAMP, but surprisingly, not by DIF-1, a soluble molecule thought to be essential for the initiation of prestalk differentiation. Thus, cAR2 appears to be a cAMP receptor present on a restricted subset of prestalk cells and whose expression does not respond typically to the prestalk inducer DIF-1. DIF-1 sensitivity may, therefore, not be characteristic of all early prestalk differentiation.