CHEMOTAXIS TO CAMP AND SLUG MIGRATION IN DICTYOSTELIUM BOTH DEPEND ONMIGA, A BTB PROTEIN

Chemotaxis in natural aggregation territories and in a chamber with an imposed gradient of cyclic AMP (cAMP) was found to be defective in a mutant strain of Dictyostelium discoideum that forms slugs unable to m igrate. This strain was selected from a population of cells mutagenize d by random insertion of plasmids facilitated by introduction of restr iction enzyme (a method termed restriction enzyme-mediated integration ). We picked this strain because it formed small misshapen fruiting bo dies. After isolation of portions of the gene as regions flanking the inserted plasmid, we were able to regenerate the original genetic defe ct in a fresh host and show that it is responsible for the development al defects. Transformation of this recapitulated mutant strain with a construct carrying the full-length migA gene and its upstream regulato ry region rescued the defects. The sequence of the full-length gene re vealed that it encodes a novel protein with a BTB domain near the N te rminus that may be involved in protein-protein interactions. The migA gene is expressed at low levels in all cells during aggregation and th en appears to be restricted to prestalk cells as a consequence of rapi d turnover in prespore cells. Although migA(-) cells have a dramatical ly reduced chemotactic index to cAMP and an abnormal pattern of aggreg ation in natural waves of cAMP, they are completely normal in size, sh ape, and ability to translocate in the absence of any chemotactic sign al. They respond behaviorally to the rapid addition of high levels of cAMP in a manner indicative of intact circuitry connecting receptor oc cupancy to restructuring of the cytoskeleton. Actin polymerization in response to cAMP is also normal in the mutant cells. The defects at bo th the aggregation and slug stage are cell autonomous. The MigA protei n therefore is necessary for efficiently assessing chemical gradients, and its absence results in defective chemotaxis and slug migration.