The internal phosphodiesterase RegA is essential for the suppression of lateral pseudopods during Dictyostelium chemotaxis

Dictyostelium strains in which the gene encoding the cytoplasmic cAMP phosp hodiesterase RegA is inactivated form small aggregates. This defect was cor rected by introducing copies of the wild-type regA gene, indicating that th e defect was solely the consequence of the loss of the phosphodiesterase. U sing a computer-assisted motion analysis system, regA(-) mutant cells were found to show little sense of direction during aggregation. When labeled wi ld-type cells were followed in a field of aggregating regA(-) cells, they a lso failed to move in an orderly direction, indicating that signaling was i mpaired in mutant cell cultures. However, when labeled regA(-) cells were f ollowed in a field of aggregating wild-type cells, they again failed to mov e in an orderly manner, primarily in the deduced fronts of waves, indicatin g that the chemotactic response was also impaired. Since wild-type cells mu st assess both the increasing spatial gradient and the increasing temporal gradient of cAMP in the front of a natural wave, the behavior of regA(-) ce lls was motion analyzed first in simulated temporal waves in the absence of spatial gradients and then was analyzed in spatial gradients in the absenc e of temporal waves. Our results demonstrate that RegA is involved neither in assessing the direction of a spatial gradient of cAMP nor in distinguish ing between increasing and decreasing temporal gradients of cAMP. However, RegA is essential for specifically suppressing lateral. pseudopod formation during the response to an increasing temporal gradient of cAMP, a necessar y component of natural chemotaxis. We discuss the possibility that RegA fun ctions in a network that regulates myosin phosphorylation by controlling in ternal cAMP levels, and, in support of that hypothesis, we demonstrate that myosin II does not localize in a normal manner to the cortex of regA(-) ce lls in an increasing temporal gradient of cAMP.