To understand how phosphatidylinositol (PI) 3-dinase (PI-3K) modulates cell structure and behavior we examined the molecular and cellular defects of a Dictyostelium mutant strain (pik 1?2?) missing two (DdPIK1 and 2) of three Pi-3K genes which are redundant homologues of the mammalian p110 subunit. DdPIK2 rescued the defects of pik 1?2?. Levels of phosphatidylinositol trisphosphate (PI93,4,5)P3) were reduced in pik 1?2? which had major defects in macropinocytosis. This was accompanied by dramatic deficits in a subset F-actin-enriched structures such as circular ruffles, actin crowns, filopodia, pseudopodia and cortical actin. Although pik1?2? were still capable of near normal motility and chemotaxis, they failed to aggragate into streams. Therefore, we conclude that PIK1 and 2, possibly through modulation of the levels of PIP3 and PI93,4)P2, regulate the reorganization of actin filaments necessary for invagination and circular ruffling during macropinocytosis, the extensio n of filopodia and pseuodopodia, and the aggregation of cells into streams but not for the regulation of cell movement during chemotaxis. Confocal microscopy studies for this project were carried out at the NCMIR. This work has been published (Zhou, Pandol, Bokoch, and Traynor-Kaplan, J. Cell Sci, 111: 283-94. 1998).