Inactivation of lmpA, encoding a LIMPII-related endosomal protein, suppresses the internalization and endosomal trafficking defects in profilin-null mutants

Profilin is a key phosphoinositide and actin-binding protein connecting and coordinating changes in signal transduction pathways with alterations in t he actin cytoskeleton. Using biochemical assays and microscopic approaches, we demonstrate that profilin-null cells are defective in macropinocytosis, fluid phase efflux, and secretion of lysosomal enzymes but are unexpectedl y more efficient in phagocytosis than wild-type cells. Disruption of the lm pA gene encoding a protein (DdLIMP) belonging to the CD36/LIMPII family sup pressed, to different degrees, most of the profilin-minus defects, includin g the increase in F-actin, but did not rescue the secretion defect. Immunof luorescence microscopy indicated that DdLIMP, which is also capable of bind ing phosphoinositides, was associated with macropinosomes but was not detec ted in the plasma membrane. Also, inactivation of the lmpA gene in wild-typ e strains resulted in defects in macropinocytosis and fluid phase efflux bu t not in phagocytosis. These results suggest an important role for profilin in regulating the internalization of fluid and particles and the movement of material along the endosomal pathway; they also demonstrate a functional interaction between profilin and DdLIMP that may connect phosphoinositide- based signaling through the actin cytoskeleton with endolysosomal membrane trafficking events.