PSMA regulates PAK phosphorylation and cytoskeletal morphology. (A) HUVECs treated with PSMA inhibitor or neutralizing antibody have decreased levels of phosphorylated PAK, while total PAK and phosphorylated extracellular signal-regulated kinase levels are unaffected. P-ERK, phospho-extracellular signal-regulated kinase (ERK). (B) HUVECs treated with PMPA show reduced levels of activated Rac, while total Rac levels remain unchanged. Quantification of the average of three Rac activation experiments is shown by the graph (n = 3; P = 0.002). **, 0.01 < P > 0.001. (C) HUVECs treated with either the PMPA inhibitor, the PSMA neutralizing antibody (α-PSMA), or siRNA-treated endothelial cells isolated from PSMA-null (PSMA−/−) animals display actin-containing protrusions extending in multiple directions. (D) quantitation of cells with protrusions extending in at least three directions reveals an approximate twofold increase in PSMA-inhibited or PSMA-null (PSMA −/−) cells (P < 0.001). *** = 0.001< P > 0.0001. Error bars indicate standard deviations.