When a cell crawls or extends pseudopods to snare a bacterium, the front edge of its membrane seethes with activity. On page 193, Brandt et al. show that a key protein that incites cell movement depends on a partner to maneuver it into the center of the action.

Polymerization of actin molecules provides the driving force for cell movement and phagocytosis. One protein that sparks this elongation is Dia1, which RhoA switches on. Previous studies have shown that, during phagocytosis, Dia1 accumulates at the advancing portion of the membrane. But researchers didn't know how it ends up there.

To find out, Brandt et al. went fishing for proteins that latch onto Dia1. They hooked one called IQGAP1 that clusters with Dia1 at the front edge of crawling cells. Depleting IQGAP1 with RNAi prevented Dia1 from homing in on these sites. IQGAP1 and Dia1 also colocalized at the phagocytic cup, the indentation created when pseudopods reach out. Blocking the interaction between the two proteins curtailed phagocytosis. The researchers also showed that IQGAP1 had no direct effect on actin extension, which suggests its job is to ferry Dia1 to active parts of the membrane. The researchers hypothesize that IQGAP1 might stabilize active Dia1, thereby increasing local actin polymerization. They now want to work out the timing of the process, such as when Dia1 arrives at an extension and how long it remains there.