Macropinocytosis is characterised by the initial extension of actin-rich, cup-shaped circular ruffles that eventually close to form endocytic vesicles. In macrophages, macropinosome formation occurs in response to stimulation with macrophage-colony-stimulating factor (M-CSF). The signalling components that activate this process, such as Rac1 and PtdIns(3,4,5)P₃, are restricted to these cup-shaped membrane domains, and the disruption of the actin cytoskeleton prevents both Rac1 activation and membrane ruffling. On page 4106, Joel Swanson and colleagues now investigate whether an actin-based diffusion barrier creates an enclosed space within circular ruffles that allows amplification of the signals required for Rac1 activation and subsequent macropinosome formation. To achieve this, they employ a plasma-membrane-localised, photoactivatable green fluorescent protein (PAGFP-MEM). Following activation of PAGFP-MEM in specific membrane regions and by analysing its diffusion pattern they find that, compared with flat membranes, circular ruffles limit diffusion. They also observe that proteins within these specific membrane domains do not mix freely with those outside this area. This retention is the result of a barrier in the ruffles at the rim of the cup. Thus, circular ruffles provide a way to concentrate and amplify the signals that are required for macropinosome formation in discrete membrane regions.

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