Figure 2. Strategies for reconstituting cellular processes.

Once the specific molecular components that are involved in a cellular process are identified, they must be organized by appropriate boundary conditions to generate functional behaviour. Three primary strategies have been used to define the boundary conditions that initiate spatially organized processes. a | The surface of microspheres can be functionalized with proteins such as actin nucleation-promoting factors (NPFs), that spatially organize actin assembly in purified protein solutions or cell extracts by a process that requires the actin-related protein 2/3 (Arp2/3) complex. The lower panel is an epi-fluorescence microscopy image showing an NPF-coated microsphere propelled by a growing actin network. b | supported lipid bilayers ensure that molecular components of the system being studied can freely diffuse. They have been used to reconstitute the immunological synapse. T cell receptor (TCR) and leukocyte function-associated antigen 1 (LFA1) on the T cell bind to intracellular adhesion molecule 1 (ICAM1) and major histocompatibility complex (MHC) peptide reconstituted on the supported lipid bilayer, to form the immunological synapse. The lower panel is an epi-fluorescence microscopy image showing TCR at the centre and ICAM1 at the periphery of the immunological synapse. c | Giant unilamellar vesicles (GUVs) represent deformable substrates, and they have been used to reconstitute filopodium-like protrusions. The lower panel is an epi-fluorescence microscopy image showing actin networks on a GUV spontaneously undergoing transitions to form inward protrusions that consist of long unbranched actin filaments. Part b is reproduced, with permission, from REF. 62 © (2005) American Association for the Advancement of Science. Part c is reproduced, with permission, from REF. 48 © (2008) Macmillan Publishers Ltd. All rights reserved.