Signaling complexes: biophysical constraints on intracellular communication.

This review surveys the kinds of protein complex that participate in cell communication and identifies, where possible, general principles by which they form and act. It also advances the notion that biophysical constraints imposed by macromolecular crowding and diffusion have had a controlling influence on the evolution of cell signaling pathways. Complexes associated with the bacterial aspartate receptor, with eucaryotic tyrosine kinase receptors, with T-cell receptors, and with focal contacts are examined together with proteins that serve as adaptors, anchors, and scaffolds for signaling complexes. The importance of diffusion in controlling the numbers and locations of signaling complexes is discussed, as is the special role played by membranes in signaling pathways.