All living cells are held together by membranes, which provide a barrier to the transport of nutrients. They are also the communication platform connecting the outside world to the cell's interior control centers. Thousands of proteins reside in these cell membranes and control the flow of select chemicals, which move across the barrier and mediate the flux of nutrients and information. Almost all of these pathways work by protein handshakes--one protein "talking" to another in order to, for example, encourage the import of a needed nutrient, to block a compound from accumulating to a toxic level, or to alert the cell's interior to changes in the outside environment.