caption: Proteins rarely solo. More often, they work by assembling into larger multiprotein complexes, some of which have the characteristics of rather complicated protein "machines." These machines, in turn, execute such major functions as protein synthesis and degradation, cell-to-cell signaling, and a host of other operations. The properties of each kind of protein, which cause it to assemble with others into machines and to execute very specific and critical reactions in the cell, are the direct consequence of the protein's amino acid sequence that dictates its final structure.