- Multicellular organisms are strictly dependent on the ability of individual cells to communicate with each other and cooperate properly to coordinate functions during embryogenesis and throughout the entire life span. A major mechanism of cellular communication is mediated by cell surface receptors that deliver signals across the plasma membrane following their engagement with cognate ligands. Since all cells express a large variety of surface receptors they can respond to many different signals provided by peptide hormones, growth factors, neurotransmitters and antigens, as well as surface molecules on neighboring cells or components of the extracellular matrix. Studies over the past two decades yielded a multitude of evidence to substantiate a concept in which both constitutive and transient protein-protein interactions, mediated by a relatively small number of evolutionary conserved protein modules, provide the underlying framework through which signaling pathways operate. Spatially and temporally regulated protein-protein interactions that occur at the ligand-occupied receptor site promote the assembly of multi-molecular complexes where posttranslational modifications regulate molecular interactions and protein functions.