Protein-protein interactions regulate signaling pathways within and between cells. They constitute the basis of the quaternary structure of multimeric proteins and represent one of the highest levels of structural organization in biology. Experimental techniques have provided atomic details of many protein-protein complexes. However, although it is feasible to extract common determinants of protein recognition from structural data, recent studies reported an extreme diversity of interaction modes. This is particularly true for transient associations that could be strengthen or weakened by specific cellular conditions, acting as an efficient regulatory mechanism. Here protein-protein interactions are classified and recurrent structural features of molecular recognition are illustrated. Moreover, metal-mediated associations are discussed to exemplify the role of cofactors in the stabilization of transient protein assemblies. In this view, the protein machinery mediating the activation of the nickel-enzyme urease from H. pylori is described. In particular, the activity of the metallo-chaperone UreE is analyzed in depth with respect to the association with its cognate UreG.