What are the types of protein-protein interactions?

Protein-protein interactions are either stable or transient, and both types of interactions may either be strong or weak. Stable interactions are associated with proteins which are purified as multi-subunit complexes; the subunits of these complexes can be identical or differ from one another. Core RNA polymerase and hemoglobin are examples of multi-subunit interactions which form stable complexes. 

Transient interactions typically control the majority of cellular processes. These interactions are temporary in nature and require a set of factors that promote the interaction such as conformational changes, phosphorylation, or localization to discrete areas of the cell. Transient interactions can be either strong or weak and fast or slow. They also associate and dissociate in vivo. When they bind to their ligands, transiently interacting proteins are involved in different processes such as transport, protein modification, apoptosis, folding, signaling and cell cycling.