PB1 domain is essential part of the mitogen-activated protein kinase kinase 5 (Map2k5, alias MEK5) one of the key member of the signaling kinases cascade which involved in angiogenesis and early cardiovascular development. The PB1 domain of Map2k5 interacts with the PB1 domain of another members of kinase cascade MEKK2 (or MEKK3). A canonical PB1-PB1 interaction, involving heterodimerization of two PB1 domain, is required for the formation of macromolecular signaling complexes ensuring specificity and fidelity during cellular signaling. The interaction between two PB1 domain depends on the type of PB1. There are three types of PB1 domains: type I which contains an OPCA motif, acidic aminoacid cluster, type II which contains a basic cluster, and type I/II which contains both an OPCA motif and a basic cluster. The Map2k5 protein contains a type I PB1 domain.