The PB1 domain is present in the two mitogen-activated protein kinase kinases MEKK2 and MEKK3 which are two members of the signaling kinase cascade involved in angiogenesis and early cardiovascular development. The PB1 domain of MEKK2 (and/or MEKK3) interacts with the PB1 domain of another member of the kinase cascade Map2k5. A canonical PB1-PB1 interaction, which involves heterodimerization of two PB1 domains, 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. Interactions of PB1 domains with other protein domains have been described as noncanonical PB1-interactions. The PB1 domain module is conserved in amoebas, fungi, animals, and plants. The MEKK2 and MEKK3 proteins contain a type II PB1 domain.