The PB1 domain is an essential part of NBR1 protein, next to BRCA1, a scaffold protein mediating specific protein-protein interaction with both titin protein kinase and with another scaffold protein p62. A canonical PB1-PB1 interaction, which involves 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 NBR1 protein contains a type I PB1 domain.