Atypical protein kinase C (aPKC) has been implicated in several signaling pathways such as cell polarity, cell survival, and cell differentiation. In contrast to other PKCs, aPKC is unique in having the PB1 (Phox and Bem 1) domain in the N terminus. The aPKC PB1 domain binds with ZIP/p62, Par6, or MEK5 through a PB1-PB1 domain interaction that controls the localization of aPKC. Here, we determined the three-dimensional structure of the PB1 domain of PKC by NMR and found that the PB1 domain adopts a ubiquitin fold. The OPCA (OPR, PC, and AID) motif inserted into the ubiquitin fold was presented as a ␣ fold in which the side chains of conserved Asp residues were oriented to the same direction to form an acidic surface. This structural feature suggested that the acidic surface of the PKC PB1 domain interacted with the basic surface of the target PB1 domains, and this was confirmed in the case of the PKC -ZIP/p62 complex by mutational analysis. Interestingly, in the PKC PB1 domain a conserved lysine residue was located on the side opposite to the OPCA motifpresenting surface, suggesting dual roles for the PKC PB1 domain in that it could interact with either the conserved lysine residue or the acidic residues on the OPCA motif of the target PB1 domains.As part of the ongoing study of atypical protein kinase C (aPKC), 1 isoform PKC was first cloned in 1988 (1), followed by the cloning of isoform PKC / (2, 3). Now, aPKC has been suggested as playing a crucial role in signal transduction pathways such as cell polarity, cell differentiation, and cell survival (4, 5). In contrast to conventional PKC, aPKC contains only a single C1 domain but is devoid of a C2 domain in the Nterminal regulatory region. A tandem repeat of the C1 domain is known to be a target of diacylglycerol, and the C2 domain is a binding site for Ca 2ϩ . However, because of the lack of a C2 domain and the incomplete C1 domain in aPKC, neither Ca 2ϩ nor diacylglycerol activates aPKC. Instead, aPKC contains a PB1 domain at the N terminus.In PKC signaling, the regulation of both the catalytic activity and the spatial localization of PKC is essential. PKC is known to translocate to a specific region in response to activation signals and then phosphorylate specific target proteins. Such spatial localization of PKC in cells is partially regulated by either PKC-binding scaffold proteins or anchoring proteins (6). In particular, aPKC binds with PB1 domain-containing proteins such as ZIP/p62 (7-9), Par6 (10 -13), or MEK5 (14) through the PB1-PB1 domain interaction. Thus, the PB1-mediated interaction between aPKC and the scaffolding or anchoring proteins plays a crucial role in exerting the biological functions of aPKC (4).ZIP/p62 homologues were isolated in different biological contexts as ZIP, p62, and EBIAP (7,15,16). Recently, it has been demonstrated that ZIP/p62 is involved in the NF B activation pathway in association with aPKC. ZIP/p62 is a scaffold protein of aPKC for the activation of NF B downstream of extracellular signals such as tumor necrosis fac...