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The cell polarity protein PAR3, conserved from the nematode to the vertebrate, forms a complex with PAR6 and atypical protein kinase C (aPKC), and the protein complex occurs at the tight junctions in mammalian epithelial cells. Here we have cloned human cDNA for a novel PAR3 homologue, designated PAR3β, whose messages are present in a variety of tissues and most abundantly expressed in the adult and fetal kidneys. The encoded protein of 1205 amino acids contains a region homologous to the aPKC-binding domain of PAR3α, another human homologue previously identified, and three PDZ domains; the first PDZ domain of PAR3α is considered to interact with PAR6. Unexpectedly, in contrast to other PAR3s found in various species, PAR3β is incapable of binding to any isotypes of PAR6 or aPKC. Nevertheless PAR3β, expressed intrinsically or extrinsically, localizes to the tight junctions, indicating that the localization does not require the ternary complex formation. [Copyright &y& Elsevier]