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Abstract: In cell polarization of Drosophila neuroblasts, Inscuteable (Insc) functions via tethering Partner of Insc (Pins) to Bazooka, homologous to human cell polarity protein Par3. However, little has been known about mammalian homologues of Insc. Here we describe cloning of two distinct cDNAs from human Insc gene, which is differentially expressed from alternative first exons: one encodes 579 amino acids, whereas the other lacks the N-terminal 47 amino acids. In contrast to human homologues for Pins and Par3, human Insc exhibits a weak homology with the Drosophila counterpart. Nevertheless, human Insc proteins bind to the human Pins homologues LGN and AGS3, and also to human Par3 and its related protein Par3β. Although LGN by itself is incapable of interacting with Par3, coexpression of human Insc leads to the interaction between LGN and Par3, indicating that human Insc plays an evolutionarily conserved role as an adaptor protein that links Pins to Par3. [Copyright &y& Elsevier]

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]