C-terminus of desmoyokin/AHNAK protein is responsible for its translocation between the nucleus and cytoplasm

We previously demonstrated that desmoyokin gene is identical to AHNAK gene, which is downregulated in neuroblastomas. Whereas desmoyokin/AHNAK protein is distributed in the nucleus and cytoplasm in nonepithelial tissues, it i s distributed in the cell membrane in epithelial tissues. It is present dif fusely in the cytoplasm and nucleus of epithelial cell lines cultured in lo w calcium condition. Low to normal calcium shift translocates it to the cel l boundary. In this study, we investigated which domain(s) of desmoyokin/AH NAK protein are responsible for its different distribution. We constructed three different eukaryotic expression plasmids, pN-DY, pM-DY, and pC-DY, wh ich expressed N-terminus, central domain, and C-terminus of this molecule, respectively, when transfected into COS-7 cells, normal human keratinocytes , and HeLa cells. In normal calcium medium, whereas N-terminus and central domain of desmoyokin/AHNAK protein were present mainly in the cytoplasm, C- terminus was present in the nucleus, cytoplasm, and weakly in the cell memb rane. In low calcium medium, C-terminus was present exclusively in the nucl eus, and a part of the molecules translocated from the nucleus to the cytop lasm, 3 h after the shift to normal calcium medium or 3 h after addition of protein kinase C activator, 12-O-tetradecanoylphorbol-13-acetate in low ca lcium medium. Calcium shift showed no effects on the distribution of N-term inus and central domain. These results suggested that C-terminus, but neith er N-terminus nor central domain, is responsible for the translocation of t his protein into the nucleus. This study may also suggest that C-terminus p lay a role in the translocation to the cell membrane, although further evid ence is necessary.