Regulation of translocation of the desmoyokin/AHNAK protein to the plasma membrane in keratinocytes by protein kinase C.

Desmoyokin was identified as a desmosomal plaque protein. We previously demonstrated that desmoyokin is identical to a protein encoded by a human gene, AHNAK, whose expression is suppressed in neuroblastoma cells. Although this protein is distributed in the cytoplasm and the nucleus in various cells, it is associated closely with the plasma membrane in keratinocytes. In keratinocytes, desmoplakin translocates from the cytoplasm to the plasma membrane following both high calcium switch and protein kinase C (PKC) activation by 12-O-tetradecanoylphorbol-13-acetate (TPA). In the low calcium medium, the desmoyokin/AHNAK protein resides diffusely in the cytoplasm and the nucleus. However, 2 h after shift to the high calcium medium, the desmoyokin/AHNAK protein localized to the cell boundary in all cells in a pattern similar to that of desmoplakin. Selective PKC inhibitors completely inhibited the calcium-induced translocation of the desmoyokin/AHNAK protein, but the inhibition of desmoplakin translocation by these inhibitors was only partial. TPA also induced translocation of both the desmoyokin/AHNAK protein and desmoplakin, which was completely inhibited by PKC inhibitors. The calcium-induced phosphorylation of the desmoyokin/AHNAK protein was confirmed by immunoprecipitation using [32P]orthophosphate-labeled keratinocytes. Furthermore, the study of extractability with non-ionic detergent indicated that desmoplakin, but not the desmoyokin/AHNAK protein, is associated with the cytoskeleton. These results suggested an involvement of PKC in the translocation of the desmoyokin/AHNAK protein in keratinocytes. It was, however, also suggested that different mechanisms are likely involved in the translocation of the desmoyokin/AHNAK protein and desmoplakin.