Identification of a novel signal sequence that targets transmembrane proteins to the nuclear envelope inner membrane

Herpesvirus maturation requires translocation of glycoprotein B homologue f rom the endoplasmic reticulum to the inner nuclear membrane. Glycoprotein B of human cytomegalovirus was used in this context as a model protein. To i dentify a specific signal sequence within human cytomegalovirus glycoprotei n B acting in a modular fashion, coding sequences were recombined with repo rter proteins. Immunofluorescence and cell fractionation demonstrated that a short sequence element within the cytoplasmic tail of human cytomegalovir us glycoprotein B was sufficient to translocate the membrane protein CD8 to the inner nuclear membrane. This carboxyl-terminal sequence had no detecta ble nuclear localization signal activity for soluble beta-Galactosidase and could not be substituted by the nuclear localization signal of SV40 T anti gen. For glycoprotein B of herpes simplex virus, a carboxyl-terminal elemen t with comparable properties was found. Further experiments showed that the amino acid sequence DRLRHR of human cytomegalovirus glycoprotein B (amino acids 885-890) was sufficient for nuclear envelope translocation. Single re sidue mutations revealed that the arginine residues in positions 4 and 6 of the DRLRHR sequence were essential for its function. These results support the view that transmembrane protein transport to the inner nuclear membran e is controlled by a mechanism different from that of soluble proteins.