A conserved internal hydrophobic domain mediates the stable membrane integration of the dengue virus capsid protein

The mature flavivirus capsid protein (virion C) is commonly thought to be free in the cytoplasm of infected cells and to form a nucleocapsid-like complex with genomic RNA in mature virus particles. There is little sequence conservation among flavivirus virion C proteins, but they are similar in size (e.g., 99 amino acids [aa] for the dengue-4 [DEN4] C) and in bearing a net positive charge. In addition, we noted that C contained a conserved internal hydrophobic segment (spanning aa 45-65 in the DEN4 C). Results of in vivo expression and in vitro translation of wt and mutant forms of the DEN4 virion C demonstrated that the conserved internal hydrophobic segment in the DEN C functioned as a membrane anchor domain. Signal peptide function of this segment was also suggested by its requirement for the entry of C into membranes. Virion C was integrated in membranes in a "hairpin" conformation; positively charged segments amino- and carboxy-terminal to the hydrophobic signal-anchor segment were accessible to protease digestion in the "cytoplasm." The net positive charge in the amino-terminal extramembraneous portion of C (aa 1-44) was one determinant of the hairpin membrane orientation; a conserved positively charged residue within the hydrophobic segment (Arg-54 in the DEN4 C) was not.