An internalized amino-terminal signal sequence retains full activity in vivo but not in vitro.

Internalization of the signal sequence of the vesicular stomatitis virus glycoprotein was accomplished by extending the amino-terminal coding sequence with sequences derived from pBR322. Such constructs were then expressed in eukaryotic cells. It was found that amino-terminal extensions consisting of 20, 61, or 102 amino acids totally unrelated to any signal peptide affected neither the function nor cleavage of the signal sequence in vivo. Subsequent transport of the glycoprotein was also not affected. Although the internalized signals functioned with wild-type efficiency in vivo, membrane insertion in vitro (as determined by proteolysis protection assays), signal cleavage, and glycosylation were only achieved when the amino-terminal presequences were short.