The effects of deleting the propeptide from human preproapolipoprotein A-I on co-translational translocation and signal peptidase processing

The two principal protein components of human high density lipoprotein particles, apoA-I and apoA-II, are initially synthesized as prepropolypeptides. The function of their NH₂-terminal prosegments is not known. We have previously shown that deletion of the pentapeptide prosegment (Ala-Leu-Val-Arg-Arg) from human preproapoA-II redirects signal peptidase cleavage to a site located between the second and third residues of the mature protein (Folz, R.J., and Gordon, J.I. (1986) J. Biol. Chem. 261, 14752-14759). The hexapeptide prosegment of human preproapoA-I differs from other NH₂-terminal propeptides in that it terminates with paired glutamine residues (Arg-His-Phe-Trp-Gln-Gln). To examine its role in the early events of protein export, we isolated a full-length human preproapoA-I cDNA and deleted its propeptide coding region by oligonucleotide-directed mutagenesis. The effects of this deletion on co-translational translocation and signal peptidase processing were assessed using an in vitro transcription/translation/microsomal membrane processing system. Propeptide deletion reduces the efficiency of co-translational translocation/processing in both reticulocyte and wheat germ lysates but does not affect the fidelity of signal peptidase cleavage. This reduced efficiency does not appear to reflect differences in signal recognition particle-nascent protein interaction as measured by a translational arrest assay. However, differences in translocation/processing rates were noted in a postinitiation translocation/cleavage assay. This assay employed the initiation inhibitor edeine to generate nascent wild type and mutant proteins of increasing chain length which were subsequently presented to canine pancreatic membranes. Unlike preproapoA-I, pre(Δpro)apoA-I retained its ability to undergo translocation and proteolytic processing even after the entire protein was synthesized. These data suggest that the propeptide in human preproapoA-I may play a role in promoting an optimal stucture for co-translational translocation and processing.