Figure 4.

TM7b and TM8 direct two distinct TM orientations for a single population of nascent chains. Oocyte homogenates containing polypeptides from plasmid S.gG.TM7b-8.P were subject to PK digestion (lanes 1–6) or carbonate extraction (lanes 7–10) and immunoprecipitated as described in Figure 2. Upward arrows identify singly glycosylated full-length chains (lanes 1 and 4). Downward double arrows indicate doubly glycosylated full-length chains (lanes 1, 2, 4, and 5) After PK digestion, all singly glycosylated chains are digested, yielding an N-terminal globin-reactive fragment (lane 2, upward arrow), whereas double-glycosylated chains are fully protected. All full-length chains are fully integrated into the bilayer (lanes 7–10). Alternate topological isoforms of these chains are indicated beneath the autoradiograms. The ratios of single-spanning chains versus double-spanning chains were 60 and 40%, respectively. Potential glycosylation sites are indicated as vertical lines, and used sites are indicated as open circles. The sequence of pertinent residues of TM7b and TM8 is indicated in Figure 1.