▲ FIGURE 16-11 Synthesis and insertion into the ER membrane of type I single-pass proteins. Step 1: After the ribosome/nascent chain complex becomes associated with a translocon in the ER membrane, the N-terminal signal sequence is cleaved. This process occurs by the same mechanism as the one for soluble secretory proteins (see Figure 16-6). Steps 12,13: The chain is elongated until the hydrophobic stop-transfer anchor sequence is synthesized and enters the translocon, where it prevents the nascent chain from extruding farther into the ER
lumen. Step 14 : The stop-transfer anchor sequence moves laterally between the translocon subunlts and becomes anchored in the phospholipid bilayer. At this time, the translocon probably closes. Step 15 : As synthesis continues, the elongating chain may loop out into the cytosol through the small space between the ribosome and translocon (see Figure 16-8). Step 6: When synthesis is complete, the ribosomal subunits are released into the cytosol, leaving the protein free to diffuse in the membrane. [See H. Do et al., 1996, Cell 85:369, and W. Mothes et al., 1997, Cell 89:523.]
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