Final answer:
A TM helix is integrated into the membrane during translation through co-translational insertion by the translocon, where specific signal and stop-transfer sequences in the polypeptide chain guide and anchor the protein in the membrane.
Step-by-step explanation:
A transmembrane (TM) helix is integrated into the membrane during translation predominantly through a mechanism known as co-translational insertion by the translocon. This process involves the ribosome synthesizing a membrane protein, which, as it elongates, begins to interact with the RER membrane, specifically a translocon, allowing the integration of the TM helix into the lipid bilayer. As the growing polypeptide chain emerges through the translocon, a stop-transfer sequence within the polypeptide halts transit and anchors the protein in the membrane. If a protein has multiple TM helices, multiple stop-transfer sequences facilitate the protein spanning the membrane more than once. Furthermore, during the translation process, signal sequences guide the ribosome to the RER, and once the protein reaches its destination, such as the membrane, the signal sequence is typically removed.