Four membrane proteins are represented schematically below. The boxes represent

Question

Four membrane proteins are represented schematically below. The boxes represent N-terminal cleaved signal sequences, internal start-transfer membrane anchors or stop-transfer membrane anchors. For each transmembrane segment, the more basic flanking residues are indicated with plus signs (independent of whether they act as cleaved N-terminal, uncleaved internal start transfers or stop transfers). Using the principles of membrane protein insertion into the ER, predict how each of the mature proteins will be arranged across the ER membrane. Indicate CLEARLY the N and C termini and LABEL the boxed regions with their original numbers AND the words "start" or "stop". Please show leftover cleaved signal sequences where appropriate.

Explanation / Answer

The vast array of integral proteins that are present in the plasma membrane, and other cellular membranes. Each such protein has a unique orientation with respect to the membrane’s phospholipid bilayer. As noted earlier, these membrane glycoproteins are synthesized on the rough ER membrane and remain membrane associated as they move to their final destinations along the same pathway followed by continuously secreted proteins. During this transport, the topology of a membrane protein is preserved; the same segments of the protein, for instance, always face the cytosol. Thus the orientation of these membrane proteins in their final sites is established during biosynthesis on the ER membrane.

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