The integration of membrane proteins requires that some parts of the polypeptide chain be translocated across the lipid bilayer whereas others are not. Despite this additional complexity, all modes of insertion of membrane proteins are simply variants of the sequence of events just described for transferring a soluble protein into the lumen of the ER. We begin by describing the three ways in which single-pass transmembrane proteins (see Figure 10–17) become inserted into the ER membrane.
In the simplest case, an N-terminal signal sequence initiates translocation, just as for a soluble protein, but an additional hydrophobic segment in the polypeptide chain stops the transfer process before the entire polypeptide chain is trans-located. This stop-transfer signal anchors the protein in the membrane after the ER signal sequence (the start-transfer signal) has been cleaved off and released from the translocator (Figure 12–42). The lateral gating mechanism transfers the stop-transfer sequence into the bilayer, where it remains as a single α-helical membrane-spanning segment, with the N-terminus of the protein on the lumenal side of the membrane and the C-terminus on the cytosolic side.
In the other two cases, the signal sequence is internal, rather than at the N-terminal end of the protein. As for an N-terminal ER signal sequence, the SRP binds to an internal signal sequence by recognizing its hydrophobic α-helical features. The SRP brings the ribosome making the protein to the ER membrane, and the ER signal sequence then serves as a start-transfer signal that initiates the protein’s translocation. After release from the translocator, the internal start-transfer sequence remains in the lipid bilayer as a single membrane-spanning α helix.
Internal start-transfer sequences can bind to the translocation apparatus in either of two orientations; this in turn determines which protein segment (the one preceding or the one following the start-transfer sequence) is moved across the membrane into the ER lumen. In one case, the resulting membrane protein has its C-terminus on the lumenal side (pathway A in Figure 12–43), while in the other, it has its N-terminus on the lumenal side (pathway B in Figure 12–43). The orientation of the start-transfer sequence depends on the distribution of nearby charged amino acids, as described in the figure legend.
Combinations of start-Transfer and stop-Transfer signals Determine the Topology of multipass Transmembrane proteins
In multipass transmembrane proteins, the polypeptide chain passes back and forth repeatedly across the lipid bilayer as hydrophobic α helices (see Figure 10–17). It is thought that an internal signal sequence serves as a start-transfer signal in these proteins to initiate translocation, which continues until the translocator encounters a stop-transfer sequence; in double-pass transmembrane proteins, for example, the polypeptide can then be released into the bilayer
Figure 12–42 How a single-pass transmembrane protein with a cleaved ER signal sequence is integrated into the ER membrane. in this protein, the co-translational translocation process is initiated by an n-terminal er signal sequence (red) that functions as a start-transfer signal, opening the translocator as in figure 12–35. in addition to this start-transfer sequence, however, the protein also contains a stop-transfer sequence (orange); when this sequence enters the translocator and interacts with a binding site within the pore, the translocator opens at the seam and discharges the protein laterally into the lipid bilayer, where the stop-transfer sequence remains to anchor the protein in the membrane. (in this figure and the two figures that follow, the ribosomes have been omitted for clarity.) (Figure 12–44). In more complex multipass proteins, in which many hydrophobic α helices span the bilayer, a second start-transfer sequence reinitiates translocation further down the polypeptide chain until the next stop-transfer sequence causes polypeptide release, and so on for subsequent start-transfer and stop-transfer sequences (Figure 12–45 and Movie 12.5).