Essential Cell Biology 5th edition

510 CHAPTER 15 Intracellular Compartments and Protein Transport
than it is for soluble proteins, as some parts of the polypeptide chain must
be translocated completely across the lipid bilayer, whereas other parts
remain fixed within the membrane.
QUESTION 15–4
A. Predict the membrane
orientation of a protein that is
synthesized with an uncleaved,
internal signal sequence (shown as
the red start-transfer sequence in
Figure 15–17) but does not contain a
stop-transfer sequence.
B. Similarly, predict the membrane
orientation of a protein that is
synthesized with an N-terminal
cleaved signal sequence followed by
a stop-transfer sequence, followed
by a start-transfer sequence.
C. What arrangement of signal
sequences would enable the
insertion of a multipass protein with
an odd number of transmembrane
segments?
In the simplest case, that of a transmembrane protein with a single
membrane-spanning segment, the N-terminal signal sequence initiates
translocation—as it does for a soluble protein. But the transfer process
is then halted by an additional sequence of hydrophobic amino acids, a
stop-transfer sequence, further along the polypeptide chain. At this point,
the protein translocator releases the growing polypeptide chain sideways
into the lipid bilayer. The N-terminal signal sequence is cleaved off,
and the stop-transfer sequence remains in the bilayer, where it forms an
α-helical membrane-spanning segment that anchors the protein in the
membrane. As a result, the protein ends up as a single-pass transmembrane
protein inserted in the membrane with a defined orientation—the
N-terminus on the lumenal side of the lipid bilayer and the C-terminus
on the cytosolic side (Figure 15–16). Once inserted into the membrane,
a transmembrane protein will never change its orientation; its cytosolic
portion will always remain in the cytosol, even if the protein is subsequently
transported to another organelle via vesicle budding and fusion
(see Figure 11−18).
In some transmembrane proteins, an internal, rather than an N-terminal,
signal sequence is used to start the protein transfer; this internal signal
sequence, called a start-transfer sequence, is never removed from
the polypeptide. This arrangement occurs in some transmembrane proteins
in which the polypeptide chain passes back and forth across the
lipid bilayer. In these cases, hydrophobic signal sequences are thought
to work in pairs: an internal start-transfer sequence serves to initiate
translocation, which continues until a stop-transfer sequence is reached;
the two hydrophobic sequences are then released into the bilayer, where
they remain as membrane-spanning α helices (Figure 15–17). In complex
multipass proteins, in which many hydrophobic α helices span the bilayer,
additional pairs of start- and stop-transfer sequences come into play: one
sequence reinitiates translocation further down the polypeptide chain,
and the other stops translocation and causes polypeptide release, and
so on for subsequent starts and stops. In this way, multipass membrane
proteins are stitched into the lipid bilayer as they are being synthesized,
by a mechanism resembling the workings of a sewing machine.
Figure 15–16 A single-pass
transmembrane protein is retained
in the lipid bilayer. An N-terminal ER
signal sequence (red ) initiates transfer
as in Figure 15–15. In addition to this
sequence, the protein also contains
a second hydrophobic sequence,
which acts as a stop-transfer sequence
(orange). When this sequence enters
the protein translocator, the growing
polypeptide chain is discharged into
the lipid bilayer. The N-terminal signal
sequence is cleaved off, leaving the
transmembrane protein anchored in
the membrane (Movie 15.4). Protein
synthesis on the cytosolic side then
continues to completion.
hydrophobic
stop-transfer
sequence
CYTOSOL
ER LUMEN
NH 2
protein translocator
ER signal
sequence
stop-transfer
sequence enters
translocator
signal
peptidase
NH 2
COOH
mature single-pass transmembrane protein
in ER membrane