1.7
Synthesis of Membrane Proteins
1.7.1
Natural Biosynthesis
In eukaryotic cells, where most MPs feature α-helical TM regions, MPs are typically identified by the
signal recognition particle (SRP) as their first hydrophobic segment emerges from the ribosome tunnel,
which halts translation. The ribosome/SRP complex, along with mRNA and the N-terminal region of
the MP, docks at the translocon, a TM complex that catalyzes insertion of the protein across the
endoplasmic reticulum (ER) membrane (Fig. 1.38). SRP is released and synthesis resumes as a
directional process in the course of which the nascent chain is inserted into a central channel in the
translocon, or perhaps a lateral groove, in which it would be partially exposed to lipids (cf. Fig. 1.38;
for reviews, see e.g. Park and Rapoport 2012; Collinson et al. 2015; Cymer et al. 2015).
Hydrophobic segments that will be TM helices in the final structure are oriented as a function of
the distribution of charged residues around their extremities (“positive-inside” rule; see von Heijne
1986), which may involve interactions with the translocon, with lipids, and/or with already inserted
helices, and released into the ER membrane as folded helices, probably singly or in hairpin pairs. The
ensemble of helices that form a TM region pack together, which may be accompanied by the formation
of reentrant loops and other non-TM features, helix reorientation, the binding of prosthetic groups or
specific lipids, oligomerization, etc., to achieve the final 3D structure (“two-stage model”; see Popot
and Engelman 1990; Engelman et al. 2003) (Fig. 1.39).
In bacteria, plasma membrane proteins, whose TM region is also α-helical, are inserted and fold
in a similar manner, except that translocation is sometimes posttranslational, in which case the protein
is temporarily kept water soluble and its insertion is guided by specialized chaperones. In either case,
translocation and insertion essentially proceed from the N- to the C-terminus of the MP, and the first
Fig. 1.37 Schematic representation of the Ca
2+ -binding sites in the calcium-bound state E1Á2Ca
2+ (A)
and calcium-free state E2 (B) of SERCA1a, the latter stabilized by the inhibitors thapsigargin (TG) and
2,5-di-tert-butyl-1,4-dihydroxybenzene (BHQ), which block the pump in a H
+ -occluded state. Cyan
spheres represent Ca
2+ ions; red ones represent water molecules. Bound protons appear as red circles.
Arrows indicate the movements of transmembrane helices in the transition E1 ! E2. Dotted arrows
indicate potential movements of water molecules. Dotted pink lines indicate hydrogen bonds, and those in
light green indicate Ca
2+ coordination. Note the presence of a PE molecule (“phospholipid”) wedged
between helices M2 and M4 in state E2. This molecule is absent in the calcium-bound state E1 (From
Obara et al. 2005. # 2005 National Academy of Sciences).
48
1 Membrane Proteins and Their Natural Environment
Synthesis of Membrane Proteins
1.7.1
Natural Biosynthesis
In eukaryotic cells, where most MPs feature α-helical TM regions, MPs are typically identified by the
signal recognition particle (SRP) as their first hydrophobic segment emerges from the ribosome tunnel,
which halts translation. The ribosome/SRP complex, along with mRNA and the N-terminal region of
the MP, docks at the translocon, a TM complex that catalyzes insertion of the protein across the
endoplasmic reticulum (ER) membrane (Fig. 1.38). SRP is released and synthesis resumes as a
directional process in the course of which the nascent chain is inserted into a central channel in the
translocon, or perhaps a lateral groove, in which it would be partially exposed to lipids (cf. Fig. 1.38;
for reviews, see e.g. Park and Rapoport 2012; Collinson et al. 2015; Cymer et al. 2015).
Hydrophobic segments that will be TM helices in the final structure are oriented as a function of
the distribution of charged residues around their extremities (“positive-inside” rule; see von Heijne
1986), which may involve interactions with the translocon, with lipids, and/or with already inserted
helices, and released into the ER membrane as folded helices, probably singly or in hairpin pairs. The
ensemble of helices that form a TM region pack together, which may be accompanied by the formation
of reentrant loops and other non-TM features, helix reorientation, the binding of prosthetic groups or
specific lipids, oligomerization, etc., to achieve the final 3D structure (“two-stage model”; see Popot
and Engelman 1990; Engelman et al. 2003) (Fig. 1.39).
In bacteria, plasma membrane proteins, whose TM region is also α-helical, are inserted and fold
in a similar manner, except that translocation is sometimes posttranslational, in which case the protein
is temporarily kept water soluble and its insertion is guided by specialized chaperones. In either case,
translocation and insertion essentially proceed from the N- to the C-terminus of the MP, and the first
Fig. 1.37 Schematic representation of the Ca
2+ -binding sites in the calcium-bound state E1Á2Ca
2+ (A)
and calcium-free state E2 (B) of SERCA1a, the latter stabilized by the inhibitors thapsigargin (TG) and
2,5-di-tert-butyl-1,4-dihydroxybenzene (BHQ), which block the pump in a H
+ -occluded state. Cyan
spheres represent Ca
2+ ions; red ones represent water molecules. Bound protons appear as red circles.
Arrows indicate the movements of transmembrane helices in the transition E1 ! E2. Dotted arrows
indicate potential movements of water molecules. Dotted pink lines indicate hydrogen bonds, and those in
light green indicate Ca
2+ coordination. Note the presence of a PE molecule (“phospholipid”) wedged
between helices M2 and M4 in state E2. This molecule is absent in the calcium-bound state E1 (From
Obara et al. 2005. # 2005 National Academy of Sciences).
48
1 Membrane Proteins and Their Natural Environment