Download
1 / 61
E N D
Chapter 8 Protein localization
8.1 Introduction8.2 Chaperones may be required for protein folding8.3 Post-translational membrane insertion depends on leader sequences 8.4 A hierarchy of sequences determines location within organelles 8.5 Signal sequences initiate translocation 8.6 How do proteins enter and leave membranes? 8.7 Anchor signals are needed for membrane residence 8.8 Bacteria use both co-translational and post-translational translocation 8.9 Pores are used for nuclear ingress and egress 8.10 Protein degradation by proteasomes
Leaderof a protein is a short N-terminal sequence responsible for passage into or through a membrane. 8.1 Introduction
8.1 Introduction Figure 8.1 Overview: proteins that are localized post-translationally are released into the cytosol after synthesis on free ribosomes. Some have signals for targeting to organelles such as the nucleus or mitochondria. Proteins that are localized cotranslationally associate with the ER membrane during synthesis, so their ribosomes are "membrane-bound". The proteins pass into the endoplasmic reticulum, along to the Golgi, and then through the plasma membrane, unless they have signals that cause retention at one of the steps on the pathway. They may also be directed to other organelles, such as endosomes or lysosomes.
8.1 Introduction Figure 8.2 Proteins synthesized on free ribosomes in the cytosol are directed after their release to specific destinations by short signal motifs.
Figure 8.3 Membrane-bound ribosomes have proteins with N-terminal sequences that enter the ER during synthesis. The proteins may flow through to the plasma membrane or may be diverted to other destinations by specific signals. 8.1 Introduction
Figure 8.4 A protein is constrained to a narrow passage as it crosses a membrane. 8.2 Chaperones may be required for protein folding
Figure 8.5 Chaperone families have eukaryotic and bacterial counterparts (named in parentheses). 8.2 Chaperones may be required for protein folding
Figure 8.6 DnaJ assists the binding of DnaK (Hsp70), which assists the folding of nascent proteins. ATP hydrolysis drives conformational change. GrpE displaces the ADP; this causes the chaperones to be released. Multiple cycles of association and dissociation may occur during the folding of a substrate protein. 8.3 The Hsp70 family is ubiquitous
Figure 8.7-1 A protein may be sequestered within a controlled environment for folding or degradation. 8.4 Hsp60/GroEL forms an oligomeric ring structure
Figure 8.7-2 GroEL forms an oligomer of two rings, each comprising a hollow cylinder made of 7 subunits. 8.4 Hsp60/GroEL forms an oligomeric ring structure
Figure 8.8 Two rings of GroEL associate back to back to form a hollow cylinder. GroES forms a dome that covers the central cavity on one side. Protein substrates bind to the cavity in the distal ring. 8.4 Hsp60/GroEL forms an oligomeric ring structure
Figure 8.9 Protein folding occurs in the proximal GroEL ring and requires ATP. Release of substrate and GroES requires ATP hydrolysis in the distal ring. 8.4 Hsp60/GroEL forms an oligomeric ring structure
Figure 8.10 Leader sequences allow proteins to recognize mitochondrial or chloroplast surfaces by a post-translational process. 8.5 Post-translational membrane insertion depends on leader sequences
Figure 8.12 The leader sequence of yeast cytochrome c oxidase subunit IV consists of 25 neutral and basic amino acids. The first 12 amino acids are sufficient to transport any attached polypeptide into the mitochondrial matrix. 8.5 Post-translational membrane insertion depends on leader sequences
8.5 Post-translational membrane insertion depends on leader sequences Figure 8.13 TOM proteins form receptor complex(es) that are needed for translocation across the mitochondrial outer membrane.
8.5 Post-translational membrane insertion depends on leader sequences Figure 8.14 Tim proteins form the complex for translocation across the mitochondrial inner membrane.
Figure 8.15 Tim9-10 takes proteins from TOM to either TIM complex, and Tim8-13 takes proteins to Tim22-54. 8.5 Post-translational membrane insertion depends on leader sequences
Figure 8.16 A translocating protein may be transferred directly from TOM to Tim22-54. 8.5 Post-translational membrane insertion depends on leader sequences
8.6 A hierarchy of sequences determines location within organelles Figure 8.17 Mitochondria have receptors for protein transport in the outer and inner membranes. Recognition at the outer membrane may lead to transport through both receptors into the matrix, where the leader is cleaved. If it has a membrane-targeting signal, it may be re-exported.
8.6 A hierarchy of sequences determines location within organelles Figure 8.18 The leader of yeast cytochrome c1 contains an N-terminal region that targets the protein to the mitochondrion, followed by a region that targets the (cleaved) protein to the inner membrane. The leader is removed by two cleavage events.
Figure 8.19 A protein approaches the chloroplast from the cytosol with a ~50 residue leader. The N-terminal half of the leader sponsors passage into the envelope or through it into the stroma. Cleavage occurs during envelope 8.6 A hierarchy of sequences determines location within organelles
Signal sequenceis the region of a protein (usually N-terminal) responsible for co-translational insertion into membranes of the endoplasmic reticulum. 8.7 Signal sequences initiate translocation
Figure 8.20 The endoplasmic reticulum consists of a highly folded sheet of membranes that extends from the nucleus. The small objects attached to the outer surface of the membranes are ribosomes. Photograph kindly provided by Lelio Orci. 8.7 Signal sequences initiate translocation
Figure 8.21 The signal sequence of bovine growth hormone consists of the N-terminal 29 amino acids and has a central highly hydrophobic region, preceded or flanked by regions containing polar amino acids. 8.7 Signal sequences initiate translocation
Figure 8.22 Ribosomes synthesizing secretory proteins are attached to the membrane via the signal sequence on the nascent polypeptide. 8.7 Signal sequences initiate translocation
Figure 8.23 The two domains of the 7S RNA of the SRP are defined by its relationship to the Alu sequence. Five of the six proteins bind directly to the 7S RNA. Each function of the SRP is associated with a particular protein(s). 8.7 Signal sequences initiate translocation
Figure 8.24 Does a signal sequence enter an aqueous tunnel created by resident ER membrane proteins? 8.8 The translocon forms a pore
Figure 8.25 The translocon consists of SRP, SRP receptor, Sec61, TRAM, and signal peptidase. 8.8 The translocon forms a pore
Figure 8.26 BiP acts as a ratchet to prevent backward diffusion of a translocating protein. 8.8 The translocon forms a pore
Integral membrane proteinis a protein (noncovalently) inserted into a membrane; it retains its membranous association by means of a stretch of ~25 amino acids that are uncharged and/or hydrophobic.Transmembrane proteinis a component of a membrane; a hydrophobic region or regions of the protein resides in the membrane, and hydrophilic regions are exposed on one or both sides of the membrane. 8.9 How do proteins enter and leave membranes?
Figure 8.27 Group I and group II transmembrane proteins have opposite orientations with regard to the membrane. 8.9 How do proteins enter and leave membranes?
Figure 8.28 The orientations of the termini of multiple membrane-spanning proteins depends on whether there is an odd or even number of transmembrane segments. 8.9 How do proteins enter and leave membranes?
Figure 8.29-1 Does a signal sequence interact directly with the hydrophobic environment of the lipid bilayer or does it directly enter an aqueous tunnel created by resident ER membrane proteins? 8.9 How do proteins enter and leave membranes?
Figure 8.29-2 How does a transmembrane protein make the transition from moving through a proteinaceous channel to interacting directly with the lipid bilayer? 8.9 How do proteins enter and leave membranes?
8.9 How do proteins enter and leave membranes? Figure 8.29-3 Proteins may be associated with one face of a membrane by acyl linkages to fatty acids.
Figure 8.30 Proteins that reside in membranes enter by the same route as secreted proteins, but transfer is halted when an anchor sequence passes into the membrane. If the anchor is at the C-terminus, the bulk of the protein passes through the membrane and is exposed on the far surface. 8.10 Anchor signals are needed for membrane residence
Figure 8.31 A combined signal-anchor sequence causes a protein to reverse its orientation, so that the N-terminus remains on the inner face and the C-terminus is exposed on the outer face of the membrane. 8.10 Anchor signals are needed for membrane residence
Figure 8.32 The signal-anchor of influenza neuraminidase is located close to the N-terminus and has a hydrophobic core. 8.10 Anchor signals are needed for membrane residence
8. 11 Bacteria use both co-translational and post-translational translocation Figure 8.33 The Tat and SecYEG ystems are used for proteins that are translocated across the inner membrane. YidC may be used ith or without SecYEG to insert proteins into the inner membrane.
8. 11 Bacteria use both co-translational and post-translational translocation Figure 8.34 SecB is a chaperone that transfers a nascent protein to SecA, which is a peripheral membrane protein associated with the integral membrane protein complex SecYEG. Translocation requires hydrolysis of ATP and a protonmotive force. Leader peptidase is an integral membrane protein that cleaves the leader sequence.
Figure 8.35 Nuclear pores are used for import and export. 8.12 Pores are used for nuclear ingress and egress
Figure 8.36 Nuclear pores appear as annular structures by electron microscopy. The bar is 0.5 mm. Photograph kindly provided by Ronald Milligan. 8.13 Nuclear pores are large symmetrical structures
Figure 8.37 A model for the nuclear pore shows 8-fold symmetry. Two rings form the upper and lower surfaces (shown in yellow); they are connected by the spokes (shown in green on the inside and blue on the outside). Photograph kindly provided by Ronald Milligan. 8.13 Nuclear pores are large symmetrical structures
Figure 8.38 The outsides of the nuclear coaxial (cytoplasmic and nucleoplasmic) rings are connected to radial arms. The interior is connected to spokes that project towards the transporter that contains the central pore. 8.13 Nuclear pores are large symmetrical structures
Figure 8.39 The nuclear pore complex spans the nuclear envelope by means of a triple ring structure. The side view shows two-fold symmetry from either horizontal or perpendicular axes. 8.13 Nuclear pores are large symmetrical structures
Figure 8.40 Nuclear localization signals have basic residues. 8.13 Nuclear pores are large symmetrical structures
Exportinsare transport receptors that bind cargo in the nucleus, and translocate into the cytoplasm where they release the cargo.Importinsare transport receptors that bind cargo in the cytoplasm, and translocate into the nucleus where they release the cargo.Nucleoporinwas originally defined to describe the components of the nuclear pore complex that bind to the inhibitory lectins, but now is used to mean any component of the basic nuclear pore complex.Translocationof a chromosome describes a rearrangement in which part of a chromosome is detached by breakage and then becomes attached to some other chromosome. 8.15 Transport receptors carry cargo proteins through the pore
Figure 8.42 There are multiple pathways for nuclear export and import. 8.15 Transport receptors carry cargo proteins through the pore
Figure 8.41 A carrier protein binds to a substrate, moves with it through the nuclear pore, is released on the other side, and must be returned for reuse. 8.15 Transport receptors carry cargo proteins through the pore
