GO:0006616 SRP-dependent cotranslational protein targeting to membrane, translocation: Protein Synthesis Pathway, Genes, Functions and Research Methods
Research-grade guide for scientists and biopharma professionals
Key Takeaways
• GO:0006616 describes the step in which a nascent polypeptide is moved across the endoplasmic reticulum (ER) membrane while still being synthesized on the ribosome, using the signal recognition particle (SRP) and its receptor to deliver the ribosome-nascent chain complex to the translocon.
• The process is conserved from bacteria to humans: SRP recognizes a hydrophobic signal sequence on the nascent chain, pauses translation, and docks the ribosome onto the membrane via the SRP receptor.
• After docking, SRP and its receptor dissociate and the nascent chain is transferred to the translocon, a protein-conducting channel that allows the growing chain to pass directly from the ribosomal exit tunnel into the ER lumen without folding in the cytosol.
• The SRP receptor and the translocon physically interact, and this interaction is critical for efficient cotranslational translocation.
• Cotranslational targeting can cooperate with posttranslational translocation pathways for certain substrates, as shown for Spc3 topogenesis.
• Defects in SRP-dependent targeting and translocation are linked to protein-misfolding diseases and are studied using ribosome profiling, proteomics, and CRISPR-engineered cell models.
Description
GO:0006616, SRP-dependent cotranslational protein targeting to membrane, translocation, is a biological process that ensures newly synthesized proteins destined for the secretory pathway are delivered across the endoplasmic reticulum (ER) membrane while they are still being translated. This mechanism is essential for the biogenesis of secreted proteins, membrane proteins, and resident ER proteins, and it is conserved across all domains of life. The process begins when the signal recognition particle (SRP) binds a hydrophobic signal sequence on the nascent polypeptide as it emerges from the ribosome, causing a pause in translation. The ribosome-nascent chain-SRP complex is then targeted to the ER membrane through the SRP receptor, where the nascent chain is transferred to the translocon, a protein-conducting channel. The elongating chain passes directly from the large ribosomal subunit into the translocon and into the ER lumen, never exposed to the cytosol and folding only after reaching the lumen. This tight coupling of translation and translocation prevents aggregation and ensures fidelity of protein targeting. Researchers study GO:0006616 to understand protein trafficking, ER homeostasis, and the molecular basis of diseases caused by defective protein targeting.
SRP-dependent cotranslational protein targeting to membrane, translocation At A Glance
| GO ID | GO:0006616 |
|---|---|
| GO term | SRP-dependent cotranslational protein targeting to membrane, translocation |
| Ontology | biological_process |
| Synonym | ER translocation; SRP-dependent cotranslational membrane targeting, translocation; SRP-dependent cotranslational protein-membrane targeting, translocation; translocation during SRP-dependent cotranslational protein targeting to membrane |
| Major function | Cotranslational delivery of nascent polypeptides across the ER membrane via SRP, SRP receptor, and the translocon |
| Cellular location | Endoplasmic reticulum membrane; ribosome; translocon |
| Key components | SRP (SRP54, SRP9/14, SRP19, SRP68/72, 7SL RNA), SRP receptor (SR alpha, SR beta), translocon (SEC61A1, SEC61B, SEC61G), ribosome |
| Conservation | Conserved from bacteria (Ffh, FtsY, SecYEG) to eukaryotes (SRP, SR, Sec61) |
| Related processes | Posttranslational translocation, tail-anchored protein insertion, ER-associated degradation |
What Is GO:0006616?
GO:0006616 is defined as the process during cotranslational membrane targeting wherein proteins move across a membrane. In this process, SRP and its receptor initiate the transfer of the nascent chain across the endoplasmic reticulum (ER) membrane; they then dissociate from the chain, which is transferred to a set of transmembrane proteins collectively called the translocon. Once the nascent chain-translocon complex is assembled, the elongating chain passes directly from the large ribosomal subunit into the centers of the translocon, a protein-lined channel within the membrane. The growing chain is never exposed to the cytosol and does not fold until it reaches the ER lumen.
Why Is SRP-dependent cotranslational protein targeting to membrane, translocation Important in Cell Biology?
GO:0006616 is fundamental to cellular proteostasis because it couples protein synthesis with membrane translocation, ensuring that secretory and membrane proteins reach the ER lumen or membrane without aggregating in the cytosol. Defects in this pathway impair the secretion of hormones, growth factors, and extracellular matrix components, and can trigger ER stress and unfolded protein responses. The process is also a target for antibiotics and is implicated in diseases ranging from cancer to neurodegeneration.
• Essential for the biogenesis of secreted proteins, membrane proteins, and ER-resident proteins.
• Prevents premature folding and aggregation of nascent chains in the cytosol.
• Conserved mechanism from bacteria to humans, making it a model for studying protein targeting.
• SRP and SRP receptor mutations are linked to protein misfolding and ER stress.
• The translocon is a hub for viral protein biogenesis and a target for antiviral and antibacterial drugs.
• Crosstalk with posttranslational translocation pathways expands substrate range.
• Dysregulation contributes to cancer cell survival via altered secretion of growth factors.
• Neurodegenerative diseases involve defective ER targeting of proteins such as prion protein.
• Ribosome profiling and proteomics enable global mapping of cotranslational translocation.
• CRISPR screens can identify genes required for SRP-dependent targeting.
What Happens During SRP-dependent cotranslational protein targeting to membrane, translocation?
Signal sequence recognition and translation arrest
In simple terms: The cell pauses protein production when it sees a shipping tag that means the protein must go to the ER.
As a nascent polypeptide emerges from the ribosomal exit tunnel, a hydrophobic signal sequence is recognized by the signal recognition particle (SRP), a ribonucleoprotein complex. SRP binding induces a pause in translation, preventing the nascent chain from folding prematurely in the cytosol. In bacteria, the SRP homolog Ffh and the 4.5S RNA perform this function. Structural studies of the ribosome-SRP-FtsY complex in the closed state reveal how SRP engages the ribosome and the nascent chain.
Targeting to the ER membrane via SRP receptor
In simple terms: The paused protein is delivered to the ER surface by a molecular taxi.
The ribosome-nascent chain-SRP complex is targeted to the ER membrane through interaction with the SRP receptor (SR), a heterodimer of SR alpha and SR beta in eukaryotes. In bacteria, the SRP receptor is FtsY. This interaction is GTP-dependent and leads to the release of SRP from the complex. The SRP receptor then hands the ribosome-nascent chain over to the translocon.
Transfer to the translocon and chain translocation
In simple terms: The protein is threaded through a tunnel into the ER while it is still being made.
Once the ribosome-nascent chain complex is delivered to the translocon, SRP and its receptor dissociate, and the nascent chain is transferred to the translocon, a protein-conducting channel composed of Sec61 alpha, beta, and gamma subunits in eukaryotes (SecYEG in bacteria). The elongating chain passes directly from the large ribosomal subunit into the center of the translocon, a protein-lined channel within the membrane. The growing chain is never exposed to the cytosol and does not fold until it reaches the ER lumen. The interaction between the SRP receptor and the translocon is critical for efficient cotranslational protein translocation.
Cooperation with posttranslational pathways
In simple terms: Some proteins can use both the co-translational and post-translational routes to get into the ER.
Cotranslational targeting can cooperate with posttranslational translocation for certain substrates. For example, Spc3 topogenesis involves both cotranslational targeting and posttranslational translocation, demonstrating flexibility in membrane protein biogenesis. This cooperation ensures that proteins with less hydrophobic signal sequences can still be efficiently inserted into the ER membrane.
Key Genes Involved in GO:0006616 SRP-dependent cotranslational protein targeting to membrane, translocation
The following genes and proteins are core components of SRP-dependent cotranslational protein targeting to membrane, translocation (GO:0006616).
| Gene | Major Role | Research Relevance |
|---|---|---|
| SRP54 | Signal recognition particle subunit that binds signal sequences and GTP | Key for signal sequence recognition and translation arrest |
| SRP9 | SRP subunit involved in elongation arrest | Studied for its role in translation pausing |
| SRP14 | SRP subunit involved in elongation arrest | Part of the Alu domain of SRP |
| SRP19 | SRP subunit required for SRP assembly | Essential for SRP RNA binding |
| SRP68 | SRP subunit involved in signal sequence binding | Component of the S domain |
| SRP72 | SRP subunit involved in signal sequence binding | Component of the S domain |
| SRPR (SR alpha) | SRP receptor subunit that interacts with SRP | Mediates docking to the ER membrane |
| SRPRB (SR beta) | SRP receptor subunit anchored in the ER membrane | Facilitates GTP hydrolysis and recycling |
| SEC61A1 | Main subunit of the translocon channel | Forms the protein-conducting pore |
| SEC61B | Translocon subunit | Regulates channel gating |
| SEC61G | Translocon subunit | Modulates translocon function |
| SEC62 | Translocon-associated protein for posttranslational translocation | Cooperates with cotranslational pathways |
| SEC63 | ER membrane protein involved in translocation | Stimulates BiP ATPase |
| FTSY (bacterial) | Bacterial SRP receptor | Model for SRP receptor function |
| FFH (bacterial) | Bacterial SRP protein component | Model for signal sequence recognition |
| SECY (bacterial) | Bacterial translocon channel | Structural homolog of Sec61 alpha |
| SECA (bacterial) | Bacterial ATPase for posttranslational translocation | Studied as a separate step from SRP targeting |
How Is SRP-dependent cotranslational protein targeting to membrane, translocation Regulated?
The SRP-dependent cotranslational targeting pathway is regulated at multiple levels. Translation arrest by SRP is relieved upon interaction with the SRP receptor and GTP hydrolysis. The interaction between the SRP receptor and the translocon is critical and may be modulated by membrane composition and accessory factors. In bacteria, the balance between SRP-dependent and SecA-dependent translocation is regulated by the signal sequence and cellular conditions. Additionally, posttranslational pathways can compensate when cotranslational targeting is compromised, as seen for Spc3. The unfolded protein response (UPR) can upregulate components of the translocon and ER chaperones to cope with increased secretory load.
SRP-dependent cotranslational protein targeting to membrane, translocation and Human Disease
| Gene | Disease / Biology | Potential Experimental Model |
|---|---|---|
| SEC61A1 | Immunodeficiency, tubulointerstitial kidney disease | Knock-in of patient mutations in HEK293 or kidney organoids |
| SRP54 | Neutropenia, pancreatic insufficiency | Knockout in hematopoietic stem cells or iPSCs |
| SRPR | ER stress-related disorders | Overexpression or point mutation in cancer cell lines |
| SEC62 | Cancer progression, prostate cancer | Knockout in prostate cancer cell lines |
| SEC63 | Polycystic liver disease | Knock-in in cholangiocytes or liver organoids |
Cancer and altered secretion
Cancer cells often exhibit increased secretion of growth factors and cytokines, which rely on SRP-dependent cotranslational translocation. Mutations in SRP or translocon components can lead to ER stress and contribute to tumor progression. Targeting this pathway is being explored for therapeutic intervention.
Neurodegeneration and protein misfolding
Defective ER targeting of proteins such as prion protein and amyloid precursor protein can lead to their mislocalization and aggregation, contributing to neurodegenerative diseases. Tail-anchored proteins that fail to insert properly can also cause neuronal dysfunction.
Ribosomopathies and ER stress
Mutations in SRP components or translocon subunits can cause ribosomopathies and chronic ER stress, affecting tissues with high secretory demand. For example, SEC61A1 mutations are linked to immunodeficiency and tubulointerstitial kidney disease.
From SRP-dependent cotranslational protein targeting to membrane, translocation-Related Genes to Experimental Models
| Research Question | Suitable Model |
|---|---|
| Does loss of SRP54 impair secretion? | SRP54 knockout in HeLa or HEK293 cells |
| Does a point mutation in SEC61A1 affect translocation? | SEC61A1 point-mutation knock-in in iPSCs |
| Can a tagged SRP receptor be used to track dynamics? | Knock-in of GFP-SRPR in U2OS cells |
| Does overexpression of SRP components enhance secretion? | Overexpression of SRP54 and SRP19 in CHO cells |
| Which genes are required for cotranslational targeting? | Genome-wide CRISPR knockout library screening in K562 cells |
| Does posttranslational translocation compensate for SRP loss? | Double knockout of SRP54 and SEC62 in HEK293 cells |
How to Study the SRP-dependent cotranslational protein targeting to membrane, translocation Process
| Method | What It Measures | Typical Application |
|---|---|---|
| Ribo-seq | Ribosome occupancy and translation pausing | Global mapping of SRP-dependent pausing |
| Proteomics | Protein abundance and secretion | Identifying cargo dependent on SRP |
| Live-cell imaging | Localization and dynamics of SRP/SR/translocon | Tracking ER targeting in real time |
| In vitro translocation | Efficiency of protein translocation | Mechanistic studies of SRP and translocon |
| CRISPR knockout screening | Gene essentiality for translocation | Identifying novel components |
| Co-immunoprecipitation | Protein-protein interactions | Mapping SRP-SR-translocon complexes |
| GTPase assays | GTP hydrolysis by SRP and SR | Studying regulation of targeting |
| Electron microscopy | Structural architecture of targeting complexes | Visualizing ribosome-SRP-FtsY |
Ribosome profiling (Ribo-seq)
Ribo-seq captures ribosome-protected mRNA fragments and can reveal translation pausing induced by SRP, as well as global changes in cotranslational targeting efficiency. It is used to map signal sequence-dependent pausing and to identify substrates of the SRP pathway.
Proteomics and secretome analysis
Mass spectrometry-based proteomics of cell lysates and conditioned media can quantify the impact of SRP or translocon perturbations on protein secretion and membrane protein abundance. This approach identifies specific cargo proteins that depend on GO:0006616.
Fluorescence microscopy and live-cell imaging
Tagging SRP, SRP receptor, or translocon subunits with fluorescent proteins enables real-time visualization of their localization and dynamics at the ER membrane. This can be combined with CRISPR knock-in to study endogenous proteins.
In vitro translocation assays
Cell-free systems using rough microsomes or purified components reconstitute SRP-dependent targeting and translocation, allowing mechanistic dissection of individual steps. These assays are used to test the effects of mutations in SRP, SR, or translocon components.
How CRISPR Can Be Used to Study GO:0006616 SRP-dependent cotranslational protein targeting to membrane, translocation
Knockout
CRISPR knockout of core genes such as SRP54, SRPR, or SEC61A1 can abolish cotranslational translocation, leading to ER stress and cell death. These models are used to identify which proteins depend on the SRP pathway and to study compensatory mechanisms.
Point Mutation
Point mutations in SEC61A1 or SRP54 identified in patients can be introduced into cell lines to study their effects on translocation efficiency and disease phenotypes. Such models help distinguish loss-of-function from dominant-negative effects.
Knock-in
Knock-in of fluorescent or affinity tags (e.g., GFP, HA) into endogenous SRP or translocon genes allows visualization and purification of native complexes. This approach preserves endogenous regulation and stoichiometry.
Overexpression
Overexpression of SRP components or translocon subunits can enhance secretory capacity or rescue partial loss-of-function phenotypes. It is used to study the effects of increased targeting capacity on protein secretion.
How EDITGENE Supports SRP-dependent cotranslational protein targeting to membrane, translocation Research
Researchers studying SRP-dependent cotranslational protein targeting to membrane, translocation-related genes often need to determine whether a candidate gene is causally involved in the pathway or merely correlated with changes in protein secretion. CRISPR-based models provide a direct way to test gene function in relevant cell types.
Contact EDITGENE today to design your custom CRISPR model for SRP-dependent cotranslational protein targeting to membrane, translocation research.
Frequently Asked Questions About SRP-dependent cotranslational protein targeting to membrane, translocation
What is GO:0006616?
GO:0006616 is the biological process of SRP-dependent cotranslational protein targeting to membrane, translocation, in which nascent proteins are moved across the ER membrane while being synthesized, using SRP, SRP receptor, and the translocon.
What genes are involved in SRP-dependent cotranslational protein targeting to membrane, translocation?
Key genes include SRP54, SRP9, SRP14, SRP19, SRP68, SRP72, SRPR, SRPRB, SEC61A1, SEC61B, SEC61G, and in bacteria FTSY, FFH, and SECY.
How does SRP-dependent cotranslational translocation work?
SRP binds a signal sequence on the nascent chain, pauses translation, and targets the ribosome to the ER membrane via the SRP receptor; the chain is then transferred to the translocon and passes into the ER lumen.
What is the role of the translocon in GO:0006616?
The translocon is a protein-conducting channel that allows the elongating polypeptide to pass directly from the ribosome into the ER lumen without exposure to the cytosol.
Why is SRP-dependent cotranslational targeting important?
It ensures proper folding and localization of secretory and membrane proteins, prevents aggregation, and is essential for ER homeostasis and cellular function.
What diseases are linked to defects in SRP-dependent translocation?
Mutations in SEC61A1 and SRP54 are linked to immunodeficiency, kidney disease, and neutropenia; defective targeting also contributes to neurodegeneration and cancer.
How can I study SRP-dependent cotranslational protein targeting to membrane, translocation?
Use Ribo-seq, proteomics, live-cell imaging, in vitro translocation assays, and CRISPR knockout or knock-in models.
Can CRISPR be used to study GO:0006616?
Yes, CRISPR knockout, point mutation, knock-in, and overexpression models allow functional dissection of SRP, SRP receptor, and translocon genes.
What is the difference between cotranslational and posttranslational translocation?
Cotranslational translocation occurs while the protein is being synthesized and requires SRP, whereas posttranslational translocation occurs after synthesis and often uses Sec62/Sec63 and BiP.
Which model systems are best for studying SRP-dependent targeting?
Human cell lines (HEK293, HeLa), yeast, and bacteria are commonly used; CRISPR-engineered cells provide precise genetic control.
Conclusion
GO:0006616, SRP-dependent cotranslational protein targeting to membrane, translocation, is a central mechanism for protein biogenesis in the secretory pathway. Its core components, SRP, SRP receptor, and the translocon, are conserved and essential, and their dysfunction is linked to a range of human diseases. Understanding this process requires integrated approaches from structural biology, genomics, and proteomics. CRISPR-based models offer powerful tools to dissect gene function and to develop therapeutic strategies targeting this pathway.
References
- 1. Steinberg R et al.. 2018. Co-translational protein targeting in bacteria.. FEMS Microbiol Lett 365(11) PMID: 29790984
- 2. Jung SJ et al.. 2021. Cotranslational Targeting and Posttranslational Translocation can Cooperate in Spc3 Topogenesis.. J Mol Biol 433(18):167109 PMID: 34153287
- 3. Neumann-Haefelin C et al.. 2000. SRP-dependent co-translational targeting and SecA-dependent translocation analyzed as individual steps in the export of a bacterial protein.. EMBO J 19(23):6419-26 PMID: 11101515
- 4. von Loeffelholz O et al.. 2015. Ribosome-SRP-FtsY cotranslational targeting complex in the closed state.. Proc Natl Acad Sci U S A 112(13):3943-8 PMID: 25775537
- 5. Müller M et al.. 2001. Protein traffic in bacteria: multiple routes from the ribosome to and across the membrane.. Prog Nucleic Acid Res Mol Biol 66:107-57 PMID: 11051763
- 6. Egea PF et al.. 2005. Targeting proteins to membranes: structure of the signal recognition particle.. Curr Opin Struct Biol 15(2):213-20 PMID: 15837181
- 7. Jiang Y et al.. 2008. An interaction between the SRP receptor and the translocon is critical during cotranslational protein translocation.. J Cell Biol 180(6):1149-61 PMID: 18347066
- 8. Mehlhorn DG et al.. 2021. Looking for a safe haven: tail-anchored proteins and their membrane insertion pathways.. Plant Physiol 187(4):1916-1928 PMID: 35235667
