Recombinant Schizosaccharomyces pombe Protein transport protein sec61 subunit beta (sbh1)

What is the primary structure and biochemical characteristics of S. pombe sbh1?

S. pombe protein transport protein sec61 subunit beta (sbh1) is a 102-amino acid protein with the following sequence: MSSTKASGSVKNSAASAPGGPKSQIRRRAAVEKNTKESNSGPAGARAAGAPGSTPTLLKLYTDEASGFKVDPVVVMVLSVGFIASVFLLHIVARILKKFASE . This protein is encoded by the sbh1 gene (ORF name: SPBC2G2.03c) and has the UniProt accession number O43002 .

Structurally, sbh1 is a tail-anchored membrane protein with a single C-terminal transmembrane domain that anchors it to the Sec61 channel . The protein contains a cytosolic domain with two distinct regions: an N-terminal intrinsically unstructured domain (approximately the first 40 amino acids) followed by a conserved membrane proximal (CMP) domain . Unlike its S. cerevisiae counterpart, S. pombe sbh1 lacks proline-flanked phosphorylation sites in its N-terminal cytosolic domain .

How does sbh1 function as part of the S. pombe Sec61 translocon complex?

In S. pombe, sbh1 functions as part of the Sec61 translocon pore complex alongside Sec61 and Sss1 proteins . While Sec61 forms the central protein-conducting channel with 10 transmembrane helices, sbh1 and Sss1 are single-spanning membrane proteins that associate peripherally with the complex . Sss1 is located at the channel periphery where it contacts both halves of Sec61 and holds them together .

Unlike S. cerevisiae and other post-whole genome duplication (WGD) species that have two homologs (Sbh1 and Sbh2), S. pombe has only a single sbh1 homolog . This means that in S. pombe, both the Sec61 and Ssh1 translocon pores share the same beta (sbh1) and gamma (Sss1) subunits, differing only in their alpha subunit (Sec61 or Ssh1) . This single sbh1 homolog is likely essential for viability in S. pombe, whereas neither Sbh1 nor Sbh2 alone is essential in S. cerevisiae .

What are the recommended storage and handling protocols for recombinant S. pombe sbh1 protein?

For optimal stability and activity of recombinant S. pombe sbh1 protein:

• Store the protein at -20°C for regular use, or at -20°C to -80°C for extended storage

• Use a Tris-based buffer with 50% glycerol optimized for protein stability

• Avoid repeated freezing and thawing cycles as this can lead to protein denaturation

• For short-term experimental work, store working aliquots at 4°C for up to one week

• When designing experiments, consider that recombinant sbh1 may contain a tag (the exact type is determined during the production process)

Methodologically, it's important to verify protein integrity by SDS-PAGE before experimental use, especially after storage periods or when transportation might have compromised protein quality.

How does the conserved membrane proximal (CMP) domain of sbh1 contribute to substrate specificity in protein translocation?

Recent research has revealed that the CMP domain of sbh1 functions as a critical gatekeeper that recognizes and guides specific substrates toward the Sec61 channel for insertion . Peptide panning experiments have demonstrated that the sbh1 CMP domain contains specific binding sites for sbh1-dependent signal peptides, suggesting a role in substrate selection .

Molecular dynamics (MD) modeling has identified a potential interaction site between the sbh1 CMP domain and the Sec61 N-terminal amphipathic helix . The methodological approach involved:

• Using MODELLER to model the Sec61 N-terminal amphipathic helix (sequence: 1MSSNRVLDLFKPFESFLPEVIAPE24) and the 36-aa N-terminal stretch of Sbh1 (sequence: 1MSSPTPPGGQRTLQKRKQGSSQKVAASAPKKNTNSN36) into α-helical conformations

• Integrating the N-terminus of Sec61p with the downstream TMD1 into a POPC lipid bilayer using CHARMM-GUI and the PPM server

• Performing 300 ns long simulated tempering molecular dynamics simulations at 11 discrete temperatures (300-350K) to enhance conformational sampling

• Extracting conformations with temperatures ≤303K and clustering them using a 2.5Å RMSD distance criterion

• Testing three potential contacting conformations between the cytosolic N-termini of Sbh1 and Sec61p with varying angles between their helical axes (0°, +30°, or -30°)

Through site-directed mutagenesis, researchers have confirmed that the CMP region controls the import of sbh1-dependent translocation substrates into the ER . This suggests that sbh1 enhances the insertion efficiency of specific translocation substrates by guiding their signal peptides into the Sec61 channel via its CMP region .

What is the significance of S. pombe sbh1 lacking proline-flanked phosphorylation sites in its N-terminal domain?

Comparative sequence analysis of sbh1 proteins from different yeast species has revealed an interesting pattern: sbh1 proteins from several pathogenic yeast species contain one or more proline-flanked phosphorylation sites in their N-terminal cytosolic domains, whereas sbh1 from non-pathogenic yeast species (including S. pombe, K. lactis, Y. lipolytica, P. pastoris, and H. polymorpha) do not contain these sites .

This structural difference may have functional implications, particularly regarding the regulation of protein secretion and virulence factor export. In pathogenic fungi like C. neoformans, sbh1 controls the entry of virulence factors into the secretory pathway, thereby regulating fungal pathogenicity . The absence of these phosphorylation sites in S. pombe sbh1 suggests different regulatory mechanisms for protein translocation in non-pathogenic versus pathogenic yeasts.

For researchers, this distinction offers valuable experimental opportunities:

• Comparative studies between S. pombe sbh1 and homologs from pathogenic species could reveal mechanisms of virulence regulation

• Introducing proline-flanked phosphorylation sites into S. pombe sbh1 through site-directed mutagenesis could provide insights into their functional significance

• Phosphoproteomic analyses across different growth conditions could reveal whether alternative phosphorylation mechanisms exist in S. pombe

How can researchers investigate the dual functionality of S. pombe sbh1 in both Sec61 and Ssh1 translocon complexes?

Since S. pombe contains only a single sbh1 homolog that must function with both Sec61 and Ssh1 translocon complexes , investigating this dual functionality presents unique research opportunities. Here are methodological approaches for such investigations:

Protein-Protein Interaction Studies:

• Co-immunoprecipitation (Co-IP) using antibodies against sbh1 to pull down associated proteins, followed by mass spectrometry to identify interactions with Sec61 versus Ssh1 complexes

• Proximity labeling techniques such as BioID or APEX to identify proteins in close proximity to sbh1 in vivo

• Fluorescence resonance energy transfer (FRET) or bimolecular fluorescence complementation (BiFC) to visualize interactions between sbh1 and components of either translocon complex

Functional Separation:

• Utilize strains with temperature-sensitive mutations in either SEC61 or SSH1 to examine sbh1 function when one complex is compromised

• Employ ribosome profiling to identify mRNAs preferentially translated at either Sec61 or Ssh1 complexes and examine how sbh1 depletion affects their translation

• Create partially functional sbh1 mutants that preferentially interact with either Sec61 or Ssh1 to dissect complex-specific functions

Structural Biology Approaches:

• Cryo-electron microscopy of purified Sec61 and Ssh1 complexes from S. pombe to determine if sbh1 adopts different conformations in each complex

• Hydrogen-deuterium exchange mass spectrometry (HDX-MS) to identify regions of sbh1 with differential solvent exposure when bound to different complexes

What experimental approaches can be used to study phosphorylation-dependent conformational changes in sbh1?

Studies have shown that phosphorylation of sbh1/Sec61β at specific residues (S3/T5) results in conformational changes in the intrinsically unstructured N-terminus . To investigate these changes in S. pombe sbh1, researchers can employ:

In Vitro Approaches:

Computational Methods:

• Molecular dynamics simulations comparing the conformational ensemble of phosphorylated and non-phosphorylated sbh1

• Machine learning approaches to predict phosphorylation-induced structural changes

Functional Studies:

• Creation of phosphomimetic (S→D or S→E) and phosphodeficient (S→A) mutants to probe the functional consequences of phosphorylation

• Quantitative proteomics to identify substrates whose translocation efficiency is affected by sbh1 phosphorylation state

Table: Recommended Experimental Approaches for Studying sbh1 Phosphorylation

What are the optimal expression systems for producing recombinant S. pombe sbh1 protein?

When designing expression systems for recombinant S. pombe sbh1, researchers should consider several factors:

Expression Hosts:

• E. coli: Suitable for high-yield expression of the cytosolic domain alone

• Yeast systems (P. pastoris, S. cerevisiae): Preferred for full-length protein with proper membrane insertion

• Insect cells (Sf9, High Five): Useful for obtaining higher eukaryotic post-translational modifications

Purification Strategies:

• For full-length sbh1:

  • Solubilization with mild detergents (DDM, LMNG) to maintain native conformation

  • Affinity purification using N-terminal tags (His, GST, MBP) since the C-terminus is embedded in the membrane

  • Size exclusion chromatography in detergent micelles or reconstitution into nanodiscs or liposomes for functional studies

• For the cytosolic domain alone:

  • Direct expression as a soluble protein with affinity tags

  • Ion exchange chromatography followed by size exclusion

Quality Control:

• Verification of proper folding using circular dichroism spectroscopy

• Mass spectrometry to confirm protein identity and detect post-translational modifications

• Functional assays to verify activity (e.g., binding to Sec61 or signal peptides)

How can researchers investigate the interaction between sbh1 and specific signal peptides?

Based on recent findings that the sbh1 CMP domain contains binding sites for specific signal peptides , several methodological approaches can be employed to study these interactions:

Binding Assays:

• Surface plasmon resonance (SPR) or bio-layer interferometry (BLI) to measure binding kinetics between immobilized sbh1 and various signal peptides

• Microscale thermophoresis (MST) to detect interactions in solution with minimal protein consumption

• Isothermal titration calorimetry (ITC) to obtain comprehensive thermodynamic parameters of binding

Structural Approaches:

• X-ray crystallography of sbh1 CMP domain in complex with signal peptides

• NMR spectroscopy to map binding interfaces and identify residues involved in signal peptide recognition

• Hydrogen-deuterium exchange mass spectrometry (HDX-MS) to identify regions protected upon signal peptide binding

Computational Methods:

• Molecular docking to predict binding modes between sbh1 CMP domain and signal peptides

• Molecular dynamics simulations to study the dynamics of sbh1-signal peptide complexes

• Machine learning approaches to predict signal peptide specificity based on sequence features

Functional Validation:

• Site-directed mutagenesis of predicted binding residues followed by in vitro binding assays

• In vivo translocation assays using reporter proteins with various signal sequences

• Crosslinking studies to capture transient interactions during translocation

What techniques can be employed to study the essentiality of sbh1 in S. pombe?

Unlike in S. cerevisiae where neither SBH1 nor SBH2 alone is essential, the single sbh1 homolog in S. pombe is likely essential for viability . To study this essentiality and its functional implications, researchers can use:

Genetic Approaches:

• Construction of conditional mutants:

  • Temperature-sensitive alleles

  • Auxin-inducible degron (AID) system for controlled protein depletion

  • Tetracycline-regulatable promoter systems to modulate expression levels

• Partial loss-of-function mutations:

  • Domain-specific deletions to separate essential from non-essential functions

  • Point mutations in conserved residues identified through sequence alignment

Complementation Studies:

• Testing whether sbh1 homologs from other species can rescue S. pombe sbh1 mutants

• Creating chimeric proteins with domains from different species to identify functionally critical regions

High-throughput Screens:

• Synthetic genetic array (SGA) analysis to identify genes that become essential in sbh1 hypomorphic backgrounds

• Chemical genetic screens to identify compounds that specifically target cells with compromised sbh1 function

Cellular Physiology:

• Ribosome profiling to examine global translational changes upon sbh1 depletion

• Proteomics analysis to identify proteins whose levels are most affected by sbh1 depletion

• Electron microscopy to examine changes in ER morphology and translocation capacity

How can molecular dynamics simulations advance our understanding of sbh1 structure-function relationships?

Molecular dynamics (MD) simulations have already provided valuable insights into sbh1 interactions with the Sec61 channel . Researchers can expand on these approaches with:

Advanced Simulation Protocols:

• Enhanced sampling techniques:

  • Replica exchange MD to overcome energy barriers and explore conformational space more efficiently

  • Metadynamics to calculate free energy landscapes of sbh1 conformational changes

  • Steered MD to study force-dependent processes during translocation

• Multi-scale modeling:

  • Coarse-grained simulations to study large-scale movements of the translocon complex

  • Quantum mechanics/molecular mechanics (QM/MM) for detailed studies of specific interactions

Specific Applications:

• Simulating conformational changes induced by phosphorylation at specific residues

• Modeling the dynamics of signal peptide recognition and binding by the CMP domain

• Investigating how sbh1 interacts with both Sec61 and Ssh1 complexes

• Exploring the dynamics of sbh1 during different stages of the translocation process

The methodology demonstrated in the literature provides a solid foundation, utilizing:

• MODELLER for initial structure prediction

• CHARMM36m force field with TIP3P water

• Simulated tempering across multiple temperatures (300-350K)

• Clustering analysis with 2.5Å RMSD criteria

What emerging technologies can enhance the study of sbh1-dependent protein translocation in S. pombe?

Several cutting-edge technologies show promise for advancing our understanding of sbh1 function:

CryoET and Subtomogram Averaging:
This approach allows visualization of the translocon complex in its native membrane environment, potentially revealing how sbh1 functions in different substrate-specific contexts.

Time-Resolved Structural Methods:

• Time-resolved cryo-EM to capture different states of the translocation process

• Time-resolved FRET to monitor conformational changes during translocation

• Single-molecule techniques to observe individual translocation events

Genome Engineering:

• CRISPR-Cas9 for precise genomic modifications of sbh1 and interacting partners

• Base editing for introducing specific mutations without double-strand breaks

• Prime editing for precise insertions and deletions

Advanced Imaging:

• Super-resolution microscopy (STORM, PALM) to visualize sbh1 distribution and dynamics at the ER membrane

• Expansion microscopy to physically enlarge samples for improved resolution

• Correlative light and electron microscopy (CLEM) to combine functional and structural information

Proteomic Approaches:

• Proximity-dependent biotin identification (BioID) or APEX2 proximity labeling to map the dynamic interactome of sbh1

• Crosslinking mass spectrometry (XL-MS) to capture transient interactions during translocation

• Targeted protein degradation (TPD) approaches to study acute loss of sbh1 function

How does S. pombe sbh1 function compare with homologs from other yeast species?

The unique evolutionary position of S. pombe as a pre-WGD organism with a single sbh1 homolog makes comparative studies particularly valuable:

Sequence Conservation Analysis:
S. pombe Sec61p shares 58.6% sequence identity with S. cerevisiae Sec61p . This moderate conservation suggests both shared and species-specific functions. Notably, S. pombe sbh1 lacks the proline-flanked phosphorylation sites found in pathogenic yeast species , suggesting different regulatory mechanisms.

Functional Complementation:
While Y. lipolytica SEC61 can complement a null mutation in S. cerevisiae, the S. pombe homolog failed to complement the respective mutant in S. cerevisiae . This failure was attributed to non-conserved amino acid substitutions within the cytoplasmic loop between transmembrane helices 4 and 5 , highlighting the importance of cytosolic interactions for Sec61 function.

Phylogenetic Classification:
Interestingly, sequence alignment shows that S. pombe Ssh1 does not align with the Ssh1 protein cluster from other species but rather with the Sec61 cluster . This raises important questions about functional complementation and evolutionary divergence.

Experimental Approaches for Comparative Studies:

• Heterologous expression of sbh1 from different species in S. pombe sbh1 conditional mutants

• Domain swapping experiments to identify regions responsible for species-specific functions

• Comparative interactome analysis to identify conserved and divergent binding partners

What can we learn from comparing the single sbh1 in S. pombe to the duplicated SBH1/SBH2 system in S. cerevisiae?

The presence of a single sbh1 in S. pombe versus the duplicated SBH1/SBH2 in post-WGD species like S. cerevisiae provides an excellent model for studying protein evolution after gene duplication:

Functional Specialization vs. Redundancy:
In S. cerevisiae, Sbh1 primarily associates with the Sec61 complex while Sbh2 associates with the Ssh1 complex . In contrast, S. pombe must use its single sbh1 for both complexes . This suggests potential functional constraints on the S. pombe protein that may have been relieved by gene duplication in S. cerevisiae.

Methodological Approaches:

• Replace S. cerevisiae SBH1 and SBH2 with S. pombe sbh1 to test functional complementation

• Create chimeric proteins combining domains from S. pombe sbh1 with S. cerevisiae Sbh1/Sbh2

• Compare the interactomes of S. pombe sbh1 with those of S. cerevisiae Sbh1 and Sbh2

• Analyze the effects of mutations in conserved residues across different species

Evolutionary Insights: Studying how S. pombe sbh1 functions with both translocon complexes can provide insights into the ancestral state before gene duplication and subsequent specialization in post-WGD species.