Recombinant Saccharomyces cerevisiae Translocation protein SEC62 (SEC62)

What is the molecular structure of Sec62p in Saccharomyces cerevisiae?

Sec62p is a 32-kDa integral membrane protein with two membrane-spanning segments. Structural analyses reveal that Sec62p spans the ER membrane twice, with both the N-terminal and C-terminal hydrophilic domains facing the cytosol . This topology positions Sec62p to interact with both cytosolic components of the translocation machinery and the membrane-embedded Sec61 channel. The protein contains specific binding domains that facilitate its interaction with other Sec-complex components, particularly Sec63p . The dual membrane-spanning topology allows Sec62p to play a critical role in controlling access to the translocation channel from both sides of the ER membrane.

What are the essential functional domains of Sec62p?

The C-terminal domain of Sec62p performs essential functions that cannot be disrupted without severely compromising cell viability . Experiments with Sec62p-invertase hybrid proteins demonstrated that constructs lacking the C-terminal domain not only failed to complement the sec62-1 mutation but actually inhibited growth of sec62-1 cells at normally permissive temperatures . In contrast, the N-terminal domain primarily serves as an interaction platform with other translocation components, particularly Sec63p . Recent cryo-EM structural studies further revealed that Sec62p plays a critical role in the stepwise gating of the Sec61 channel, as absence of Sec62p results in the Sec61 translocation pore remaining closed by the plug domain, rendering the channel inactive .

What experimental approaches are most effective for studying Sec62p membrane topology?

Researchers have successfully employed multiple complementary techniques to elucidate Sec62p membrane topology:

• Protease digestion of intact microsomes: This approach identifies which domains are accessible to proteases, indicating their cytosolic orientation .

• Analysis of oligosaccharide content in hybrid proteins: By creating Sec62p-invertase hybrid proteins and analyzing their glycosylation patterns, researchers determined which domains reside in the ER lumen where glycosylation occurs .

• Subcellular fractionation and membrane extraction: These techniques confirmed Sec62p's intimate association with the ER membrane, distinguishing it from peripheral membrane proteins .

• Indirect immunofluorescence: This visualized Sec62p's localization within the cell, confirming its ER membrane association .

Combining these approaches provided conclusive evidence for Sec62p's dual-spanning membrane topology with both termini facing the cytosol.

How can researchers assess the functional impact of Sec62p mutations?

Several methodologies have proven effective for evaluating Sec62p function:

• In vivo competition assays: These assays allow researchers to characterize and dissect physical and functional interactions between Sec62p and components of the Sec-complex .

• Complementation studies: Testing mutant versions of Sec62p for their ability to rescue sec62 temperature-sensitive mutants provides insights into domain functionality .

• Dominant negative approaches: Some Sec62p-invertase hybrid proteins dramatically inhibit growth of sec62-1 cells at normally permissive temperatures, providing a powerful tool to study Sec62p function .

• Translocation efficiency measurements: Assessing the impact of Sec62p mutations on translocation of specific substrate proteins, particularly those with moderately hydrophobic signal sequences .

• Cryo-EM structural analysis: Recent advances in cryo-EM have enabled visualization of the Sec61–Sec62–Sec63 complex in different conformational states, revealing mechanistic insights into how Sec62p contributes to channel gating .

What methods are available for studying the interaction between signal sequences and Sec62p?

Researchers can employ several approaches to investigate signal sequence-Sec62p interactions:

• Systematic variation of signal sequence hydrophobicity: By creating a set of model proteins with signal anchor sequences of controlled hydrophobicity, researchers can evaluate the hydrophobicity-dependent targeting efficiency and pathway preference .

• Comparison of translocation in wild-type versus Sec62-defective cells: This approach revealed that moderately hydrophobic signal anchor proteins specifically require Sec62p for proper membrane insertion and orientation .

• Assessment of membrane protein topology: Investigating how Sec62p defects specifically affect N in-C out membrane orientation provides mechanistic insights into its role in membrane protein topogenesis .

• Combined mutation analysis: Testing how mutations in both signal sequences and Sec62p affect translocation can identify specific interaction points and requirements.

How does Sec62p contribute to the gating of the Sec61 channel?

Recent cryo-EM structures of Sec61–Sec62–Sec63 complexes have revealed a stepwise gating mechanism involving both Sec62p and Sec63p :

• Without Sec62p, the translocation pore of Sec61 remains closed by the plug domain, rendering the channel completely inactive .

• The lateral gate of Sec61 must first be partially opened through interactions between Sec61 and Sec63 in both cytosolic and luminal domains .

• Simultaneous disruption of these interactions completely closes the channel, demonstrating their essential nature .

• Structural and molecular dynamics simulations suggest that Sec62p may also prevent lipids from invading the channel through the open lateral gate .

• Sec63 and Sec62 work together in a hierarchical manner to activate Sec61 for post-translational protein translocation .

This stepwise activation mechanism explains why both Sec62p and Sec63p are essential for post-translational protein translocation, as they perform complementary functions in channel activation.

What determines whether a protein utilizes Sec62p-dependent post-translational translocation?

The choice between SRP-dependent co-translational and Sec62p-dependent post-translational translocation pathways depends on several factors:

Interestingly, these pathways are not mutually exclusive for signal anchor proteins, and moderately hydrophobic ones require both SRP and Sec62p for proper targeting and translocation to the ER . Defects in Sec62p selectively reduce signal sequences inserted in an N in-C out membrane orientation, suggesting a specific role in regulating membrane topogenesis of moderately hydrophobic signal anchor proteins .

How do the N-terminal and C-terminal domains of Sec62p differentially contribute to function?

The N-terminal and C-terminal cytosolic domains of Sec62p serve distinct but complementary functions:

• N-terminal domain:

  • Serves as the major interaction site with other Sec-complex components

  • Directly binds to the C-terminal 14 residues of Sec63p

  • Establishes the primary connection to the translocation machinery

• C-terminal domain:

  • Performs essential functions that cannot be disrupted

  • May be involved in signal sequence recognition

  • When mutated or deleted, causes dominant negative effects that inhibit cell growth

  • Its deletion cannot be complemented by overexpression of other components

The functional differentiation between these domains explains why full-length Sec62p is required for proper translocation, and why partial constructs often act as dominant negatives by disrupting the balanced interaction with other Sec-complex components.

How do we reconcile different observations about Sec62p function in various experimental systems?

Several factors explain apparent contradictions in Sec62p function across different studies:

• Signal sequence hydrophobicity effects: Studies demonstrate that Sec62p dependence varies with signal sequence hydrophobicity - highly hydrophobic sequences show less dependence on Sec62p than moderately hydrophobic ones .

• Substrate-specific requirements: Sec62p shows client-specific roles in ER protein import, such as the post-translational ER import of presecretory proteins with fewer than 100 amino acid residues .

• Pathway overlap: The discovery that SRP-dependent co-translational and SRP-independent post-translational translocation pathways are not mutually exclusive explains why some substrates show partial dependence on both pathways .

• Experimental conditions: Different in vitro and in vivo systems may emphasize different aspects of Sec62p function, particularly when comparing reconstituted systems versus cellular environments.

• Species-specific differences: While the core functions are conserved, mammalian Sec62 has evolved additional roles beyond those seen in yeast, including interactions with the ribosome and roles in ER stress recovery .

What explains the variability in Sec62p requirement based on signal sequence properties?

Research has revealed a nuanced understanding of how signal sequence properties determine Sec62p dependence:

• Proteins with highly hydrophobic signal sequences predominantly use the SRP-dependent pathway and show minimal Sec62p dependence .

• Proteins with moderately hydrophobic signal sequences require both SRP and Sec62p for optimal translocation efficiency .

• Small proteins (<100 amino acids) with less hydrophobic signal sequences rely heavily on the Sec62p-dependent post-translational pathway .

• Sec62p plays a specific role in facilitating N in-C out membrane topology, suggesting its involvement in orienting signal anchor sequences during membrane insertion .

This variability reflects the evolutionary adaptation of multiple translocation pathways to handle the diverse secretory proteome efficiently, with Sec62p serving as a critical factor for specific substrate classes.

What structural insights inform our understanding of Sec62p function in the Sec complex?

Recent cryo-EM studies of Sec61–Sec62–Sec63 complexes from Saccharomyces cerevisiae and Thermomyces lanuginosus have provided detailed structural insights into Sec62p function :

• Sec62p and Sec63p induce opening of the Sec61 channel in a coordinated manner .

• The dataset revealed distinct subpopulations of complexes with or without Sec62p (referred to as Sec62+ and Sec62−), allowing direct comparison of their conformational states .

• Without Sec62p, the Sec61 translocation pore remains closed by the plug domain .

• Sec62p appears to stabilize the partially open lateral gate created by Sec61-Sec63 interactions .

• Molecular dynamics simulations suggest Sec62p may prevent lipids from entering the channel through the open lateral gate .

These structural findings explain the essential nature of Sec62p in post-translational translocation and provide a mechanistic framework for understanding its functional contributions.

How does yeast Sec62p differ from its mammalian counterpart?

While the core structure and function are conserved, several notable differences exist between yeast and mammalian Sec62:

Mammalian Sec62 has evolved additional functions beyond protein translocation, including participation in cellular stress responses and calcium homeostasis, reflecting the increased complexity of ER functions in higher eukaryotes .

What insights can be gained from studying the Sec complex in different fungal species?

Comparative studies of the Sec complex across fungal species provide evolutionary insights:

• Cryo-EM structures of Sec61–Sec62–Sec63 complexes from both Saccharomyces cerevisiae and Thermomyces lanuginosus show conserved core structures and mechanisms .

• The stepwise gating mechanism involving Sec62p and Sec63p appears to be evolutionarily conserved across fungal species .

• Comparative analysis can identify both highly conserved regions critical for core functions and more variable regions that may reflect species-specific adaptations.

• Studying thermophilic fungi like Thermomyces lanuginosus provides structural insights into the Sec complex under different physiological conditions, potentially revealing additional conformational states.

• Cross-species complementation experiments can identify functionally interchangeable domains versus species-specific elements.

How have SEC62 functions diversified in higher eukaryotes?

The evolutionary trajectory of SEC62 from yeast to mammals shows functional diversification:

• Mammalian Sec62 maintains the core translocation function but has acquired additional roles in ER homeostasis .

• In mammals, Sec62 shows client-specific functions in ER protein import, including both post-translational import of small presecretory proteins and co-translational import of certain large precursor polypeptides .

• Mammalian Sec62 can interact with the ribosome and recruit BiP to the Sec61 channel, facilitating channel opening for precursors with weak signal peptides .

• A unique function of mammalian Sec62 in recovery from ER stress has been identified, which is activated by demand and possibly regulated by phosphorylation or calcium binding .

• The amplification of SEC62 in multiple human cancer types suggests it may have acquired functions related to cell proliferation and survival that are not present in yeast .

This functional diversification reflects the increased complexity of protein quality control and stress response systems in higher eukaryotes, where Sec62 has evolved from a dedicated translocation component to a multifunctional protein involved in various aspects of ER homeostasis.