Identification of novel protein-protein interactions at the cytosolic surface of the Sec63 complex in the yeast ER membrane

Emerging View on the Molecular Functions of Sec62 and Sec63 in Protein Translocation

Most secreted and membrane proteins are targeted to and translocated across the endoplasmic reticulum (ER) membrane through the Sec61 protein-conducting channel. Evolutionarily conserved Sec62 and Sec63 associate with the Sec61 channel, forming the Sec complex and mediating translocation of a subset of proteins. For the last three decades, it has been thought that ER protein targeting and translocation occur via two distinct pathways: signal recognition particle (SRP)-dependent co-translational or SRP-independent, Sec62/Sec63 dependent post-translational translocation pathway. However, recent studies have suggested that ER protein targeting and translocation through the Sec translocon are more intricate than previously thought. This review summarizes the current understanding of the molecular functions of Sec62/Sec63 in ER protein translocation.

Rapid inactivation of the yeast Sec complex selectively blocks transport of post-translationally translocated proteins

The yeast endoplasmic reticulum has three distinct protein translocation channels. The heterotrimeric Sec61 and Ssh1 complexes, which bind translating ribosomes, mediate cotranslational translocation of proteins targeted to the endoplasmic reticulum by the signal recognition particle (SRP) and SRP receptor targeting pathway, whereas the heptameric Sec complex has been proposed to mediate ribosome-independent post-translational translocation of proteins with less hydrophobic signal sequences that escape recognition by the SRP. However, multiple reports have proposed that the Sec complex may function cotranslationally and be involved in translocation or integration of SRP-dependent protein translocation substrates. To provide insight into these conflicting views, we induced expression of the tobacco etch virus protease to achieve rapid inactivation of the Sec complex by protease-mediated cleavage within the cytoplasmic domain of the Sec63 protein. Protein translocation assays conducted after tobacco etch virus protease induction revealed a complete block in translocation of two well-characterized substrates of the Sec complex, carboxypeptidase Y (CPY) and Gas1p, when the protease cleavage sites were located at structural domain boundaries in Sec63. However, integration of SRP-dependent membrane protein substrates was not detectably impacted. Moreover, redirecting CPY to the cotranslational pathway by increasing the hydrophobicity of the signal sequence rendered translocation of CPY insensitive to inactivation of the Sec complex. We conclude that the Sec complex is primarily responsible for the translocation of yeast secretome proteins with marginally hydrophobic signal sequences.

Stepwise gating of the Sec61 protein-conducting channel by Sec63 and Sec62

Many proteins are transported into the endoplasmic reticulum by the universally conserved Sec61 channel. Post-translational transport requires two additional proteins, Sec62 and Sec63, but their functions are poorly defined. Here, we determined cryo-EM structures of several variants of Sec61–Sec62–Sec63 complexes from Saccharomyces cerevisiae and Thermomyces lanuginosus and show that Sec62 and Sec63 induce opening of the Sec61 channel. Without Sec62, the translocation pore of Sec61 remains closed by the plug domain, rendering the channel inactive. We further show that the lateral gate of Sec61 must first be partially opened by interactions between Sec61 and Sec63 in cytosolic and lumenal domains, a simultaneous disruption of which completely closes the channel. The structures and molecular dynamics simulations suggest that Sec62 may also prevent lipids from invading the channel through the open lateral gate. Our study shows how Sec63 and Sec62 work together in a hierarchical manner to activate Sec61 for post-translational protein translocation.

The ER luminal C-terminus of AtSec62 is critical for male fertility and plant growth in Arabidopsis thaliana

Protein translocation into the endoplasmic reticulum (ER) occurs either co- or post-translationally through the Sec translocation system. The Arabidopsis Sec post-translocon is composed of the protein-conducting Sec61 complex, the chaperone-docking protein AtTPR7, the J-domain-containing proteins AtERdj2A/B and the yet uncharacterized AtSec62. Yeast Sec62p is suggested to mainly function in post-translational translocation, whereas mammalian Sec62 also interacts with ribosomes. In Arabidopsis, loss of AtSec62 leads to impaired growth and drastically reduced male fertility indicating the importance of AtSec62 in protein translocation and subsequent secretion in male gametophyte development. Moreover, AtSec62 seems to be divergent in function as compared with yeast Sec62p, since we were not able to complement the thermosensitive yeast mutant sec62-ts. Interestingly, AtSec62 has an additional third transmembrane domain in contrast to its yeast and mammalian counterparts resulting in an altered topology with the C-terminus facing the ER lumen instead of the cytosol. In addition, the AtSec62 C-terminus has proven to be indispensable for AtSec62 function, since a construct lacking the C-terminal region was not able to rescue the mutant phenotype in Arabidopsis. We thus propose that Sec62 acquired a unique topology and function in protein translocation into the ER in plants.

Proteomic response to linoleic acid hydroperoxide in Saccharomyces cerevisiae

Yeast AP-1 transcription factor (Yap1p) and the enigmatic oxidoreductases Oye2p and Oye3p are involved in counteracting lipid oxidants and their unsaturated breakdown products. In order to uncover the response to linoleic acid hydroperoxide (LoaOOH) and the roles of Oye2p, Oye3p and Yap1p, we carried out proteomic analysis of the homozygous deletion mutants oye3Δ, oye2Δ and yap1Δ alongside the diploid parent strain BY4743. The findings demonstrate that deletion of YAP1 narrowed the response to LoaOOH, as the number of proteins differentially expressed in yap1Δ was 70% of that observed in BY4743. The role of Yap1p in regulating the major yeast peroxiredoxin Tsa1p was demonstrated by the decreased expression of Tsa1p in yap1Δ. The levels of Ahp1p and Hsp31p, previously shown to be regulated by Yap1p, were increased in LoaOOH-treated yap1Δ, indicating their expression is also regulated by another transcription factor(s). Relative to BY4743, protein expression differed in oye3Δ and oye2Δ under LoaOOH, underscored by superoxide dismutase (Sod1p), multiple heat shock proteins (Hsp60p, Ssa1p, and Sse1p), the flavodoxin-like protein Pst2p and the actin stabiliser tropomyosin (Tpm1p). Proteins associated with glycolysis were increased in all strains following treatment with LoaOOH. Together, the dataset reveals, for the first time, the yeast proteomic response to LoaOOH, highlighting the significance of carbohydrate metabolism, as well as distinction between the roles of Oye3p, Oye2p and Yap1p.

Riboflavin deficiency induces a significant change in proteomic profiles in HepG2 cells

Riboflavin deficiency is widespread in many regions over the world, especially in underdeveloped countries. In this study, we investigated the effects of riboflavin deficiency on protein expression profiles in HepG2 cells in order to provide molecular information for the abnormalities induced by riboflavin deficiency. HepG2 cells were cultured in media containing different concentrations of riboflavin. Changes of cell viability and apoptosis were assessed. A comparative proteomic analysis was performed using a label-free shotgun method with LC-MS/MS to investigate the global changes of proteomic profiles in response to riboflavin deficiency. Immunoblotting test was used to validate the results of proteomic approach. The cell viability and apoptosis tests showed that riboflavin was vital in maintaining the cytoactivity of HepG2 cells. The label-free proteomic analysis revealed that a total of 37 proteins showing differential expression (±2 fold, p < 0.05) were identified after riboflavin deficiency. Bioinformatics analysis indicated that the riboflavin deficiency caused an up-regulation of Parkinson's disease pathway, steroid catabolism, endoplasmic reticulum stress and apoptotic process, while the fatty acid metabolism, tricarboxylic citrate cycle, oxidative phosphorylation and iron metabolism were down-regulated. These findings provide a molecular basis for the elucidation of the effects caused by riboflavin deficiency.

The Sec62-Sec63 translocon facilitates translocation of the C-terminus of membrane proteins

The Sec62-Sec63 complex mediates post-translational translocation of a subset of primarily secretory proteins into the endoplasmic reticulum (ER) in yeast. Therefore, it has been thought that membrane proteins, which are mainly co-translationally targeted into the ER, are not handled by the Sec62-Sec63 translocon. By systematic analysis of single and multi-spanning membrane proteins with broad sequence context [with differing hydrophobicity, flanking charged residues and orientation of transmembrane (TM) segments], we show that mutations in the N-terminal cytosolic domain of yeast Sec62 impair its interaction with Sec63 and lead to defects in membrane insertion and translocation of the C-terminus of membrane proteins. These results suggest that there is an unappreciated function of the Sec62-Sec63 translocon in regulating topogenesis of membrane proteins in the eukaryotic cell.

The Sec63p J-domain is required for ERAD of soluble proteins in yeast

How misfolded proteins are exported from the ER to the cytosol for degradation (ER-associated Degradation, ERAD) and which proteins are participating in this process is not understood. Several studies using a single, leaky mutant indicated that Sec63p might be involved in ERAD. More recently, Sec63p was also found strongly associated with proteasomes attached to the protein-conducting channel in the ER membrane which presumably form part of the export machinery. These observations prompted us to reinvestigate the role of Sec63p in ERAD by generating new mutants which were selected in a screen monitoring the intracellular accumulation of the ERAD substrate CPY*. We show that a mutation in the DnaJ-domain of Sec63p causes a defect in ERAD, whereas mutations in the Brl, acidic, and transmembrane domains only affect protein import into the ER. Unexpectedly, mutations in the acidic domain which mediates interaction of Sec63p with Sec62p also caused defects in cotranslational import. In contrast to mammalian cells where SEC63 expression levels affect steady-state levels of multi-spanning transmembrane proteins, the sec63 J-domain mutant was only defective in ERAD of soluble substrates.

The secretory pathway: exploring yeast diversity

Protein secretion is an essential process for living organisms. In eukaryotes, this encompasses numerous steps mediated by several hundred cellular proteins. The core functions of translocation through the endoplasmic reticulum membrane, primary glycosylation, folding and quality control, and vesicle-mediated secretion are similar from yeasts to higher eukaryotes. However, recent research has revealed significant functional differences between yeasts and mammalian cells, and even among diverse yeast species. This review provides a current overview of the canonical protein secretion pathway in the model yeast Saccharomyces cerevisiae, highlighting differences to mammalian cells as well as currently unresolved questions, and provides a genomic comparison of the S. cerevisiae pathway to seven other yeast species where secretion has been investigated due to their attraction as protein production platforms, or for their relevance as pathogens. The analysis of Candida albicans, Candida glabrata, Kluyveromyces lactis, Pichia pastoris, Hansenula polymorpha, Yarrowia lipolytica, and Schizosaccharomyces pombe reveals that many - but not all - secretion steps are more redundant in S. cerevisiae due to duplicated genes, while some processes are even absent in this model yeast. Recent research obviates that even where homologous genes are present, small differences in protein sequence and/or differences in the regulation of gene expression may lead to quite different protein secretion phenotypes.

All roads lead to Rome (but some may be harder to travel): SRP-independent translocation into the endoplasmic reticulum

Translocation into the endoplasmic reticulum (ER) is the first biogenesis step for hundreds of eukaryotic secretome proteins. Over the past 30 years, groundbreaking biochemical, structural and genetic studies have delineated one conserved pathway that enables ER translocation- the signal recognition particle (SRP) pathway. However, it is clear that this is not the only pathway which can mediate ER targeting and insertion. In fact, over the past decade, several SRP-independent pathways have been uncovered, which recognize proteins that cannot engage the SRP and ensure their subsequent translocation into the ER. These SRP-independent pathways face the same challenges that the SRP pathway overcomes: chaperoning the preinserted protein while in the cytosol, targeting it rapidly to the ER surface and generating vectorial movement that inserts the protein into the ER. This review strives to summarize the various mechanisms and machineries which mediate these stages of SRP-independent translocation, as well as examine why SRP-independent translocation is utilized by the cell. This emerging understanding of the various pathways utilized by secretory proteins to insert into the ER draws light to the complexity of the translocational task, and underlines that insertion into the ER might be more varied and tailored than previously appreciated.

Analysis of Sec61p and Ssh1p interactions in the ER membrane using the split-ubiquitin system

BACKGROUND

The split-ubiquitin system monitors interactions of transmembrane proteins in yeast. It is based on the formation of a quasi-native ubiquitin structure upon interaction of two proteins to which the N- and C-terminal halves of ubiquitin have been fused. In the system we use here ubiquitin formation leads to proteolytic cleavage liberating a transcription factor (PLV) from the C-ubiquitin (C) fusion protein which can then activate reporter genes. Generation of fusion proteins is, however, rife with problems, and particularly in transmembrane proteins often disturbs topology, structure and function.

RESULTS

We show that both the Sec61 protein which forms the principal protein translocation channel in the endoplasmic reticulum (ER) membrane, and its non-essential homologue, Ssh1p, when fused C-terminally to CPLV are inactive. In a heterozygous diploid Sec61-CPLV is present in protein translocation channels in the ER membrane without disturbing their function and displays a limited set of protein-protein interactions similar to those found for the wildtype protein using biochemical methods. Although its expression level is similar, Ssh1-CPLV interactions are less strong, and, in contrast to Sec61p, Ssh1p does not distinguish between Sbh1p and Sbh2p. We show that interactions can be monitored by reporter gene activity or directly by PLV cleavage, which is more sensitive, but leads to quantitatively different results.

CONCLUSIONS

We conclude that the split-ubiquitin system we used here has high fidelity, but low sensitivity and is of limited use for detection of new, transient interactions with protein translocation channels in the ER membrane.

Mutation of sec63 in zebrafish causes defects in myelinated axons and liver pathology

Mutations in SEC63 cause polycystic liver disease in humans. Sec63 is a member of the endoplasmic reticulum (ER) translocon machinery, although it is unclear how mutations in SEC63 lead to liver cyst formation in humans. Here, we report the identification and characterization of a zebrafish sec63 mutant, which was discovered in a screen for mutations that affect the development of myelinated axons. Accordingly, we show that disruption of sec63 in zebrafish leads to abnormalities in myelinating glia in both the central and peripheral nervous systems. In the vertebrate nervous system, segments of myelin are separated by the nodes of Ranvier, which are unmyelinated regions of axonal membrane containing a high density of voltage-gated sodium channels. We show that sec63 mutants have morphologically abnormal and reduced numbers of clusters of voltage-gated sodium channels in the spinal cord and along peripheral nerves. Additionally, we observed reduced myelination in both the central and peripheral nervous systems, as well as swollen ER in myelinating glia. Markers of ER stress are upregulated in sec63 mutants. Finally, we show that sec63 mutants develop liver pathology. As in glia, the primary defect, detectable at 5 dpf, is fragmentation and swelling of the ER, indicative of accumulation of proteins in the lumen. At 8 dpf, ER swelling is severe; other pathological features include disrupted bile canaliculi, altered cytoplasmic matrix and accumulation of large lysosomes. Together, our analyses of sec63 mutant zebrafish highlight the possible role of ER stress in polycystic liver disease and suggest that these mutants will serve as a model for understanding the pathophysiology of this disease and other abnormalities involving ER stress.

Hph1 and Hph2 are novel components of the Sec63/Sec62 posttranslational translocation complex that aid in vacuolar proton ATPase biogenesis

Hph1 and Hph2 are homologous integral endoplasmic reticulum (ER) membrane proteins required for Saccharomyces cerevisiae survival under environmental stress conditions. To investigate the molecular functions of Hph1 and Hph2, we carried out a split-ubiquitin-membrane-based yeast two-hybrid screen and identified their interactions with Sec71, a subunit of the Sec63/Sec62 complex, which mediates posttranslational translocation of proteins into the ER. Hph1 and Hph2 likely function in posttranslational translocation, as they interact with other Sec63/Sec62 complex subunits, i.e., Sec72, Sec62, and Sec63. hph1Δ hph2Δ cells display reduced vacuole acidification; increased instability of Vph1, a subunit of vacuolar proton ATPase (V-ATPase); and growth defects similar to those of mutants lacking V-ATPase activity. sec71Δ cells exhibit similar phenotypes, indicating that Hph1/Hph2 and the Sec63/Sec62 complex function during V-ATPase biogenesis. Hph1/Hph2 and the Sec63/Sec62 complex may act together in this process, as vacuolar acidification and Vph1 stability are compromised to the same extent in hph1Δ hph2Δ and hph1Δ hph2Δ sec71Δ cells. In contrast, loss of Pkr1, an ER protein that promotes posttranslocation assembly of Vph1 with V-ATPase subunits, further exacerbates hph1Δ hph2Δ phenotypes, suggesting that Hph1 and Hph2 function independently of Pkr1-mediated V-ATPase assembly. We propose that Hph1 and Hph2 aid Sec63/Sec62-mediated translocation of specific proteins, including factors that promote efficient biogenesis of V-ATPase, to support yeast cell survival during environmental stress.

Congenital disorders of glycosylation in hepatology: the example of polycystic liver disease

Autosomal dominant polycystic liver disease (PCLD) is a rare progressive disorder characterized by an increased liver volume due to many (>20) fluid-filled cysts of biliary origin. Disease causing mutations in PRKCSH or SEC63 are found in approximately 25% of the PCLD patients. Both gene products function in the endoplasmic reticulum, however, the molecular mechanism behind cyst formation remains to be elucidated. As part of the translocon complex, SEC63 plays a role in protein import into the ER and is implicated in the export of unfolded proteins to the cytoplasm during ER-associated degradation (ERAD). PRKCSH codes for the beta-subunit of glucosidase II (hepatocystin), which cleaves two glucose residues of Glc(3)Man(9)GlcNAc(2) N-glycans on proteins. Hepatocystin is thereby directly involved in the protein folding process by regulating protein binding to calnexin/calreticulin in the ER. A separate group of genetic diseases affecting protein N-glycosylation in the ER is formed by the congenital disorders of glycosylation (CDG). In distinct subtypes of this autosomal recessive multisystem disease specific liver symptoms have been reported that overlap with PCLD. Recent research revealed novel insights in PCLD disease pathology such as the absence of hepatocystin from cyst epithelia indicating a two-hit model for PCLD cystogenesis. This opens the way to speculate about a recessive mechanism for PCLD pathophysiology and shared molecular pathways between CDG and PCLD. In this review we will discuss the clinical-genetic features of PCLD and CDG as well as their biochemical pathways with the aim to identify novel directions of research into cystogenesis.

Evolutionary gain of function for the ER membrane protein Sec62 from yeast to humans

We characterized interactions between the human proteins Sec62 and Sec63 as well as the putative interaction of human Sec62 with ribosomes. The data demonstrate evolutionary conservation of Sec62/Sec63 interaction and indicate that in the course of evolution Sec62 of vertebrates has gained the additional function to interact with ribosomes.

Because of similarity to their yeast orthologues, the two membrane proteins of the human endoplasmic reticulum (ER) Sec62 and Sec63 are expected to play a role in protein biogenesis in the ER. We characterized interactions between these two proteins as well as the putative interaction of Sec62 with ribosomes. These data provide further evidence for evolutionary conservation of Sec62/Sec63 interaction. In addition, they indicate that in the course of evolution Sec62 of vertebrates has gained an additional function, the ability to interact with the ribosomal tunnel exit and, therefore, to support cotranslational mechanisms such as protein transport into the ER. This view is supported by the observation that Sec62 is associated with ribosomes in human cells. Thus, the human Sec62/Sec63 complex and the human ER membrane protein ERj1 are similar in providing binding sites for BiP in the ER-lumen and binding sites for ribosomes in the cytosol. We propose that these two systems provide similar chaperone functions with respect to different precursor proteins.

Sec-dependent protein translocation across biological membranes: evolutionary conservation of an essential protein transport pathway (Review)

All living organisms, no matter how simple or complex, possess the ability to translocate proteins across biological membranes and into different cellular compartments. Although a range of membrane transport processes exist, the major pathway used to translocate proteins across the bacterial cytoplasmic membrane or the eukaryotic endoplasmic reticulum membrane is conserved and is known as the Sec or Sec61 pathway, respectively. Over the past two decades the Sec and Sec61 pathways have been studied extensively and are well characterised at the genetic and biochemical levels. However, it is only now with the recent structural determination of a number of the key elements of the pathways that the translocation complex is beginning to give up its secrets in exquisite molecular detail. This article will focus on the routes of Sec- and Sec61-dependent membrane targeting and the nature of the translocation channel in bacteria and eukaryotes.

The uterine expression of SEC63 gene is up-regulated at implantation sites in association with the decidualization during the early pregnancy in mice

BACKGROUND

Sec63 is a key component of the protein translocation machinery in the mammalian endoplasmic reticulum (ER), and involved in the post-translation processing of secretory proteins. The aim of this study was to determine the expression pattern of SEC63 gene in mouse uterus during the early pregnancy.

METHODS

Real-time quantitative PCR and Western blot analyses were used to evaluate the alteration in levels of uterine SEC63 gene expression during the peri-implantation period in mice. Further, both in situ hybridization and immunohistochemical analyses were performed to examine the spatial localization of SEC63 gene expression in mouse uterine tissues. The presence of Sec63 protein in human uterine tissue was also detected by immunohistochemical analysis. Statistical analysis was carried out using Tukey test.

RESULTS

Uterine SEC63 gene expression was up-regulated and predominantly localized in mouse decidual cells during days 5-8 of pregnancy. More interestingly, Sec63 protein was also detected in human decidua of 10-week pregnancy, whereas was not observed in human endometrial tissues both at proliferative and secretory phases of menstrual cycle.

CONCLUSION

The pattern of SEC63 gene expression is consistent with a possible role for SEC63 in decidualization.

Sec61p is required for ERAD-L: genetic dissection of the translocation and ERAD-L functions of Sec61P using novel derivatives of CPY

Misfolded proteins in the endoplasmic reticulum (ER) are exported to the cytosol for degradation by the proteasome in a process known as ER-associated degradation (ERAD). CPY* is a well characterized ERAD substrate whose degradation is dependent upon the Hrd1 complex. However, although the functions of some of the components of this complex are known, the nature of the protein dislocation channel remains obscure. Sec61p has been suggested as an obvious candidate because of its role as a protein-conducting channel through which polypeptides are initially translocated into the ER. However, it has not yet been possible to functionally dissect any role for Sec61p in dislocation from its essential function in translocation. By changing the translocation properties of a series of novel ERAD substrates, we are able to separate these two events and find that functional Sec61p is essential for the ERAD-L pathway.

Role of protein translocation pathways across the endoplasmic reticulum in Trypanosoma brucei

The translocation of secretory and membrane proteins across the endoplasmic reticulum (ER) membrane is mediated by co-translational (via the signal recognition particle (SRP)) and post-translational mechanisms. In this study, we investigated the relative contributions of these two pathways in trypanosomes. A homologue of SEC71, which functions in the post-translocation chaperone pathway in yeast, was identified and silenced by RNA interference. This factor is essential for parasite viability. In SEC71-silenced cells, signal peptide (SP)-containing proteins traversed the ER, but several were mislocalized, whereas polytopic membrane protein biogenesis was unaffected. Surprisingly trypanosomes can interchangeably utilize two of the pathways to translocate SP-containing proteins except for glycosylphosphatidylinositol-anchored proteins, whose level was reduced in SEC71-silenced cells but not in cells depleted for SRP68, an SRP-binding protein. Entry of SP-containing proteins to the ER was significantly blocked only in cells co-silenced for the two translocation pathways (SEC71 and SRP68). SEC63, a factor essential for both translocation pathways in yeast, was identified and silenced by RNA interference. SEC63 silencing affected entry to the ER of both SP-containing proteins and polytopic membrane proteins, suggesting that, as in yeast, this factor is essential for both translocation pathways in vivo. This study suggests that, unlike bacteria or other eukaryotes, trypanosomes are generally promiscuous in their choice of mechanism for translocating SP-containing proteins to the ER, although the SRP-independent pathway is favored for glycosylphosphatidylinositol-anchored proteins, which are the most abundant surface proteins in these parasites.

Saccharomyces cerevisiae S288C genome annotation: a working hypothesis

The S. cerevisiae genome is the most well-characterized eukaryotic genome and one of the simplest in terms of identifying open reading frames (ORFs), yet its primary annotation has been updated continually in the decade since its initial release in 1996 (Goffeau et al., 1996). The Saccharomyces Genome Database (SGD; www.yeastgenome.org) (Hirschman et al., 2006), the community-designated repository for this reference genome, strives to ensure that the S. cerevisiae annotation is as accurate and useful as possible. At SGD, the S. cerevisiae genome sequence and annotation are treated as a working hypothesis, which must be repeatedly tested and refined. In this paper, in celebration of the tenth anniversary of the completion of the S. cerevisiae genome sequence, we discuss the ways in which the S. cerevisiae sequence and annotation have changed, consider the multiple sources of experimental and comparative data on which these changes are based, and describe our methods for evaluating, incorporating and documenting these new data.

The Brl domain in Sec63p is required for assembly of functional endoplasmic reticulum translocons

Protein translocation into the endoplasmic reticulum occurs at pore-forming structures known as translocons. In yeast, two different targeting pathways converge at a translocation pore formed by the Sec61 complex. The signal recognition particle-dependent pathway targets nascent precursors co-translationally, whereas the Sec62p-dependent pathway targets polypeptides post-translationally. In addition to the Sec61 complex, both pathways also require Sec63p, an integral membrane protein of the Hsp40 family, and Kar2p, a soluble Hsp70 located in the ER lumen. Using a series of mutant alleles, we demonstrate that a conserved Brl (Brr2-like) domain in the COOH-terminal cytosolic region of Sec63p is essential for function both in vivo and in vitro. We further demonstrate that this domain is required for assembly of two oligomeric complexes of 350 and 380 kDa, respectively. The larger of these corresponds to the heptameric "SEC complex" required for post-translational translocation. However, the 350-kDa complex represents a newly defined hexameric SEC' complex comprising Sec61p, Sss1p, Sbh1p, Sec63p, Sec71p, and Sec72p. Our data indicate that the SEC' complex is required for co-translational protein translocation across the yeast ER membrane.

Protein kinase CK2 phosphorylates Sec63p to stimulate the assembly of the endoplasmic reticulum protein translocation apparatus

The heterotetrameric Sec62/63 complex associates with the heterotrimeric Sec61 complex to form the heptameric Sec complex. This complex is necessary and sufficient for post-translational protein translocation across the membrane of the endoplasmic reticulum. We show that Sec63p is phosphorylated at its C-terminal domain by the protein kinase CK2 and that this phosphorylation strengthens the interaction between the cytosolic domains of Sec63p and Sec62p. Exchanging either threonine 652 or threonine 654 against the nonphosphorylatable alanines in Sec63p impairs the binding to Sec62p and interferes with the efficient translocation of proteins across the membrane of the endoplasmic reticulum. These findings show that phosphorylation of Sec63p is required for tightly recruiting the putative signal-sequence-binding subunit Sec62p to the Sec complex.