Nonlethal sec71-1 and sec72-1 mutations eliminate proteins associated with the Sec63p-BiP complex from S. cerevisiae

A Potent Kalihinol Analogue Disrupts Apicoplast Function and Vesicular Trafficking in P. falciparum Malaria

Here we report the discovery of MED6-189, a new analogue of the kalihinol family of isocyanoterpene (ICT) natural products. MED6-189 is effective against drug-sensitive and -resistant P. falciparum strains blocking both intraerythrocytic asexual replication and sexual differentiation. This compound was also effective against P. knowlesi and P. cynomolgi. In vivo efficacy studies using a humanized mouse model of malaria confirms strong efficacy of the compound in animals with no apparent hemolytic activity or apparent toxicity. Complementary chemical biology, molecular biology, genomics and cell biological analyses revealed that MED6-189 primarily targets the parasite apicoplast and acts by inhibiting lipid biogenesis and cellular trafficking. Genetic analyses in P. falciparum revealed that a mutation in PfSec13, which encodes a component of the parasite secretory machinery, reduced susceptibility to the drug. The high potency of MED6-189 in vitro and in vivo, its broad range of efficacy, excellent therapeutic profile, and unique mode of action make it an excellent addition to the antimalarial drug pipeline.

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.

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.

Sec66-Dependent Regulation of Yeast Spindle-Pole Body Duplication Through Pom152

In closed mitotic systems such as Saccharomyces cerevisiae, the nuclear envelope (NE) does not break down during mitosis, so microtubule-organizing centers such as the spindle-pole body (SPB) must be inserted into the NE to facilitate bipolar spindle formation and chromosome segregation. The mechanism of SPB insertion has been linked to NE insertion of nuclear pore complexes (NPCs) through a series of genetic and physical interactions between NPCs and SPB components. To identify new genes involved in SPB duplication and NE insertion, we carried out genome-wide screens for suppressors of deletion alleles of SPB components, including Mps3 and Mps2. In addition to the nucleoporins POM152 and POM34, we found that elimination of SEC66/SEC71/KAR7 suppressed lethality of cells lacking MPS2 or MPS3. Sec66 is a nonessential subunit of the Sec63 complex that functions together with the Sec61 complex in import of proteins into the endoplasmic reticulum (ER). Cells lacking Sec66 have reduced levels of Pom152 protein but not Pom34 or Ndc1, a shared component of the NPC and SPB. The fact that Sec66 but not other subunits of the ER translocon bypass deletion mutants in SPB genes suggests a specific role for Sec66 in the control of Pom152 levels. Based on the observation that sec66∆ does not affect the distribution of Ndc1 on the NE or Ndc1 binding to the SPB, we propose that Sec66-mediated regulation of Pom152 plays an NPC-independent role in the control of SPB duplication.

Protein transport into the human endoplasmic reticulum

Protein transport into the endoplasmic reticulum (ER) is essential for all eukaryotic cells and evolutionary related to protein transport into and across the cytoplasmic membrane of eubacteria and archaea. It is based on amino-terminal signal peptides in the precursor polypeptides plus various transport components in cytosol plus ER and can occur either cotranslationally or posttranslationally. The two mechanisms merge at the heterotrimeric Sec61 complex in the ER membrane, which forms an aqueous polypeptide-conducting channel. Since the mammalian ER is also the main intracellular calcium storage organelle, the Sec61 complex is tightly regulated in its dynamics between the open and closed conformations by various ligands, such as precursor polypeptides at the cytosolic face and the Hsp70-type molecular chaperone BiP at the ER lumenal face (Hsp, heat shock protein). Furthermore, BiP binding to the incoming precursor polypeptide contributes to unidirectionality and efficiency of transport. Recent insights into the structural dynamics of the Sec61 complex and related complexes in eubacteria and archaea have various mechanistic and functional implications.

Embracing the void--how much do we really know about targeting and translocation to the endoplasmic reticulum?

In order for a protein to enter the secretory pathway, two crucial steps must occur: it first needs to be targeted to the cytosolic surface of the endoplasmic reticulum (ER), and then be translocated across the ER membrane. Although for many years studies of targeting focused on the signal recognition particle, recent findings reveal that several alternative targeting pathways exist, some still undescribed, and some only recently elucidated. In addition, many genes implicated in the translocation step have not been assigned a specific function. Here, we will focus on the open questions regarding ER targeting and translocation, and discuss how combining classical biochemistry with systematic approaches can promote our understanding of these essential cellular steps.

AtTPR7 as part of the Arabidopsis Sec post-translocon

The secretory system in eukaryotic organisms ensures targeting of proteins to their place of function after they entered the endoplasmic reticulum either co- or post-translationally. Thereby proteins are translocated through the Sec translocon into the endoplasmic reticulum. In the Arabidopsis genome homologs for the three major components of the Sec translocon, the central pore Sec61α and the auxiliary proteins Sec62 and Sec63 are present. Phylogenetic analyses show Sec61α to be the most conserved subunit within the Sec translocon whereas Sec62 and Sec63 show less homology but contain the same functional domains among all organisms. We recently characterized a novel tetratricopeptide repeat domain containing protein, AtTPR7, as part of the Arabidopsis Sec translocon which is probably involved in chaperone assisted post-translational import. In this study we investigated the interaction of AtTPR7 with Sec62 as well as the cytosolic chaperones HSP70 and HSP90 not only in vitro but also in vivo to further strengthen the hypothesis of AtTPR7 being a chaperone docking protein of the Sec translocon for secretory preproteins in Arabidopsis.

Protein translocation across the rough endoplasmic reticulum

The rough endoplasmic reticulum is a major site of protein biosynthesis in all eukaryotic cells, serving as the entry point for the secretory pathway and as the initial integration site for the majority of cellular integral membrane proteins. The core components of the protein translocation machinery have been identified, and high-resolution structures of the targeting components and the transport channel have been obtained. Research in this area is now focused on obtaining a better understanding of the molecular mechanism of protein translocation and membrane protein integration.

AtTPR7 is a chaperone-docking protein of the Sec translocon in Arabidopsis

Chaperone-assisted sorting of post-translationally imported proteins is a general mechanism among all eukaryotic organisms. Interaction of some preproteins with the organellar membranes is mediated by chaperones, which are recognised by membrane-bound tetratricopeptide repeat (TPR) domain containing proteins. We have characterised AtTPR7 as an endoplasmic reticulum protein in plants and propose a potential function for AtTPR7 in post-translational protein import. Our data demonstrate that AtTPR7 interacts with the heat shock proteins HSP90 and HSP70 via a cytosol-exposed TPR domain. We further show by in vitro and in vivo experiments that AtTPR7 is associated with the Arabidopsis Sec63 homologue, AtERdj2. Interestingly, AtTPR7 can functionally complement a Δsec71 yeast mutant that is impaired in post-translational protein transport. These data strongly suggest that AtTPR7 not only has a role in chaperone binding but also in post-translational protein import into the endoplasmic reticulum, pointing to a general mechanism of chaperone-mediated post-translational sorting between the endoplasmic reticulum, mitochondria and chloroplasts in plant cells.

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.

Protein translocation across the ER membrane

Protein translocation into the endoplasmic reticulum (ER) is the first and decisive step in the biogenesis of most extracellular and many soluble organelle proteins in eukaryotic cells. It is mechanistically related to protein export from eubacteria and archaea and to the integration of newly synthesized membrane proteins into the ER membrane and the plasma membranes of eubacteria and archaea (with the exception of tail anchored membrane proteins). Typically, protein translocation into the ER involves cleavable amino terminal signal peptides in precursor proteins and sophisticated transport machinery components in the cytosol, the ER membrane, and the ER lumen. Depending on the hydrophobicity and/or overall amino acid content of the precursor protein, transport can occur co- or posttranslationally. The respective mechanism determines the requirements for certain cytosolic transport components. The two mechanisms merge at the level of the ER membrane, specifically, at the heterotrimeric Sec61 complex present in the membrane. The Sec61 complex provides a signal peptide recognition site and forms a polypeptide conducting channel. Apparently, the Sec61 complex is gated by various ligands, such as signal peptides of the transport substrates, ribosomes (in cotranslational transport), and the ER lumenal molecular chaperone, BiP. Binding of BiP to the incoming polypeptide contributes to efficiency and unidirectionality of transport. Recent insights into the structure of the Sec61 complex and the comparison of the transport mechanisms and machineries in the yeast Saccharomyces cerevisiae, the human parasite Trypanosoma brucei, and mammals have various important mechanistic as well as potential medical implications. This article is part of a Special Issue entitled Protein translocation across or insertion into membranes.

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.

DIVERSITY AND EVOLUTION OF PROTEIN TRANSLOCATION

Cells need to translocate proteins into and across hydrophobic membranes in order to interact with the extracellular environment. Although a subset of proteins are thought to spontaneously insert into lipid bilayers, translocation of most transported proteins requires additional cellular components. Such components catalyze efficient lateral transport into or across cellular membranes in prokaryotes and eukaryotes. These include, among others, the conserved YidC/Oxa1/Alb3 proteins as well as components of the Sec and the Tat pathways. Our current knowledge of the function and distribution of these components and their corresponding pathways in organisms of the three domains of life is reviewed. On the basis of this information, the evolution of protein translocation is discussed.

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

Precursors of secretory proteins are targeted to the membrane of the endoplasmic reticulum by specific protein complexes that recognize their signal sequence. All eukaryotic cells investigated so far have been found to possess the signal recognition particle (SRP) that targets the majority of precursors to the translocation machinery. In Saccharomyces cerevisiae a number of proteins are translocated independently of SRP. These precursors rely on a different signal sequence-binding complex, which includes Sec62p, Sec63p, Sec71p and Sec72p. Identifying interactions between individual components of this tetrameric protein complex is important in the understanding of its function. We demonstrate a specific interaction between the only two essential proteins in this complex, Sec62p and Sec63p. Second, we show evidence of homodimerization of Sec72p molecules and further identify the YLR301w gene product as a novel in vivo interacting partner of Sec72p. Finally, we determine the authentic N-terminus of Sec62p and describe interacting subdomains of both Sec62p and Sec63p.

Protein translocation across membranes

Cellular membranes act as semipermeable barriers to ions and macromolecules. Specialized mechanisms of transport of proteins across membranes have been developed during evolution. There are common mechanistic themes among protein translocation systems in bacteria and in eukaryotic cells. Here we review current understanding of mechanisms of protein transport across the bacterial plasma membrane as well as across several organelle membranes of yeast and mammalian cells. We consider a variety of organelles including the endoplasmic reticulum, outer and inner membranes of mitochondria, outer, inner, and thylakoid membranes of chloroplasts, peroxisomes, and lysosomes. Several common principles are evident: (a) multiple pathways of protein translocation across membranes exist, (b) molecular chaperones are required in the cytosol, inside the organelle, and often within the organelle membrane, (c) ATP and/or GTP hydrolysis is required, (d) a proton-motive force across the membrane is often required, and (e) protein translocation occurs through gated, aqueous channels. There are exceptions to each of these common principles indicating that our knowledge of how proteins translocate across membranes is not yet complete.

Mammalian Sec61 is associated with Sec62 and Sec63

In yeast, efficient protein transport across the endoplasmic reticulum (ER) membrane may occur co-translationally or post-translationally. The latter process is mediated by a membrane protein complex that consists of the Sec61p complex and the Sec62p-Sec63p subcomplex. In contrast, in mammalian cells protein translocation is almost exclusively co-translational. This transport depends on the Sec61 complex, which is homologous to the yeast Sec61p complex and has been identified in mammals as a ribosome-bound pore-forming membrane protein complex. We report here the existence of ribosome-free mammalian Sec61 complexes that associate with two ubiquitous proteins of the ER membrane. According to primary sequence analysis both proteins display homology to the yeast proteins Sec62p and Sec63p and are therefore named Sec62 and Sec63, respectively. The probable function of the mammalian Sec61-Sec62-Sec63 complex is discussed with respect to its abundance in ER membranes, which, in contrast to yeast ER membranes, apparently lack efficient post-translational translocation activity.

Molecular characterization of a novel mammalian DnaJ-like Sec63p homolog

We identified a human cDNA sequence encoding a polypeptide of 760 amino acids that shares 53% homology and 25.6% identity with the yeast DnaJ-like endoplasmic reticulum (ER) translocon component Sec63p. Three epitope-specific antisera revealed a protein of an apparent molecular mass of 83 kDa, both in human cell extracts and in dog pancreatic microsomes. Biochemical analyses show that it is an integral membrane protein of the rough ER, which has the DnaJ domain located in the ER lumen. The novel Sec63 protein could thus represent a key component of the mammalian ER protein translocation machinery.

Disruption of YHC8, a member of the TSR1 gene family, reveals its direct involvement in yeast protein translocation

Genetic studies of Saccharomyces cerevisiae have identified many components acting to deliver specific proteins to their cellular locations. Genome analysis, however, has indicated that additional genes may also participate in such protein trafficking. The product of the yeast Yarrowia lipolytica TSR1 gene promotes the signal recognition particle-dependent translocation of secretory proteins through the endoplasmic reticulum. Here we describe the identification of a new gene family of proteins that is well conserved among different yeast species. The TSR1 genes encode polypeptides that share the same protein domain distribution and, like Tsr1p, may play an important role in the early steps of the signal recognition particle-dependent translocation pathway. We have identified five homologues of the TSR1 gene, four of them from the yeast Saccharomyces cerevisiae and the other from Hansenula polymorpha. We generated a null mutation in the S. cerevisiae YHC8 gene, the closest homologue to Y. lipolytica TSR1, and used different soluble (carboxypeptidase Y, alpha-factor, invertase) and membrane (dipeptidyl-aminopeptidase) secretory proteins to study its phenotype. A large accumulation of soluble protein precursors was detected in the mutant strain. Immunofluorescence experiments show that Yhc8p is localized in the endoplasmic reticulum. We propose that the YHC8 gene is a new and important component of the S. cerevisiae endoplasmic reticulum membrane and that it functions in protein translocation/insertion of secretory proteins through or into this compartment.

Genetic interactions between KAR7/SEC71, KAR8/JEM1, KAR5, and KAR2 during nuclear fusion in Saccharomyces cerevisiae

During mating of Saccharomyces cerevisiae, two nuclei fuse to produce a single diploid nucleus. Two genes, KAR7 and KAR8, were previously identified by mutations that cause defects in nuclear membrane fusion. KAR7 is allelic to SEC71, a gene involved in protein translocation into the endoplasmic reticulum. Two other translocation mutants, sec63-1 and sec72Delta, also exhibited moderate karyogamy defects. Membranes from kar7/sec71Delta and sec72Delta, but not sec63-1, exhibited reduced membrane fusion in vitro, but only at elevated temperatures. Genetic interactions between kar7 and kar5 mutations were suggestive of protein-protein interactions. Moreover, in sec71 mutants, Kar5p was absent from the SPB and was not detected by Western blot or immunoprecipitation of pulse-labeled protein. KAR8 is allelic to JEMI, encoding an endoplasmic reticulum resident DnaJ protein required for nuclear fusion. Overexpression of KAR8/JEM1 (but not SEC63) strongly suppressed the mating defect of kar2-1, suggesting that Kar2p interacts with Kar8/Jem1p for nuclear fusion. Electron microscopy analysis of kar8 mutant zygotes revealed a nuclear fusion defect different from kar2, kar5, and kar7/sec71 mutants. Analysis of double mutants suggested that Kar5p acts before Kar8/Jem1p. We propose the existence of a nuclear envelope fusion chaperone complex in which Kar2p, Kar5p, and Kar8/Jem1p are key components and Sec71p and Sec72p play auxiliary roles.

Translocation of proteins across the endoplasmic reticulum membrane

Secretory protein biogenesis begins with the insertion of a preprotein into the lumen of the endoplasmic reticulum (ER). This insertion event, known as ER protein translocation, can occur either posttranslationally, in which the preprotein is completely synthesized on cytosolic ribosomes before being translocated, or cotranslationally, in which membrane-associated ribosomes direct the nascent polypeptide chain into the ER concomitant with polypeptide elongation. In either case, preproteins are targeted to the ER membrane through specific interactions with cytosolic and/or ER membrane factors. The preprotein is then transferred to a multiprotein translocation machine in the ER membrane that includes a pore through which the preprotein passes into the ER lumen. The energy required to drive protein translocation may derive either from the coupling of translation to translocation (during cotranslational translocation) or from ER lumenal molecular chaperones that may harness the preprotein or regulate the translocation machinery (during posttranslational translocation).

The Tim54p-Tim22p complex mediates insertion of proteins into the mitochondrial inner membrane

We have identified a new protein, Tim54p, located in the yeast mitochondrial inner membrane. Tim54p is an essential import component, required for the insertion of at least two polytopic proteins into the inner membrane, but not for the translocation of precursors into the matrix. Several observations suggest that Tim54p and Tim22p are part of a protein complex in the inner membrane distinct from the previously characterized Tim23p-Tim17p complex. First, multiple copies of the TIM22 gene, but not TIM23 or TIM17, suppress the growth defect of a tim54-1 temperature-sensitive mutant. Second, Tim22p can be coprecipitated with Tim54p from detergent-solubilized mitochondria, but Tim54p and Tim22p do not interact with either Tim23p or Tim17p. Finally, the tim54-1 mutation destabilizes the Tim22 protein, but not Tim23p or Tim17p. Our results support the idea that the mitochondrial inner membrane carries two independent import complexes: one required for the translocation of proteins across the inner membrane (Tim23p-Tim17p), and the other required for the insertion of proteins into the inner membrane (Tim54p-Tim22p).

Binding of secretory precursor polypeptides to a translocon subcomplex is regulated by BiP

The translocation of a secretory precursor protein across the ER membrane comprises three phases: docking of the precursor at the membrane, insertion into the translocation pore, and exit from the pore into the ER lumen. We demonstrate that Sec62p, Sec71p and Sec72p form a translocon subcomplex that engages secretory precursors at the membrane site of the ER translocation machinery. Binding of a precursor to the subcomplex depends on the presence of an intact signal sequence and occurs only in the absence of ATP. In the presence of ATP, the precursor is released from the subcomplex in a reaction mediated by the lumenal hsp70, BiP. This release reaction, which is specific to BiP and requires interaction between BiP and the DnaJ homolog Sec63p, defines a role for BiP and Sec63p early in the ER translocation process.

Polypeptide translocation machinery of the yeast endoplasmic reticulum

Proteins enter the secretory pathway by two general routes. In one, the complete polypeptide is made in the cytoplasm and held in an incompletely folded state by chaperoning adenosine triphosphatases (ATPases) such as hsp70. In Saccharomyces cerevisiae, fully synthesized secretory precursors engage the endoplasmic reticulum (ER) membrane by interaction with a set of Sec proteins comprising the polypeptide translocation apparatus (Sec61p, Sec62p, Sec63p, Sec71p, Sec72p). Productive interaction requires displacement of hsp70 from the precursor, a reaction that is facilitated by Ydj1p, a homologue of the Escherichia coli DnaJ protein. Both DnaJ and Ydj1p regulate chaperone activity by stimulating the ATPase activity of their respective hsp70 partners (E. coli DnaK and S. cerevisiae Ssa1p, respectively). In the ER lumen, another hsp70 chaperone, BiP, binds ATP and interacts with the ER membrane via its contact with a peptide loop of Sec63p. This loop represents yet another DnaJ homologue in that it contains a region of approximately 70 residue similarity to the 'J box', the most conserved region of the DnaJ family of proteins. In the presence of ATP, under conditions in which BiP can bind to Sec63p, the secretory precursor passes from the cytosol into the lumen through a membrane channel formed by Sec61p. A second route to the membrane pore that is used by many other secretory precursors, particularly in mammalian cells, requires that the polypeptide engage the ER membrane as the nascent chain emerges from the ribosome. Such cotranslational translocation bypasses the need for certain Sec proteins, instead utilizing an alternate set of cytosolic and membrane factors that allows the nascent chain to be inserted directly into the Sec61p channel.

The homologue of mammalian SPC12 is important for efficient signal peptidase activity in Saccharomyces cerevisiae

The multisubunit signal peptidase catalyzes the cleavage of signal peptides and the degradation of some membrane proteins within the endoplasmic reticulum (ER). The only subunit of this enzyme functionally examined to date, yeast Sec11p, is related to signal peptidase I from bacteria. Since bacterial signal peptidase is capable of processing both prokaryotic and eukaryotic signal sequences as a monomer, it is unclear why the analogous enzyme in the ER contains proteins unrelated to signal peptidase I. To address this issue, the gene encoding Spc1p, the yeast homologue to mammalian SPC12, is isolated from the yeast Saccharomyces cerevisiae. Spc1p co-purifies and genetically interacts with Sec11p, but unlike Sec11p, Spc1p is not required for cell growth or the proteolytic processing of tested proteins in yeast. This indicates that only a subset of the ER signal peptidase subunits is required for signal peptidase and protein degradation activities in vivo. Through both genetic and biochemical criteria, Spc1p appears, however, to be important for efficient signal peptidase activity.

A second trimeric complex containing homologs of the Sec61p complex functions in protein transport across the ER membrane of S. cerevisiae

Yeast microsomes contain a heptameric Sec complex involved in post-translational protein transport that is composed of a heterotrimeric Sec61p complex and a tetrameric Sec62-Sec63 complex. The trimeric Sec61p complex also exists as a separate entity that probably functions in co-translational protein transport, like its homolog in mammals. We have now discovered in the yeast endoplasmic reticulum membrane a second, structurally related trimeric complex, named Ssh1p complex. It consists of Ssh1p1 (Sec sixty-one homolog 1), a rather distant relative of Sec61p, of Sbh2p, a homolog of the Sbh1p subunit of the Sec61p complex, and of Sss1p, a component common to both trimeric complexes. In contrast to Sec61p, Ssh1p is not essential for cell viability but it is required for normal growth rates. Sbh1p and Sbh2p individually are also not essential, but cells lacking both proteins are impaired in their growth at elevated temperatures and accumulate precursors of secretory proteins; microsomes isolated from these cells also exhibit a reduced rate of post-translational protein transport. Like the Sec61p complex, the Ssh1p complex interacts with membrane-bound ribosomes, but it does not associate with the Sec62-Sec63p complex to form a heptameric Sec complex. We therefore propose that it functions exclusively in the co-translational pathway of protein transport.

A mutation affecting signal peptidase inhibits degradation of an abnormal membrane protein in Saccharomyces cerevisiae

Signal peptidase removes amino-terminal signal peptides from precursor proteins during or immediately following their translocation to the lumen of the endoplasmic reticulum (ER) and may participate in ER degradation, a poorly defined process whereby abnormal proteins are rapidly degraded early in the secretory pathway. Here, the involvement of signal peptidase in ER degradation is examined through the use of two chimeric membrane proteins that lack amino-terminal signal peptides: A189invHD, which contains sequences derived from arginine permease and histidinol dehydrogenase, and AHDK2, containing the ER-resident protein Kar2p fused to the carboxyl terminus of A189invHD. Degradation of approximately 95% of A189invHD is observed in yeast cells expressing enzymatically active signal peptidase, whereas only 60% undergoes rapid degradation in a sec11 mutant bearing a temperature-sensitive mutation in the gene encoding the 18-kDa subunit (Sec11p) of the signal peptidase complex. AHDK2 is proteolyzed in a reaction yielding at least two fragments in wild-type cells and in the sec11 mutant containing a plasmid bearing the SEC11 gene. The proteolytic reaction is catalyzed in a temperature-dependent manner in the sec11 mutant, with AHDK2 remaining stable at the nonpermissive temperature. Using conditional mutants defective in protein translocation into and out of the ER and in vitro protease protection studies, the site of degradation for AHDK2 is localized to the ER lumen. The data therefore indicate (i) A189invHD is degraded through both signal peptidase-dependent and independent processes; (ii) signal peptidase, specifically the Sec11p subunit, is required for the proteolysis of AHDK2; and (iii) the Kar2 fragment at the carboxyl terminus of AHDK2 permits detection of proteolytic intermediates.

Protein translocation at the membrane of the endoplasmic reticulum

The process of insertion into and translocation across the ER membrane is a significant step in the biosynthesis of a membrane or secretory protein. This commits the protein to a destination within the "secretory pathway" (Palade, 1975) and is part of a complex series of events involving protein targeting, translocation, maturation and sorting, which finally results in a biologically-active protein being delivered to its correct subcellular location. The focus for this review has been the initial events of this process. Proteins which constitute at least a part of the actual translocation site across the ER membrane have been identified and the minimum components required to reconstitute ER translocation in vitro have been defined. A detailed description of the architecture of the ER translocation site and the molecular events occurring during translocation and membrane insertion remain goals for the future. The process occurring in vivo may be more complex since (i) each translocation site may only promote a single round of translocation in vitro whereas in vivo the sites must operate catalytically and go through many cycles of translocation and insertion (see Gilmore, 1993) and (ii) the in vivo requirement for a translocation site which is impermeable to small molecules (in order not to dissipate chemical gradients and the redox potential) is unlikely to be important for in vitro assays. Thus, other components which play a vital role in protein translocation and membrane insertion in vivo may remain to be identified. Our future aim must be to place a detailed understanding of the molecular events of the translocation process into the context of the normal cellular environment.