Secretion in yeast: preprotein binding to a membrane receptor and ATP-dependent translocation are sequential and separable events in vitro

Protein translocation into the endoplasmic reticulum: a light at the end of the tunnel

Intracellular transport of secretory of proteins and many membrane proteins in eukaryotic cells commences with their translocation into or across the membrane of the rough endoplasmic reticulum. Several components of the cellular machinery that mediates this process have been elucidated using in vitro assays or by genetic means. An analysis of how they function will depend on the ability to reassemble them into translocation-competent lipid vesicles.

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.

Posttranslational protein translocation across the membrane of the endoplasmic reticulum

Posttranslational protein translocation across the membrane of the endoplasmic reticulum is mediated by the Sec complex. This complex includes a transmembrane channel formed by multiple copies of the Sec61 protein. Translocation of a polypeptide begins when the signal sequence binds at a specific site within the channel. Binding results in the insertion of the substrate into the channel, possibly as a loop with a small segment exposed to the lumen. While bound, the signal sequence is in contact with both protein components of the channel and the lipid of the membrane. Subsequent movement of the polypeptide through the channel occurs when BiP molecules interact transiently with a luminal domain of the Sec complex, hydrolyze ATP, and bind to the substrate. Bound BiP promotes translocation by preventing the substrate from diffusing backwards through the channel, and thus acts as a molecular ratchet.

Sac1p plays a crucial role in microsomal ATP transport, which is distinct from its function in Golgi phospholipid metabolism

Analysis of microsomal ATP transport in yeast resulted in the identification of Sac1p as an important factor in efficient ATP uptake into the endoplasmic reticulum (ER) lumen. Yet it remained unclear whether Sac1p is the authentic transporter in this reaction. Sac1p shows no homology to other known solute transporters but displays similarity to the N-terminal non-catalytic domain of a subset of inositol 5'-phosphatases. Furthermore, Sac1p was demonstrated to be involved in inositol phospholipid metabolism, an activity whose absence contributes to the bypass Sec14p phenotype in sac1 mutants. We now show that purified recombinant Sac1p can complement ATP transport defects when reconstituted together with sac1Delta microsomal extracts, but is unable to catalyze ATP transport itself. In addition, we demonstrate that sac1Delta strains are defective in ER protein translocation and folding, which is a direct consequence of impaired ATP transport function and not related to the role of Sac1p in Golgi inositol phospholipid metabolism. These data suggest that Sac1p is an important regulator of microsomal ATP transport providing a possible link between inositol phospholipid signaling and ATP-dependent processes in the yeast ER.

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).

Protein transport by purified yeast Sec complex and Kar2p without membranes

Posttranslational protein translocation across the endoplasmic reticulum membrane of yeast requires a seven-component transmembrane complex (the Sec complex) in collaboration with the lumenal Kar2 protein (Kar2p). A translocation substrate was initially bound to the cytosolic face of the purified Sec complex in a signal-sequence-dependent but Kar2p- and nucleotide-independent manner. In a subsequent reaction, in which Kar2p interacted with the lumenal face of the Sec complex and hydrolyzed adenosine triphosphate, the substrate moved through a channel formed by the Sec complex and was released at the lumenal end. Movement through the channel occurred in detergent solution in the absence of a lipid bilayer.

Site-directed mutagenesis of hepatitis A virus protein 3A: effects on membrane interaction

Due to a stretch of hydrophobic amino acids, protein 3A of hepatitis A virus (HAV) has been suggested to act as a membrane anchor or a carrier of the genome-linked protein 3B (VPg) during viral RNA synthesis. Mutagenesis analysis was performed in order to elucidate the role of the N- and C-terminal tracts of protein 3A in cell membrane interaction. Expression of the mutated proteins in E. coli cells demonstrated that the presence of positively charged residues at the C-terminus is not required for membrane anchoring. Changes in the primary sequence involving charged amino acids at the N- and C-termini critically influenced the ability of the protein 3A of a cytopathic strain of HAV to change bacterial membrane permeability. This result demonstrates the strict correlation between the structure and pore-forming potential of HAV protein 3A.

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.

Sac1p mediates the adenosine triphosphate transport into yeast endoplasmic reticulum that is required for protein translocation

Protein translocation into the yeast endoplasmic reticulum requires the transport of ATP into the lumen of this organelle. Microsomal ATP transport activity was reconstituted into proteoliposomes to characterize and identify the transporter protein. A polypeptide was purified whose partial amino acid sequence demonstrated its identity to the product of the SAC1 gene. Accordingly, microsomal membranes isolated from strains harboring a deletion in the SAC1 gene (sac1 delta) were found to be deficient in ATP-transporting activity as well as severely compromised in their ability to translocate nascent prepro-alpha-factor and preprocarboxypeptidase Y. Proteins isolated from the microsomal membranes of a sac1 delta strain were incapable of stimulating ATP transport when reconstituted into the in vitro assay system. When immunopurified to homogeneity and incorporated into artificial lipid vesicles, Sac1p was shown to reconstitute ATP transport activity. Consistent with the requirement for ATP in the lumen of the ER to achieve the correct folding of secretory proteins, the sac1 delta strain was shown to have a severe defect in transport of procarboxypeptidase Y out of the ER and into the Golgi complex in vivo. The collective data indicate an intimate role for Sac1p in the transport of ATP into the ER lumen.

Interaction between BiP and Sec63p is required for the completion of protein translocation into the ER of Saccharomyces cerevisiae

To clarify the roles of Kar2p (BiP) and Sec63p in translocation across the ER membrane in Saccharomyces cerevisiae, we have utilized mutant alleles of the essential genes that encode these proteins: kar2-203 and sec63-1. Sanders et al. (Sanders, S. L., K. M. Whitfield, J. P. Vogel, M. D. Rose, and R. W. Schekman. 1992. Cell. 69:353-365) showed that the translocation defect of the kar2-203 mutant lies in the inability of the precursor protein to complete its transit across the membrane, suggesting that the lumenal hsp70 homologue Kar2p (BiP) binds the transiting polypeptide in order to facilitate its passage through the pore. We now show that mutation of a conserved residue (A181-->T) (Nelson, M. K., T. Kurihara, and P. Silver. 1993. Genetics. 134:159-173) in the lumenal DnaJ box of Sec63p (sec63-1) results in an in vitro phenotype that mimics the precursor stalling defect of kar2-203. We demonstrate by several criteria that this phenotype results specifically from a defect in the lumenal interaction between Sec63p and BiP: Neither a sec62-1 mutant nor a mutation in the cytosolically exposed domain of Sec63p causes precursor stalling, and interaction of the sec63-1 mutant with the membranebound components of the translocation apparatus is unimpaired. Additionally, dominant KAR2 suppressors of sec63-1 partially relieve the stalling defect. Thus, proper interaction between BiP and Sec63p is necessary to allow the precursor polypeptide to complete its transit across the membrane.

Posttranslational protein transport in yeast reconstituted with a purified complex of Sec proteins and Kar2p

We have reproduced the posttranslational mode of protein translocation across the endoplasmic reticulum membrane with reconstituted proteoliposomes containing a purified complex of seven yeast proteins. This Sec complex includes a heterotrimeric Sec61p complex, homologous to that in mammals, as well as all other membrane proteins found in genetic screens for translocation components. Efficient posttranslational translocation also requires the addition of lumenal Kar2p (BiP) and ATP. The trimeric Sec61p complex also exists as a separate entity that, in contrast with the large Sec complex, is associated with membrane-bound ribosomes. We therefore hypothesize that distinct membrane protein complexes function in co- and posttranslational translocation pathways.

The membrane proteins TRAMp and sec61 alpha p may be involved in post-translational transport of presecretory proteins into mammalian microsomes

The presecretory protein ppcecDHFR, a hybrid between preprocecropinA and dihydrofolate reductase, is transported into mammalian microsomes post-translationally, i.e. independent of ribosome and signal recognition particle. Here, the involvement of microsomal proteins in ribonucleoparticle-independent transport of ppcecDHFR was analyzed by transport into trypsin-pretreated microsomes and by transport of a truncated version of ppcecDHFR and subsequent chemical cross-linking. We observed that post-translational transport of ppcecDHFR can occur into microsomes which had been pretreated with trypsin (final concentration, 100 micrograms/ml) and that of the known transport components only TRAMp and sec61 alpha p are still present under these conditions. Furthermore, we found that the truncated ppcecDHFR, ppcecDHFR-98mer', can be cross-linked to 36 kDa microsomal membrane proteins during post-translational transport. Therefore, the two microsomal membrane proteins with molecular masses of about 36 kDa, TRAMp and sec61 alpha p, appear to be involved in the post-translational transport of ppcecDHFR and ppcecDHFR-98mer.

A stably folded presecretory protein associates with and upon unfolding translocates across the membrane of mammalian microsomes

The presecretory protein ppcecDHFR, a hybrid between preprocecropin A and dihydrofolate reductase, is transported into mammalian microsomes post-translationally, i.e. independently of ribosome and signal recognition particle. Upon staging the transport process, stably folded ppcecDHFR bound to mammalian microsomes and subsequently translocated across the membrane. Membrane association depended on the signal peptide but involved neither ATP nor an N-ethylmaleimide-sensitive microsomal protein. Membrane insertion of bound ppcecDHFR did not necessitate unfolding of the DHFR domain but depended on ATP and an N-ethylmaleimide-sensitive microsomal protein. Completion of translocation relied on unfolding of the DHFR domain. Thus mammalian microsomes have the capability of transporting a bound and folded precursor protein, i.e. to trigger unfolding of a precursor protein on the membrane surface.

The initial association of a truncated form of the Neurospora plasma membrane H(+)-ATPase and of the precursor of yeast invertase with microsomes are distinct processes

Translocation and integration activities were assessed in Neurospora microsomes (nRM) after modification either by a sulfhydryl alkylating reagent or by a proteinase. A Neurospora in vitro system was programmed with RNA transcripts that encode the amino-terminal 194 amino-acid residues of the Neurospora plasma membrane H(+)-ATPase (pma194+) or the 262 amino-acid residues of the precursor of yeast invertase (preinv262). The processing of preinv262 was blocked in N-phenylmaleimide- and in trypsin-pretreated nRM. In contrast, the binding of preinv262 to microsomes was unaffected in the chemically alkylated nRM, but was affected in the trypsin-pretreated nRM. In the chemically alkylated vesicles, the integration of the pma194+ was not affected, but was partially blocked in the trypsin-pretreated vesicles. These data imply that trypsin-sensitive components are required for these activities in nRM, and that binding, translocation and integration can be differentiated by their sensitivity to chemical alkylation of sulfhydryl groups in nRM. Evaluated also were the effects of temperature on translocation and integration activities in the nRM. These were maximal at 20 degrees C, whereas the binding of preinv262 was maximal at 0 degree C. Taken together, these data demonstrate that the processing of preinv262 by nRM can be resolved into two steps: binding of the precursor protein to nRM and subsequent translocation into the lumen of the vesicles. Whereas, the integration of the pma194+ into nRM could not be resolved into separable steps. Taken together, these results are interpreted to imply that the initial association of truncated forms of the pma+ and the precursor of invertase to the surface of the nRM are distinct processes.

DIDS (4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid) inhibits an early step of protein translocation across the mammalian ER membrane

Protein translocation across the endoplasmic reticulum (ER) membrane of yeast can be inhibited by agents believed to specifically affect the transport of ATP through the membrane (Mayinger, P. and Meyer, D.I. (1993) EMBO J. 12, 659-666), suggesting the involvement of a translocation component in the lumen of the ER that binds ATP. We demonstrate that one of the inhibitors, 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid (DIDS), also affects the translocation of proteins into mammalian microsomes. Translocation is blocked at the point of transfer of the nascent chain from the signal recognition particle (SRP) into the ER-membrane. We also confirm that photoaffinity-labelling of microsomes with 8-azido-ATP inhibits the same early step of protein translocation. Since this step is reported to not require ATP, these results raise the possibility that, in both cases, factor(s) other than ATP-binding components of the translocation machinery are perturbed.

Protein secretion in Bacillus species

Bacilli secrete numerous proteins into the environment. Many of the secretory proteins, their export signals, and their processing steps during secretion have been characterized in detail. In contrast, the molecular mechanisms of protein secretion have been relatively poorly characterized. However, several components of the protein secretion machinery have been identified and cloned recently, which is likely to lead to rapid expansion of the knowledge of the protein secretion mechanism in Bacillus species. Comparison of the presently known export components of Bacillus species with those of Escherichia coli suggests that the mechanism of protein translocation across the cytoplasmic membrane is conserved among gram-negative and gram-positive bacteria differences are found in steps preceding and following the translocation process. Many of the secretory proteins of bacilli are produced industrially, but several problems have been encountered in the production of Bacillus heterologous secretory proteins. In the final section we discuss these problems and point out some possibilities to overcome them.

Sec61p and BiP directly facilitate polypeptide translocation into the ER

Secretory proteins are segregated from cytosolic proteins by their translocation into the endoplasmic reticulum (ER). A modified secretory protein trapped during translocation across the ER membrane can be crosslinked to two previously identified proteins, Sec61p and BiP (Kar2p). The dependence of this cross-linking upon proteins and small molecules was examined. Mutations in SEC62 and SEC63 decrease the ability of Sec61p to be cross-linked to the secretory polypeptide trapped in translocation. ATP is also required for interaction of Sec61p with the secretory protein. Three kar2 alleles display defective translocation in vitro. Two of these alleles also decrease the ability of Sec61p to be cross-linked to the secretory protein. The third allele, while exhibiting a severe translocation defect, does not affect the interaction of Sec61p with the secretory protein. These results suggest that Sec61p is directly involved in translocation and that BiP acts at two stages of the translocation cycle.

Yeast Sec proteins interact with polypeptides traversing the endoplasmic reticulum membrane

We show by photocross-linking that nascent secretory proteins, during their passage through the endoplasmic reticulum membrane of S. cerevisiae, are in physical contact with Sec61p and Sec62p, two genetically identified membrane proteins that are essential for in vivo translocation. Sec61p seems to be in continuous contact, whereas Sec62p is involved only transiently. Translocation comprises both ATP-dependent and -independent phases of interaction with the Sec proteins. The results suggest a direct role of the Sec proteins in translocation.

Protein export in prokaryotes and eukaryotes. Theme with variations

Protein export in prokaryotes as well as in eukaryotes can be defined as protein transport across the plasma membrane. In both types of organisms there are various apparently ATP-dependent transport mechanisms which can be distinguished from one another and which show similarities when the prokaryotic mechanism is compared with the respective eukaryotic mechanism. First, one can distinguish between transport mechanisms which involve so-called signal or leader peptides and those which do not. The latter mechanisms seem to employ ATP-dependent transport systems which belong to the family of oligopeptide permeases and multiple drug resistance proteins. Second, in signal or leader peptide-dependent transport one can distinguish between transport mechanisms which involve ribonucleoparticles and those which employ molecular chaperones. Both mechanisms appear to converge at the level of ATP-dependent translocases.

Protein retention in yeast rough endoplasmic reticulum: expression and assembly of human ribophorin I

The RER retains a specific subset of ER proteins, many of which have been shown to participate in the translocation of nascent secretory and membrane proteins. The mechanism of retention of RER specific membrane proteins is unknown. To study this phenomenon in yeast, where no RER-specific membrane proteins have yet been identified, we expressed the human RER-specific protein, ribophorin I. In all mammalian cell types examined, ribophorin I has been shown to be restricted to the membrane of the RER. Here we ascertain that yeast cells correctly target, assemble, and retain ribophorin I in their RER. Floatation experiments demonstrated that human ribophorin I, expressed in yeast, was membrane associated. Carbonate (pH = 11) washing and Triton X-114 cloud-point precipitations of yeast microsomes indicated that ribophorin I was integrated into the membrane bilayer. Both chromatography on Con A and digestion with endoglycosidase H were used to prove that ribophorin I was glycosylated once, consistent with its expression in mammalian cells. Proteolysis of microsomal membranes and subsequent immunoblotting showed ribophorin I to have assumed the correct transmembrane topology. Sucrose gradient centrifugation studies found ribophorin I to be included only in fractions containing rough membranes and excluded from smooth ones that, on the basis of the distribution of BiP, included smooth ER. Ribosome removal from rough membranes and subsequent isopycnic centrifugation resulted in a shift in the buoyant density of the ribophorin I-containing membranes. Furthermore, the rough and density-shifted fractions were the exclusive location of protein translocation activity. Based on these studies we conclude that sequestration of membrane proteins to rough domains of ER probably occurs in a like manner in yeast and mammalian cells.