The Rab7 effector protein RILP controls lysosomal transport by inducing the recruitment of dynein-dynactin motors

Many intracellular compartments, including MHC class II-containing lysosomes, melanosomes, and phagosomes, move along microtubules in a bidirectional manner and in a stop-and-go fashion due to the alternating activities of a plus-end directed kinesin motor and a minus-end directed dynein-dynactin motor. It is largely unclear how motor proteins are targeted specifically to different compartments. Rab GTPases recruit and/or activate several proteins involved in membrane fusion and vesicular transport. They associate with specific compartments after activation, which makes Rab GTPases ideal candidates for controlling motor protein binding to specific membranes. We and others [7] have identified a protein, called RILP (for Rab7-interacting lysosomal protein), that interacts with active Rab7 on late endosomes and lysosomes. Here we show that RILP prevents further cycling of Rab7. RILP expression induces the recruitment of functional dynein-dynactin motor complexes to Rab7-containing late endosomes and lysosomes. Consequently, these compartments are transported by these motors toward the minus end of microtubules, effectively inhibiting their transport toward the cell periphery. This signaling cascade may be responsible for timed and selective dynein motor recruitment onto late endosomes and lysosomes.

The major substrates for TAP in vivo are derived from newly synthesized proteins

The transporter associated with antigen processing (TAP) is a member of the family of ABC transporters that translocate a large variety of substrates across membranes. TAP transports peptides from the cytosol into the endoplasmic reticulum for binding to MHC class I molecules and for subsequent presentation to the immune system. Here we follow the lateral mobility of TAP in living cells. TAP's mobility increases when it is inactive and decreases when it translocates peptides. Because TAP activity is dependent on substrate, the mobility of TAP is used to monitor the intracellular peptide content in vivo. Comparison of the diffusion rates in peptide-free and peptide-saturated cells indicates that normally about one-third of all TAP molecules actively translocate peptides. However, during an acute influenza infection TAP becomes fully employed owing to the production and degradation of viral proteins. Furthermore, TAP activity depends on continuing protein translation. This implies that MHC class I molecules mainly sample peptides that originate from newly synthesized proteins, to ensure rapid presentation to the immune system.

Mice lacking the MHC class II-associated invariant chain

The invariant chain (li) has aroused much interest because of its close association with major histocompatibility complex (MHC) class II molecules. Various functions have been proposed for it; several of these have received experimental support, but most have not been definitively proven, owing largely to uncertainties inherent in the experimental systems employed. We have now generated a line of mice devoid of the invariant chain by introducing a drastic mutation into the li gene. Cells from mutant animals show aberrant transport of MHC class II molecules, resulting in reduced levels of class II complexes at the surface, and these do not have the typical compact conformation indicative of tight peptide binding. Consequently, mutant cells present protein antigens very poorly and mutant mice are deficient in producing and at negatively selecting CD4+ T cells.

Dynamics of proteasome distribution in living cells

Proteasomes are proteolytic complexes involved in non-lysosomal degradation which are localized in both the cytoplasm and the nucleus. The dynamics of proteasomes in living cells is unclear, as is their targeting to proteins destined for degradation. To investigate the intracellular distribution and mobility of proteasomes in vivo, we generated a fusion protein of the proteasome subunit LMP2 and the green fluorescent protein (GFP). The LMP2-GFP chimera was quantitatively incorporated into catalytically active proteasomes. The GFP-tagged proteasomes were located within both the cytoplasm and the nucleus. Within these two compartments, proteasomes diffused rapidly, and bleaching experiments demonstrated that proteasomes were transported slowly and unidirectionally from the cytoplasm into the nucleus. During mitosis, when the nuclear envelope has disintegrated, proteasomes diffused rapidly throughout the dividing cell without encountering a selective barrier. Immediately after cell division, the restored nuclear envelope formed a new barrier for the diffusing proteasomes. Thus, proteasomes can be transported unidirectionally over the nuclear membrane, but can also enter the nucleus upon reassembly during cell division. Since proteasomes diffuse rapidly in the cytoplasm and nucleus, they may perform quality control by continuous collision with intracellular proteins, and degrading those proteins that are properly tagged or misfolded.

Recycling MHC class I molecules and endosomal peptide loading

MHC class I molecules usually present peptides derived from endogenous antigens that are bound in the endoplasmic reticulum. Loading of exogenous antigens on class I molecules, e.g., in cross-priming, sometimes occurs, but the intracellular location where interaction between the antigenic fragment and class I takes place is unclear. Here we show that measles virus F protein can be presented by class I in transporters associated with antigen processing-independent, NH(4)Cl-sensitive manner, suggesting that class I molecules are able to interact and bind antigen in acidic compartments, like class II molecules. Studies on intracellular transport of green fluorescent protein-tagged class I molecules in living cells confirmed that a small fraction of class I molecules indeed enters classical MHC class II compartments (MIICs) and is transported in MIICs back to the plasma membrane. Fractionation studies show that class I complexes in MIICs contain peptides. The pH in MIIC (around 5.0) is such that efficient peptide exchange can occur. We thus present evidence for a pathway for class I loading that is shared with class II molecules.

Proteasome subunits encoded by the major histocompatibility complex are not essential for antigen presentation

Major histocompatibility complex (MHC) class I molecules bind and deliver peptides derived from endogenously synthesized proteins to the cell surface for survey by cytotoxic T lymphocytes. It is believed that endogenous antigens are generally degraded in the cytosol, the resulting peptides being translocated into the endoplasmic reticulum where they bind to MHC class I molecules. Transporters containing an ATP-binding cassette encoded by the MHC class II region seem to be responsible for this transport. Genes coding for two subunits of the '20S' proteasome (a multicatalytic proteinase) have been found in the vicinity of the two transporter genes in the MHC class II region, indicating that the proteasome could be the unknown proteolytic entity in the cytosol involved in the generation of MHC class I-binding peptides. By introducing rat genes encoding the MHC-linked transporters into a human cell line lacking both transporter and proteasome subunit genes, we show here that the MHC-encoded proteasome subunit are not essential for stable MHC class I surface expression, or for processing and presentation of antigenic peptides from influenza virus and an intracellular protein.

Multivesicular body morphogenesis requires phosphatidyl-inositol 3-kinase activity

Multivesicular bodies are endocytic compartments containing multiple small vesicles that originate from the invagination and 'pinching off' of the limiting membrane into the luminal space [1] [2] [3]. The molecular mechanisms responsible for the formation of these compartments are unknown. In the human melanoma cell line Mel JuSo, newly synthesised major histocompatibility complex (MHC) class II molecules accumulate in multivesicular early lysosomes [4]. The phosphatidylinositol (PI) 3-kinase inhibitor wortmannin induced the transient vacuolation of early MHC class II compartments, but also of early and late endosomes. We demonstrate that endocytic membrane influx is required for the wortmannin-induced swelling of vesicles. The wortmannin-induced vacuoles contained a reduced number of intraluminal vesicles that were linked to the limiting membrane by membraneous connections. These data suggest that wortmannin inhibits the invagination and/or pinching off of intraluminal vesicles and provide evidence of a role for PI 3-kinase in multivesicular body morphogenesis. We propose that the wortmannin-induced vacuolation occurs as a result of the inability of multivesicular bodies to store endocytosed membranes as intraluminal vesicles thereby causing the formation of large 'empty' vacuoles.

Direct vesicular transport of MHC class II molecules from lysosomal structures to the cell surface

Newly synthesized MHC class II molecules are sorted to lysosomal structures where peptide loading can occur. Beyond this point in biosynthesis, no MHC class II molecules have been detected at locations other than the cell surface. We studied this step in intracellular transport by visualizing MHC class II molecules in living cells. For this purpose we stably expressed a modified HLA-DR1 beta chain with the Green Fluorescent Protein (GFP) coupled to its cytoplasmic tail (beta-GFP) in class II-expressing Mel JuSo cells. This modification of the class II beta chain does not affect assembly, intracellular distribution, and peptide loading of the MHC class II complex. Transport of the class II/ beta-GFP chimera was studied in living cells at 37 degrees C. We visualize rapid movement of acidic class II/beta-GFP containing vesicles from lysosomal compartments to the plasma membrane and show that fusion of these vesicles with the plasma membrane occurs. Furthermore, we show that this transport route does not intersect the earlier endosomal pathway.

A single residue exchange within a viral CTL epitope alters proteasome-mediated degradation resulting in lack of antigen presentation

CTL epitope (KSPWFTTL) encoded by AKV/MCF type of murine leukemia virus (MuLV) differs from the sequence in Friend/Moloney/Rauscher (FMR) type in one residue (RSPWFTTL). CTL experiments indicated defective processing of the FMR peptide in tumor cells. Proteasome-mediated digestion of AKV/MCF-type 26-mer peptides resulted in the early generation and higher levels of epitope-containing fragments than digestion of FMR-type peptides, explained by prominent cleavage next to R in the FMR sequence. The fragments were identified as 10- and 11-mer peptides and were efficiently translocated by TAP. The naturally presented AKV/MCF peptide is the 8-mer, indicating ER peptide trimming. In conclusion, a single residue exchange can cause CTL epitope destruction by specific proteasomal cleavage.

TAP-translocated peptides specifically bind proteins in the endoplasmic reticulum, including gp96, protein disulfide isomerase and calreticulin

The endoplasmic reticulum (ER) membrane-embedded transporter associated with antigen processing (TAP) associates with peptides in the cytosol and translocates these into the ER lumen. Here, MHC class I molecules bind a subset of these peptides and the remainder is either removed or degraded, or may be retained in the ER in association with other proteins. We have visualized peptide-binding proteins in the ER using radioactive peptides with a photoreactive group. Besides TAP, two proteins were identified as gp96 and protein disulfide isomerase (PDI). Calreticulin, previously found in complex with TAP, only binds glycosylated peptides. In addition, two as yet unidentified, ER luminal glycoproteins (gp120 and gp170) were visualized. The effects of peptide size and sequence on binding to the ER-resident proteins were studied by using partially degenerated peptides with photoreactive side chains. All identified proteins were able to bind peptides within the size range of peptides translocated by TAP, from 8 to more than 20 amino acids. Whereas PDI associated with all peptides tested, gp96 and gp120 showed a clear sequence preference for non-charged amino acids at positions 2 and 9 in 9mer peptides. Thus various ER proteins, other than the MHC class I heterodimer and TAP, are able to interact with peptides albeit with a different substrate selectivity.

Trimming of TAP-translocated peptides in the endoplasmic reticulum and in the cytosol during recycling

Cytosolic peptides are translocated to the endoplasmic reticulum (ER) lumen by the transporters associated with antigen processing (TAP), where major histocompatibility complex (MHC) class I molecules associate with peptides of about 8-10 amino acids. TAP translocates peptides of 9-13 amino acids with the highest relative affinity but also longer and shorter peptides. The fate of the peptides that fail to associate with class I molecules because of incorrect sequence or length, is unknown. Here we show that the bulk of the translocated peptides are rapidly released from the ER by a mechanism that requires adenosine triphosphate (ATP) and that could not be inhibited by GTP gamma S. TAP does not appear to be involved in this process. Whereas free peptides are slowly trimmed in the ER lumen, they are rapidly degraded in the cytosol. A fraction of the peptides released from the ER escapes complete degradation in the cytosol and recycles back to the ER in a TAP-dependent fashion. These results suggest that peptides that are too long for binding to class I molecules in the ER can be trimmed further in the ER lumen or, alternatively, can be transported back to the cytosol where a fraction of the peptides is trimmed to a size suitable for association to MHC class I molecules and recycles back to the ER.

HLA-DO is a negative modulator of HLA-DM-mediated MHC class II peptide loading

BACKGROUND

Class II molecules of the major histocompatibility complex become loaded with antigenic peptides after dissociation of invariant chainderived peptides (CLIP) from the peptide-binding groove. The human leukocyte antigen (HLA)-DM is a prerequisite for this process, which takes place in specialised intracellular compartments. HLA-DM catalyses the peptide-exchange process, simultaneously functioning as a peptide 'editor', favouring the presentation of stably binding peptides. Recently, HLA-DO, an unconventional class II molecule, has been found associated with HLA-DM in B cells, yet its function has remained elusive.

RESULTS

The function of the HLA-DO complex was investigated by expression of both chains of the HLA-DO heterodimer (either alone or fused to green fluorescent protein) in human Mel JuSo cells. Expression of HLA-DO resulted in greatly enhanced surface expression of CLIP via HLA-DR3, the conversion of class II complexes to the SDS-unstable phenotype and reduced antigen presentation to T-cell clones. Analysis of peptides eluted from HLA-DR3 demonstrated that CLIP was the major peptide bound to class II in the HLA-DO transfectants. Peptide exchange assays in vitro revealed that HLA-DO functions directly at the level of class II peptide loading by inhibiting the catalytic action of HLA-DM.

CONCLUSIONS

HLA-DO is a negative modulator of HLA-DM. By stably associating with HLA-DM, the catalytic action of HLA-DM on class II peptide loading is inhibited. HLA-DO thus affects the peptide repertoire that is eventually presented to the immune system by MHC class II molecules.

Association between HLA-DM and HLA-DR in vivo

The exchange of HLA class II-associated invariant chain peptides (CLIP) for cognate peptide is catalyzed by HLA-DM under acidic conditions in vitro by an unknown mechanism. Here, we show an association between HLA-DM and HLA-DR in vivo by coprecipitation of the two heterodimers. The association is favored at low pH and in the nonionic detergent digitonin. Most DM-DR complexes are isolated from dense subcellular fractions. Recovery of HLA-DM by the conformation-dependent DR3 monoclonal antibody 16.23 suggests an association with HLA-DR heterodimers beyond the stage at which CLIP is released. The additional N-linked glycan on mutant DR3 molecules isolated from the 10.24.6 cell line, which interferes with DM-enhanced CLIP release from DR3 in vitro, also affects the DM-DR interaction.

Export of antigenic peptides from the endoplasmic reticulum intersects with retrograde protein translocation through the Sec61p channel

Antigenic peptides are translocated by the TAP peptide transporter from the cytosol into the endoplasmic reticulum (ER) for loading onto MHC class I molecules. Peptides that fail to bind need to be removed from the ER. Here we provide evidence that peptide export utilizes the Sec61p translocon as demonstrated by blocking this channel with bacterial exotoxin. Peptide export interferes with the retrotranslocation of beta2-microglobulin from the ER to the cytosol, suggesting similar pathways for the disposal of proteins and oligopeptides. Peptide export requires ATP supply to the ER lumen but is independent of ATP hydrolysis.

Point mutations in the alpha 2 domain of HLA-A2.1 define a functionally relevant interaction with TAP

BACKGROUND

Glycoproteins encoded by the major histocompatibility complex class I region (MHC class I) present peptide antigens to cytotoxic T cells (CTLs). Peptides are delivered to the site of MHC class I assembly by the transporter associated with antigen processing (TAP), and cell lines that lack this transporter are unable to present endogenous antigens to CTLs. Although it has been shown that a fraction of newly synthesized class I molecules are in physical association with TAP, it is not known whether this interaction is functionally relevant, or where on the class I molecule the TAP binding site might be.

RESULTS

C1R cells transfected with a mutant HLA-A2.1 heavy chain (HC), where threonine at position 134 in the alpha 2 domain is changed to lysine (T134K), are unable to present endogenous antigens to CTLs. We have studied the biochemistry of this mutant in C1R cells, and found that a large pool of unstable empty class I HC-beta 2m (beta-2 microglobulin) heterodimers exist that are rapidly transported to the cell surface. The T134K mutant seemed to bind peptide antigens and assemble with beta 2m as efficiently as wild-type HLA-A2.1. However, we show here that the inefficiency with which T134K presents intracellular antigen is associated with its inability to interact with the TAP heterodimer.

CONCLUSIONS

These experiments establish that the class I-TAP interaction is obligatory for the presentation of peptide epitopes delivered to the endoplasmic reticulum (ER) by TAP. Wild-type HLA-A2.1 molecules in TAP-deficient cells are retained in the ER, whereas T134K is rapidly released to the cell surface, but is unstable, suggesting a role for the TAP complex as an intracellular checkpoint that only affects the release of class I molecules with stably bound peptide ligands.

The proteasome-specific inhibitor lactacystin blocks presentation of cytotoxic T lymphocyte epitopes in human and murine cells

We describe the effect of the proteasome specific inhibitor lactacystin on the metabolic stability of influenza nucleoprotein (NP) and on the generation of antigens presented by human and murine class I molecules of the major histocompatibility complex to cytotoxic T lymphocytes (CTL). We show that cells treated with lactacystin fail to present influenza antigens to influenza-specific CTL, but retain the capacity to present defined epitopes expressed as peptides intracellularly by recombinant vaccinia viruses. This block in antigen presentation can be overcome by expressing the viral protein within the lumen of the endoplasmic reticulum, confirming the specificity of lactacystin for cytosolic proteases. We also show that the effect of lactacystin on antigen presentation correlates with the block of breakdown of a rapidly degraded form of the influenza NP linked to ubiquitin. These results demonstrate that proteasome-dependent degradation plays an important role in the cytosolic generation of CTL epitopes.

Presentation of cytosolic glycosylated peptides by human class I major histocompatibility complex molecules in vivo

Antigens presented by class I major histocompatibility complex (MHC) molecules for recognition by cytotoxic T lymphocytes consist of 8-10-amino-acid-long cytosolic peptides. It is not known whether posttranslationally modified peptides are also presented by class I MHC molecules in vivo. Many different posttranslational modifications occur on cytoplasmic proteins, including a cytosolic O-beta-linked glycosylation of serine and threonine residues with N-acetylglucosamine (GlcNAc). Using synthetic glycopeptides carrying the monosaccharide O-beta-GlcNAc substitution on serine residues, we have shown that glycopeptides bind efficiently to class I MHC molecules and elicit a glycopeptide-specific cytotoxic T lymphocyte response in mice. In this study, we provide evidence that peptides presented by human class I MHC molecules in vivo encompass a small, significant amount of glycopeptides, constituting up to 0.1% of total peptide. Furthermore, we find that carbohydrate structures present on glycopeptides isolated from class I MHC molecules are dominated by the cytosolic O-beta-GlcNAc substitution, and synthetic peptides carrying this substitution are efficiently transported by TAP (transporter associated with antigen presentation) into the endoplasmic reticulum. Thus, in addition to unmodified peptides, posttranslationally modified cytosolic peptides carrying O-beta-linked GlcNAc can be presented by class I MHC molecules to the immune system.

Analysis of the fine specificity of rat, mouse and human TAP peptide transporters

Prior to their association with major histocompatibility complex (MHC) class I molecules, peptides generated from cytosolic antigens need to be translocated by the MHC-encoded peptide transporter (TAP) into the lumen of the endoplasmic reticulum (ER). While class I molecules possess well-known binding characteristics for peptides, the fine specificity of TAP for its peptide substrates has not been analyzed in detail. Previously, we have studied the effect of amino acid variations at the N-terminal, the C-terminal, and the penultimate residue on the efficiency of peptide translocation. Using permeabilized cells, we have shown that TAP pre-selects peptides in an allele- and species-specific manner, for which only the C-terminal residue is crucial. This finding is confirmed in the present study by using microsomes containing different TAP. The influence of amino acid substitutions at positions 2 to 7 of 9-residue model peptides on TAP-dependent peptide translocation is systematically examined. Only a few amino acid substitutions at these positions affect the efficiency of peptide translocation significantly, e.g. Pro at position 2 or 3 negatively influences transport whereas Glu at positions 6 and 7 enhances transport. The differences in translocation by the rat TAP alleles a or u, mouse TAP and human TAP are, however, minor for the peptide with internal substitutions used in this study. These results show that the C-terminal residue essentially governs the species-specific substrate specificity of TAP.

Major histocompatibility complex class II molecules induce the formation of endocytic MIIC-like structures

During biosynthesis, major histochompatibility complex class II molecules are transported to the cell surface through a late endocytic multilaminar structure with lysosomal characteristics. This structure did not resemble any of the previously described endosomal compartments and was termed MIIC. We show here that continuous protein synthesis is required for the maintenance of MIIC in B cells. Transfection of class II molecules in human embryonal kidney cells induces the formation of multilaminar endocytic structures that are morphologically analogous to MIIC in B cells. Two lysosomal proteins (CD63 and lamp-1), which are expressed in MIIC of B cells, are also present in the structures induced by expression of major histocompatibility complex class II molecules. Moreover, endocytosed HRP enters the induced structures defining them as endocytic compartments. Exchanging the transmembrane and cytoplasmic tail of the class II alpha and beta chains for that of HLA-B27 does not result in the induction of multilaminar structures, and the chimeric class II molecules are now located in multivesicular structures. This suggests that expression of class II molecules is sufficient to induce the formation of characteristic MIIC-like multilaminar structures.

Modulation of the major histocompatibility complex class II-associated peptide repertoire by human histocompatibility leukocyte antigen (HLA)-DO

Antigen presentation by major histocompatibility complex class II molecules is essential for antibody production and T cell activation. For most class II alleles, peptide binding depends on the catalytic action of human histocompatibility leukocyte antigens (HLA)-DM. HLA-DO is selectively expressed in B cells and impedes the activity of DM, yet its physiological role remains unclear. Cell surface iodination assays and mass spectrometry of major histocompatibility complex class II-eluted peptides show that DO affects the antigenic peptide repertoire of class II. DO generates both quantitative and qualitative differences, and inhibits presentation of large-sized peptides. DO function was investigated under various pH conditions in in vitro peptide exchange assays and in antigen presentation assays using DO(-) and DO(+) transfectant cell lines as antigen-presenting cells, in which effective acidification of the endocytic pathway was prevented with bafilomycin A(1), an inhibitor of vacuolar ATPases. DO effectively inhibits antigen presentation of peptides that are loaded onto class II in endosomal compartments that are not very acidic. Thus, DO appears to be a unique, cell type-specific modulator mastering the class II-mediated immune response induced by B cells. DO may serve to increase the threshold for nonspecific B cell activation, restricting class II-peptide binding to late endosomal compartments, thereby affecting the peptide repertoire.

Opposing motor activities of dynein and kinesin determine retention and transport of MHC class II-containing compartments

MHC class II molecules exert their function at the cell surface by presenting to T cells antigenic fragments that are generated in the endosomal pathway. The class II molecules are targetted to early lysosomal structures, termed MIIC, where they interact with antigenic fragments and are subsequently transported to the cell surface. We previously visualised vesicular transport of MHC class II-containing early lysosomes from the microtubule organising centre (MTOC) region towards the cell surface in living cells. Here we show that the MIIC move bidirectionally in a ‘stop-and-go’ fashion. Overexpression of a motor head-deleted kinesin inhibited MIIC motility, showing that kinesin is the motor that drives its plus end transport towards the cell periphery. Cytoplasmic dynein mediates the return of vesicles to the MTOC area and effectively retains the vesicles at this location, as assessed by inactivation of dynein by overexpression of dynamitin. Our data suggest a retention mechanism that determines the perinuclear accumulation of MIIC, which is the result of dynein activity being superior over kinesin activity. The bidirectional nature of MIIC movement is the result of both kinesin and dynein acting reciprocally on the MIIC during its transport. The motors may be the ultimate targets of regulatory kinases since the protein kinase inhibitor staurosporine induces a massive release of lysosomal vesicles from the MTOC region that is morphologically similar to that observed after inactivation of the dynein motor.

CIIV, MIIC and other compartments for MHC class II loading

Virtually every endocytic compartment has been claimed to be an MIIC, a site where class II molecules accumulate. Here, it is argued that the definition of MIIC is not accurate and often pointless. MIIC can better be used as a working title for a collection of late endocytic compartments that contain the goodies necessary for efficient peptide loading of class II molecules.

Coronin is involved in uptake of Mycobacterium bovis BCG in human macrophages but not in phagosome maintenance

By applying density gradient electrophoresis (DGE) to human macrophages infected with Mycobacterium bovis BCG, we were able to separate three different bacterial fractions representing arrested phagosomes, phagolysosomes and mycobacterial clumps. After further purification of the phagosomal population, we found that isolated phagosomes containing live BCG were arrested in maturation as they exhibited only low amounts of the lysosomal glycoprotein LAMP-1 and processing of the lysosomal hydrolase cathepsin D was blocked. In addition, low amounts of MHC class I and class II molecules and the absence of HLA-DM suggest sequestration of mycobacterial phagosomes from antigen-processing pathways. We further investigated the involvement of the actin-binding protein coronin in intracellular survival of mycobacteria and showed that human coronin, as well as F-actin, were associated with early stages of mycobacterial phagocytosis but not with phagosome maintenance. Therefore, we conclude that the unique DGE migration pattern of arrested phagosomes is not as a result of retention of coronin, but that there are other proteins or lipids responsible for the block in maturation in human macrophages.

Identification of novel peptide binding proteins in the endoplasmic reticulum: ERp72, calnexin, and grp170

Transient interactions between molecular chaperones and nascent polypeptide chains assist protein folding in the endoplasmic reticulum. In an experimental setting that resembles the ER, we have used peptides as model substrates to identify and compare substrate specificities of ER-resident chaperones. The ER-located peptide transporter TAP was used to introduce peptides into the lumen of microsomes. In addition to PDI and gp96, previously identified as peptide-binding chaperones in the ER, we show that ERp72, calnexin, and grp170 interact with TAP-translocated peptides. The chaperones that have been identified can all bind peptide substrates that range from 8 to 40 amino acids in a manner independent of ATP. In addition, these chaperones exhibit broad and largely overlapping, however not identical, substrate selectivities. Our data indicate that peptide translocation into microsomes via TAP can be used as a method to monitor substrate selectivities of ER-resident chaperones. The implications of the observed preferences for chaperone-substrate interactions and for chaperones applied as vehicles in peptide-based vaccination strategies will be discussed.

Phosphorylation of the oestrogen receptor alpha at serine 305 and prediction of tamoxifen resistance in breast cancer

Phosphorylation of oestrogen receptor alpha at serine 305 (ERalphaS305-P) induces tamoxifen resistance in experimental studies, but does not influence response to other endocrine agents, such as fulvestrant. We evaluated ERalphaS305-P using immunohistochemistry in 377 breast carcinomas from premenopausal participants of a randomized trial (n=248) and patients with advanced disease (n=129). Among the premenopausal patients, adjuvant tamoxifen improved recurrence-free survival (RFS) for ERalphaS305-P-negative tumours (multivariate HR=0.53, 95% CI 0.32-0.86, p=0.010), but not for ERalphaS305-P-positive tumours (multivariate HR=1.01, 95% CI 0.33-3.05, p=0.99) (interaction p=0.131). Notably, ERalphaS305-P was not significantly associated with RFS in patients not treated with tamoxifen (multivariate HR=0.64, 95% CI 0.30-1.37, p=0.248), indicating that ERalphaS305-P is a marker for treatment outcome rather than tumour progression. Given the direct experimental link between ERalphaS305-P and tamoxifen resistance and these first clinical data suggesting that premenopausal patients with ERalphaS305-P-positive breast cancer are resistant to adjuvant tamoxifen, further research is encouraged to study whether alternative endocrine treatment should be considered for this subgroup.