(Received for publication, September 22, 1995; and in revised form, November 27, 1995)
From the
In mammalian cells, resident luminal and type I transmembrane proteins of the endoplasmic reticulum usually contain KDEL and KKXX at the carboxyl terminus. These sequences induce retrieval from compartments located downstream in the secretory pathway. It has been suggested that the retrieval may occur from multiple sites, ranging from the intermediate compartment to the trans-Golgi network. To compare the retrieval of luminal and type I membrane proteins, we have used different forms of a single reporter, the human CD8 glycoprotein, stably expressed in FRT cells. Metabolic labeling and oligosaccharide analysis show that the mechanism based on the KDEL signal is leaky. With time, the KDEL-containing CD8 form reaches the trans/trans-Golgi network compartments, where the protein is terminally glycosylated. At this stage, the retrieval mechanism stops being effective and the protein is consequently secreted. Conversely, the mechanism based on the KKXX signal guarantees that most of the KKXX-containing CD8 form resides in the endoplasmic reticulum, little in the Golgi complex and undetectable levels at the plasma membrane. The O-glycosylation of this protein comprises for the vast majority the sole addition of peptide-bound GalNAc that occurs in an early Golgi compartment.
Many endoplasmic reticulum, luminal and type I transmembrane
proteins (carboxyl terminus in the cytosol) contain carboxyl-terminal
sequences of the prototype KDEL and KKXX, respectively (Munro
and Pelham, 1987; Nilsson et al., 1989). These sequences are
necessary and sufficient to determine ER ()localization of
modified secretory and plasma membrane proteins (Pelham, 1988; Jackson et al., 1990). This localization is achieved by retrieval
mechanisms, understood only in part. Luminal as well as membrane
reporter proteins show Golgi post-translational modifications (Pelham,
1988; Pelham et al., 1988; Dean and Pelham, 1990; Jackson et al., 1993; Gaynor et al., 1994; Townsley and
Pelham, 1994); a receptor for KDEL-containing proteins, the ERD 2
transmembrane protein, has been identified and localized in the Golgi
complex of yeast and mammalian cells (Semenza et al., 1990;
Lewis et al., 1990; Lewis and Pelham, 1990; Tang et
al., 1993; Griffiths et al., 1994). Binding to the ligand
would induce receptor oligomerization and recycling to the ER, where
the ligand will be discharged, while the monomeric receptor would
return to the Golgi (Lewis and Pelham, 1992; Townsley et al.,
1993). Less is known about the retrieval of membrane proteins. In
vitro, the KKXX sequence binds the coatomer protein
complex (Cosson and Letourneur, 1994), and triggers microtubule
polymerization (Dahllöf et al., 1991).
Yeast coatomer mutants show defective retrieval (Letourneur et
al., 1994). How these interesting findings relate to a retrieval
function remains to be determined. However, the importance of the
cytosolic location of the signal for transmembrane proteins is
supported by the observation that some type II proteins (amino terminus
in the cytosol), contain an amino-terminal double arginine motif that
maintains them in the ER (Schutze et al., 1994).
Current evidence favors the hypothesis that retrieval of luminal and membrane proteins resident in the ER from post ER compartments represents a salvage mechanism for proteins erroneously included in the membrane flow of the secretory pathway (Munro and Pelham, 1987; Mazzarella et al., 1990; Jackson et al., 1993; Pelham and Munro, 1993; Nilsson and Warren, 1994). Resident proteins would be largely excluded from vesicular export from this organelle because their high local concentration favors intermolecular interactions or the presence of retention signals (Sönnichsen et al., 1994).
A number of studies suggests that retrieval may occur from multiple locations, ranging from the IC to the TGN (Pelham 1991; Peter et al., 1992; Jackson et al., 1993; Griffiths et al., 1994; Miesenböck and Rothman, 1995). However, it is difficult to generalize these data and to compare the retrieval of KDEL- and KKXX-containing reporter proteins. Either different proteins have been used in several host cell lines, or transient transfection experiments have been performed, thus greatly limiting the biochemical and morphological analysis. To overcome these problems, we decided to study a single reporter protein, not naturally resident in the ER, in stably transfected clones expressing comparable levels of its modified versions. We chose the human CD8 protein, a uniquely O-glycosylated type I membrane protein, and the FRT cell line (Littman et al., 1985; Nitsch et al., 1985). CD8 has been extensively used to define the sequence of the KKXX signal (Jackson et al., 1990), and the retrieval of KKXX- and KDEL-containing proteins in HeLa and NIH 3T3 cells (Jackson et al., 1993). In addition, much information has been obtained on CD8 structure (Leahy et al., 1992), and on its biosynthesis and O-glycosylation in FRT cells (Pascale et al., 1992a; Pascale et al., 1992b). O-Glycosylation is a relatively simple multistep process, compartmentalized in the Golgi complex (Roth, 1987; Perez-Vilar et al., 1991; Pascale et al., 1992b). Thus, the analysis of the carbohydrate moiety carried by the different reporter protein forms would contribute significantly to reveal the extent of their penetration in the Golgi complex.
Figure 1:
Schematic drawing of the different
CD8 forms used. Human CD8 (white bar) has 162 amino acids in
the ectodomain (114 in the NH-terminal immunoglobulin
domain and 48 in the linker), followed by 26 residues in the
transmembrane and in the cytosolic domains (Littman et al.,
1985). The cytosolic domain of CD8-E19 protein (black bar)
consists of the 15 residues of the corresponding domain of E19 protein,
the last 4 amino acids being KKMP (Nilsson et al., 1989).
CD8-S has the first 146 residues of CD8 ectodomain (Leahy et
al., 1992). CD8-K consists of the first 159 residues of CD8
ectodomain followed by SEKDEL (Jackson et al.,
1993).
Throughout this work, the CD8 forms bearing the KDEL and KKXX sequences have been compared to the 146-residue anchorless form (CD8-S) and to the wild-type transmembrane form of CD8, respectively. Apart from the localization signals, these two control proteins also differ slightly from their tagged counterpart (see Fig. 1). However, previous experiments have shown that the anchorless form of CD8, which matches CD8-K but lacks the KDEL sequence, and the CD8-E19 form mutagenized in either one of the two crucial lysine residues are glycosylated and transported in the same way as the shorter CD8-S and unmodified CD8 used in this study (Jackson et al., 1990, 1993). We chose the latter forms, due to the extensive information available concerning their structure, biosynthesis, glycosylation, and intracellular transport (Leahy et al., 1992; Pascale et al. 1992a, 1992b).
Figure 2: Intracellular localization of CD8-S and CD8-K proteins. Cells were treated for double indirect immunofluorescence light microscopy as indicated under ``Experimental Procedures.'' a and b, CD8-S-expressing cells; c-f, CD8-K-expressing cells. a, c, and e, anti-CD8 OKT8 mAb; b and d, anti-mannosidase II polyclonal antibody; f, anti-SSRP polyclonal antibody.
Figure 3: Intracellular localization of CD8-E19 and CD8 proteins. a-d, CD8-E19-expressing cells; e and f, CD8-expressing cells. a, c, and e, anti-CD8 OKT8 mAb; b and f, anti-mannosidase II polyclonal antibody; d, anti-SSRp polyclonal antibody.
Figure 4: CD8-E19 is not present on the surface of the cell. a-d, CD8-E19-expressing cells. a and b, permeabilized cells; c and d, non-permeabilized cells. a and c, anti-CD8 OKT8 mAb; b and d, anti-A. hypogea polyclonal antibody. To detect Gal exposing glycans (Goldstein and Hayes, 1978), cells were incubated for 20 min with 1 mg/ml A. hypogea before exposure to the primary and secondary antibodies.
The markers of the ER and Golgi complex gave the same results when tested in the parental cell line or in the transfected clones (data not shown). However, it has been reported that overexpression of KDEL-bearing proteins may result in the transfer of the KDEL receptor from the Golgi complex to the ER (Lewis and Pelham, 1992). Thus we controlled the distribution of the mammalian equivalent of yeast ERD-2 receptor, the p23 protein (Tang et al., 1993), in FRT-, CD8-K-, and CD8-E19-expressing cells. In all cell lines, p23 was mostly localized in the Golgi complex (data not shown).
On the basis of this information, we
first performed pulse-chase analysis on the clones expressing CD8-S and
CD8-K protein. As shown in Fig. 5A,
[S]cysteine labeling revealed that CD8-S is
synthesized as a 19-kDa precursor that is converted to a 29-kDa mature
form. Intermediate forms in the maturation are detected at low level,
and only the mature form is secreted. Conversely, CD8-K is synthesized
initially as a 20-kDa form (Fig. 5B), which is slowly
and progressively converted to several intermediate forms; little of
the 30-kDa mature form is detectable intracellularly at all chase time
points, but it is clearly present in the extracellular medium (Fig. 5B). Densitometric analysis of the autoradiograms
of several pulse-chase experiments showed that the half-times of the
disappearance of newly synthesized form, and of secretion, were
approximately 1 and 2 h for CD8-S, and 16 and 20 h for CD8-K,
respectively (data not shown). These results suggested that the delay
in processing and secretion time of CD8-K is not proportionally
distributed, but more severe for the early events of the secretory
pathway.
Figure 5:
Biosynthesis and secretion of CD8-S and
CD8-K proteins. SDS-PAGE analysis of the immunoprecipitated products
after pulse-chase labeling with [S]cysteine. A, CD8-S-expressing cells, pulse time 20 min. B,
CD8-K-expressing cells, pulse time 1 h. The decrease in the amount of
the mature form of CD8-K in the extracellular medium after 48 h of
chase is a random event. Numbers on the bottom indicate the chase times in h. Numbers on the left indicate the position on the same gel of a 30-kDa marker protein.
Only the relevant portion of the gel is
shown.
To better evaluate this hypothesis, we moved to labeling
experiments with [H]GlcN, which is incorporated
in O-glycans as GalNAc, GlcNAc, and NeuNAc, and
[
H]Gal. As shown in Fig. 6, only the
intermediate and mature forms were visualized. The intracellular mature
forms were labeled with both sugar precursors, the intermediate forms
of CD8-S were detectable with [
H]GlcN but not
with [
H]Gal, and the intermediate forms of CD8-K
were labeled anyhow, but [
H]GlcN revealed more
discrete bands. Finally, both labeling conditions confirmed that only
the mature forms of CD8-S and CD8-K were secreted. Next, we performed
pulse-chase labeling experiments with these sugar precursors. With
[
H]Gal, the mature forms of both CD8-S and CD8-K
were quickly secreted, whereas the intermediate forms of CD8-K lasted
intracellularly for long time (data not shown). To amplify this result,
we labeled CD8-K-expressing cells with both
[
H]GlcN and [
H]Gal. As
shown in Fig. 7(panels A and B), the mature
form of CD8-K moved quickly from the cell to the medium (some mature
form is already secreted in the pulse time). Conversely, the
intermediate forms lasted even 48 h after the pulse, showing a slow
decrease in electrophoretic mobility (Fig. 7A). That
these intermediates do not represent a dead end product is shown in Fig. 7C. The bulk of secretion of the mature form did
occur during the first 6 h of chase, but significant secretion took
place up to 48 h. Clearly, new mature form is generated from the
intermediate forms during the chase, and it is not detectable
intracellularly because it is rapidly secreted.
Figure 6:
Intracellular accumulation of the
initially glycosylated, intermediate forms of CD8-K. SDS-PAGE analysis
of the immunoprecipitated products after 3 h of labeling with
[H]GlcN (lanes 1, 2, 5, and 6) or [
H]Gal (lanes
3, 4, 7, and 8) from CD8-S- and
CD8-K-expressing cells. Lanes 1, 3, 5, and 7, intracellular products; lanes 2, 4, 6, and 8, extracellular products. Brackets on the left indicate the initially glycosylated,
intermediate forms of CD8-S and CD8-K. Numbers on the left indicate the position on the same gel of a 30-kDa marker protein.
Only the relevant portion of the gel is
shown.
Figure 7:
Delayed terminal glycosylation, but
subsequent rapid secretion, of CD8-K protein. SDS-PAGE analysis of the
immunoprecipitated products after pulse-chase labeling with both
[H]GlcN and [
H]Gal of
CD8-K-expressing cells. A, intracellular products. B,
extracellular products. Pulse time 1 h; numbers indicate the hours of
chase. C, the extracellular products that accumulate in the
following time intervals (in h) after 1 h of labeling: 0-6 (lane 1), 6-12 (lane 2), 12-24 (lane
3), 24-36 (lane 4), 36-48 (lane 5).
In lane 6, the intracellular products after 48 h of chase. Numbers on the left indicate the position on the same
gel of a 30-kDa marker protein. Only the relevant portion of the gel is
shown.
In conclusion, the
labeling experiments performed with [S]cysteine
or [
H]sugars indicated that processing and
secretion of CD8-K is much slower than CD8-S. In addition, it appeared
that most of the delay was due to a slow arrival of newly synthesized
CD8-K to the late Golgi compartments.
Next we performed pulse-chase
labeling experiments on CD8-E19-expressing cells. As shown in Fig. 8, CD8-E19 is initially synthesized as a 24/25-kDa doublet
form, which is slowly converted to a discrete 28-kDa form. The doublet
does not represent an initial glycosylation step (see below), but it is
due to an unknown reversible modification (Jackson et al.,
1993). In spite of the low increment of the apparent molecular mass,
the 28-kDa form was the final maturation product of CD8-E19 visualized
by long labeling with [H]GlcN and less
efficiently with [
H]Gal (compare lanes 7 and 8 of Fig. 8), and the only one detectable by
Western blot analysis (Fig. 8, lane 9). As expected,
CD8-E19 was not found in the extracellular medium (data not shown).
Interestingly, very prolonged chase time showed that the 28-kDa form is
remarkably stable, having a half-time for degradation greater than 3
days. All together, these results strongly suggested that CD8-E19
undergoes a slow and only partial maturation process.
Figure 8:
Biosynthesis and partial maturation of
CD8-E19 protein. SDS-PAGE analysis of the immunoprecipitated products
after pulse-chase labeling of CD8-E19-expressing cells with
[S]cysteine. Pulse time 20 min (lane
1); chase times of 1, 2, 4, 6, and 24 h (lanes 2-6,
respectively). In lanes 7 and 8, the
immunoprecipitated products after 16 h of labeling with
[
H]GlcN or 16 h labeling with
[
H]Gal. In lane 9, the intracellular
products detected by Western blot analysis. The number on the left indicates the position on the same gel of a 30-kDa marker
protein. Only the relevant portion of the gel is
shown.
We decided to characterize the carbohydrate chains of the intermediate and mature forms of all CD8-derived proteins to ascertain if the modified peptide moiety changed the general pattern of O-glycosylation, and at which stage the glycan processing was arrested by the KDEL and KKXX motifs with a consequent intracellular accumulation of forms that are not fully processed.
As
mentioned before, the branching by GlcNAc represents a crucial step in
the O-glycan processing, which very likely coincides with the
routing to a late compartment of the Golgi complex. Therefore, we
compared the amounts of [H]GlcNAc and
[
H]GalNAc incorporated in the intermediate and
mature forms of CD8, CD8-S and CD8-K, and the single glycosylated of
form of CD8-E19. As expected, the ratio between
[
H]GlcNAc and [
H]GalNAc was
very low in the intermediate forms and much higher in the mature forms,
indicating that the branching does parallel the maturation process (Table 1). The relative proportion of
[
H]GlcNAc in CD8i and CD8-Si was the same,
whereas less [
H]GlcNAc was found in CD8-Ki and
even less in CD8-E19. To characterize the carbohydrate carried by
CD8-Ki and CD8-E19, the reduced O-glycans, generated by mild
alkaline borohydryde treatment of Pronase-glycopeptides, were analyzed
by HPLC. Table 2shows that in both cases the predominant
carbohydrate was, by far, non-elongated GalNAc; it represented 85 and
67% of the [
H]GlcNAc-derived radioactivity
incorporated in CD8-E19 and CD8-Ki, respectively. Very similar results
have been obtained with a separate preparation. The relative
heterogeneity in the elongated O-glycans of CD8-Ki and CD8-E19
may account for their migration pattern, i.e. for the discrete
bands visualized by SDS-PAGE (see Fig. 7and Fig. 8).
Finally, we measured the relative amounts of neutral versus sialylated mature forms. No significant difference was found in the degree of sialylation of mature forms of CD8, CD8-S and CD8-K, whereas exclusively neutral glycans were found in CD8-E19, as well as sialic acid (data not shown).
On the basis of these results, we concluded that (i) the O-glycosylation process of CD8-S and CD8-K is similar to that of CD8, despite the difference in the intracellular transit rate; (ii) the intracellular accumulation of CD8-Ki corresponds to the arrest of O-glycosylation at a stage in which the peptide-bound GalNAc is predominantly non elongated (this arrest is not complete, so that CD8-Ki is eventually processed to CD8-Km by the addition of GlcNAc and Gal and by sialylation); (iii) the O-glycosylation of CD8-E19 is more drastically restricted, such that the vast majority of O-linked carbohydrate on CD8-E19 consists of a single GalNAc residue, with only traces of branched core-2 glycans. Remarkably, no sialylated glycans were found.
The main outcome of this study is summarized in Fig. 9. The retrieval mechanism based on the KDEL signal is leaky. With time, CD8-K reaches the trans-Golgi/TGN region, from where retrieval is no longer effective. As a result, the protein is secreted as a fully glycosylated form. Conversely, the mechanism based on the KKXX signal is ``leak-proof''; it retains the bulk of CD8-E19 in the ER as a partially processed glycoprotein. The principal results in support of our conclusions are as follows. (a) The mature form of CD8-K is secreted with a 10-fold delay with respect to CD8-S. Almost all the delay is due to events occurring before CD8-K arrives in the trans-Golgi/TGN region. Both the unglycosylated and the initially glycosylated forms of CD8-K are intracellularly located longer than the respective CD8-S forms. Eventually, CD8-K is partially routed out of the ER/cis-medial Golgi compartments; it is terminally glycosylated in the trans/TGN Golgi region similarly to unmodified CD8 and CD8-S, and is rapidly secreted. Indeed, CD8-K and CD8-S proteins isolated from cell media carry as major O-glycan the same core-2 branched tetrasaccharide found in unmodified CD8 (Serafini-Cessi et al., 1995). (b) CD8-E19 accumulates mostly in the ER, but carries nascent O-linked GalNAc acquired in the cis/medial Golgi compartments. The typical core-2 branching structure of CD8 O-glycans manufactured by FRT cells is practically absent in the intracellularly accumulated CD8-E19. This protein is not detectable at the cell surface, although it lasts several days within the cell, whereas unmodified CD8 is terminally glycosylated and reaches the plasma membrane with a half-time of 30 min (Pascale et al., 1992a).
Figure 9: Schematic drawing of the intracellular routing of CD8-K and CD8-E19. Rightward pointing black arrows indicate anterograde transport; leftward pointing white arrows indicate retrograde transport toward the ER. Question marks hypothesize transport steps of the two reporter proteins that are not demonstrated by the present work. The size of the arrows reflects the balance of anterograde versus retrograde movement of the proteins.
Several other observations reinforce these results. The clones analyzed in detail were indistinguishable from the parental cells for all the criteria examined (doubling time, expression of epithelial phenotype, ER and Golgi complex ultrastructure, distribution of marker proteins, Golgi localization of the p23 KDEL receptor). Furthermore, different clones confirmed the slow secretion of CD8-K and the tight ER retention of CD8-E19. Thus, we had no indication that the expression of significant amounts of the different CD8 forms altered the normal physiology or the retrieval functions of FRT cells.
Numerous reports confirm part of the results we have obtained and largely support their general interpretation. Transient transfection experiments revealed a delayed secretion of KDEL-containing rat growth hormone and human chorionic gonadotrophin in COS and HeLa cells (Zagouras and Rose, 1989). Similarly, a small amount of CD8-K protein is secreted at the end of a 24-h chase in transiently transfected HeLa cells (Jackson et al., 1993). Some KDEL-containing proteins are secreted in exocrine pancreatic cells (Takemoto et al., 1992). The distribution in the Golgi complex of the p23 KDEL receptor has been studied by immunoelectron microscopy. Under physiological conditions, the receptor was mostly present in the IC and in the cis- and medial Golgi; under stress conditions a shift to the trans/TGN regions was observed (Griffiths et al., 1994). None of these data contradict the hypothesis that the efficiency of the retrieval process for KDEL-containing luminal protein decreases from the cis/medial to the trans/TGN region of the Golgi complex. This effect could be due to a decreasing concentration gradient of the receptor, or to a decreased affinity of ligand binding. Minor changes in the luminal enviroment of the Golgi compartments may affect the affinity of binding.
Less is known about KKXX-containing membrane proteins. CD8-E19 and CD4-E19 transiently transfected in HeLa and NIH 3T3 cells appeared localized mainly in the ER by immunofluorescence microscopy (Jackson et al., 1990). By cell fractionation of transiently transfected HeLa cells, the bulk of sialylated as well as only GalNAc modified CD8-E19 was seen cosedimenting with an ER marker (Jackson et al., 1993). This finding suggests retrieval from both the cis- and the trans-Golgi compartment. We did not detect sialylation of stably expressed CD8-E19, and the amount of fully galactosylated oligosaccharide chains was very limited. Our data are then insufficient to indicate the routing of CD8-E19 to the trans/trans-Golgi network area (see below). An immunoelectron microscopical analysis of CD8-E19-expressing cells could provide the answer to this question. In conclusion, the evidence available suggests that KKXX-containing reporter proteins are restricted to the ER by means of an efficient retrieval mechanism. This efficiency could be the result of a cytosol-based mechanism, either peculiarly able to operate from the cis/medial Golgi, as our data may indicate, or equally able to interact with KKXX-exposing proteins in all Golgi compartments.
It could be argued that the results obtained with CD8-K and CD8-E19 are due to a severe impairment in the folding and oligomerization of the newly synthesized proteins. Indeed, it is well known that only properly folded/oligomerized secretory and plasma membrane proteins are exported from the ER (Doms et al., 1993; Helenius, 1994), but once exported they proceed toward the surface of the cell without other identified ``quality control'' steps. Clearly, this hypothesis would explain only the delay in the initial glycosylation events of CD8-K and CD8-E19, not the behavior of CD8-K intermediate forms or the accumulation in the ER of glycosylated CD8-E19. In newly synthesized CD8, proper folding can be easily monitored from the expression of the epitope recognized by OKT8 mAb (a conformation-specific antibody), and by the formation of disulfide-bond homodimers (Verde et al., 1995). Newly synthesized CD8-K and CD8-E19 (as well as CD8-S) are detected by OKT8 mAb, and we have evidence that CD8-E19 and CD8-S rapidly form disulfide-linked homodimers (data not shown). It is difficult to ascertain the oligomerization status of CD8-K. The entire anchorless ectodomain of CD8 forms homodimers not linked by disulfide bridge, even in the absence of the KDEL sequence (Leahy et al., 1992). Preliminary results obtained by FPLC analysis showed that the secreted form of CD8-K was homodimeric, whereas part of the intracellular forms migrated as multimeric complexes. Interestingly, it has been suggested that occupied KDEL receptors oligomerize, forming protein patches that migrated to the ER (Townsley et al., 1993).
It has been suggested that the IC between ER and Golgi complex is a site of retrieval to the ER (Lewis and Pelham, 1990; Jackson et al., 1993; Griffiths et al., 1994). This hypothesis can not be tested by biochemical assays because of the lack of a specific post-translational modification marker for the IC. The immunofluorescence pattern we observed with CD8-K, p23 KDEL receptor, and CD8-E19 do not exclude their presence in the IC. However, these patterns are largely different from the ones given by proteins enriched in the IC (Saraste and Svensson, 1991; Schweizer et al., 1990; Schweizer et al., 1993; Lotti et al., 1992). We have observed that newly synthesized CD8-K and CD8-E19, in contrast to CD8-S and unmodified CD8, require several hours to undergo the initial O-glycosylation events. There is compelling evidence that these events occur in the cis/medial Golgi compartments (Roth et al., 1994; Piller et al., 1989; Verde et al., 1995; Vassard et al., 1995). This finding may thus support the view of efficient retrieval occurring upstream of these compartments. A cell fractionation-based approach could help to resolve this issue.