Deficiency of microsomal glucose-6-phosphatase
(G6Pase), the key enzyme in glucose homeostasis, causes glycogen
storage disease type 1a, an autosomal recessive disorder.
Characterization of the transmembrane topology of G6Pase should
facilitate the identification of amino acid residues contributing to
the active site and broaden our understanding of the effects of
mutations that cause glycogen storage disease type 1a. Using N- and
C-terminal tagged G6Pase, we show that in intact microsomes, the N
terminus is resistant to protease digestion, whereas the C terminus is
sensitive to such treatment. Our results demonstrate that G6Pase
possesses an odd number of transmembrane helices, with its N and C
termini facing the endoplasmic reticulum lumen and the cytoplasm,
respectively. During catalysis, a phosphoryl-enzyme intermediate is
formed, and the phosphoryl acceptor in G6Pase is a His residue.
Sequence alignment suggests that mammalian G6Pases, lipid phosphatases, acid phosphatases, and a vanadium-containing chloroperoxidase (whose
tertiary structure is known) share a conserved phosphatase motif.
Active-site alignment of the vanadium-containing chloroperoxidase and
G6Pases predicts that Arg-83, His-119, and His-176 in G6Pase contribute
to the active site and that His-176 is the residue that covalently
binds the phosphoryl moiety during catalysis. This alignment also
predicts that Arg-83, His-119, and His-176 reside on the same side of
the endoplasmic reticulum membrane, which is supported by the recently
predicted nine-transmembrane helical model for G6Pase. We have
previously shown that Arg-83 is involved in positioning the phosphate
during catalysis and that His-119 is essential for G6Pase activity.
Here we demonstrate that substitution of His-176 with structurally
similar or dissimilar amino acids inactivates the enzyme, suggesting
that His-176 could be the phosphoryl acceptor in G6Pase during
catalysis.
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INTRODUCTION |
Glucose-6-phosphatase
(G6Pase1; EC 3.1.3.9), which
catalyzes the terminal step in gluconeogenesis and glycogenolysis, is the key enzyme in glucose homeostasis (1). In humans, deficiency in
microsomal G6Pase causes glycogen storage disease type 1a (GSD-1a), also known as von Gierke's disease (2). It is an autosomal recessive
disorder with clinical manifestations of severe hypoglycemia, growth
retardation, hepatomegaly, kidney enlargement, hyperlipidemia, hyperuricemia, and lactic acidemia (2, 3). G6Pase is tightly associated
with the endoplasmic reticulum (ER) membranes (1), and enzymatic
activity in intact hepatic microsomes is resistant to limited
proteolysis, suggesting that the active site of G6Pase is not exposed
to the cytoplasm (4-7). To understand the biology and pathophysiology
of GSD-1a, we have characterized the murine (8) and human (9)
G6Pase cDNAs and genes and showed that mammalian G6Pases
are hydrophobic proteins of 357 amino acids. Analysis of the hydropathy
profiles by the algorithm used in the PC/Gene program (10) predicted
that mammalian G6Pases are anchored in the ER membrane by six putative
transmembrane helices (8, 9). During catalysis, a phosphoryl moiety is
transferred from Glu-6-P to a His residue in G6Pase, forming a
phosphoryl-enzyme intermediate (11-13). Structure-function studies
suggest that Arg-83 in G6Pase is involved in stabilizing the
phosphoryl-enzyme intermediate formed during catalysis (14). These
studies also show that His-119 is essential for G6Pase activity,
suggesting that this His residue could be the phosphoryl acceptor
(14).
Recently, the tertiary structure of a vanadium-containing
chloroperoxidase from the fungus Curvularia inaequalis has
been determined (15). Vanadate, which is a strong competitive inhibitor for G6Pase (16), is structurally similar to phosphate. Moreover, apochloroperoxidase can function as a phosphatase (17). Sequence analysis indicates that the amino acids contributing to the active site
of the vanadium-containing chloroperoxidase are also conserved in lipid
phosphatases, acid phosphatases, and mammalian G6Pases (17, 18), even
though their overall amino acid identities are very low. Therefore, the
vanadium-containing chloroperoxidase structure could provide clues to
the structure of G6Pase. Alignment of the active-site residues of the
vanadium-containing chloroperoxidase with the proposed
phosphate-binding site of G6Pase (19) supports our earlier proposal
(14) that Arg-83 is a candidate for positioning the phosphoryl moiety
during catalysis. However, this alignment predicts that His-176 is the
residue that covalently binds the phosphoryl moiety (19), not His-119
as suggested earlier (14). If His-176 is the phosphoryl acceptor, the
six-transmembrane helical model of G6Pase must be re-evaluated because
it places His-176 on the opposite side of the ER membrane from Arg-83
and His-119 (14). The hydropathy profiles analyzed by a newly developed algorithm (20) predict that G6Pase contains nine transmembrane helices,
which would place Arg-83, His-119, and His-176 on the same side of the
ER membrane (19).
The number of transmembrane helices dictates the luminal or cytoplasmic
location of the N or C terminus of G6Pase. The nine-transmembrane helical model predicts that only one terminus of G6Pase faces the
cytoplasm, which will be sensitive to protease digestion. On the other
hand, the six-transmembrane helical model predicts that both the N and
C termini of G6Pase face either the ER lumen or the cytoplasm, and
thus, both termini should have the same sensitivity to protease
digestion. In this study, we performed protease protection assays using
N- and C-terminal tagged G6Pase and showed that G6Pase contains an odd
number of transmembrane helices, with its N terminus facing the ER
lumen and its C terminus facing the cytoplasm. To examine the role of
His-176 in G6Pase during catalysis, we substituted codon 176 with amino
acids of different structures by site-directed mutagenesis and analyzed G6Pase activity after transient expression of wild-type (WT) and mutant
G6Pase cDNAs in COS-1 cells.
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MATERIALS AND METHODS |
Generation of Mutant G6Pase Constructs--
The
phG6Pase-DraIII construct, which contains an additional
DraIII site at nucleotides 614-622 but retains the primary
amino acid sequence of WT human G6Pase and exhibits WT enzymatic
activity (14), was used as a template for mutant construction by
polymerase chain reaction. The eight-amino acid FLAG marker peptide
DYKDDDDK (Kodak Scientific Imaging Systems) was used to tag the N and C termini of G6Pase. The 5'-primer for the N-terminal FLAG-tagged G6Pase
(G6Pase-5'FLAG) contained an ATG initiation codon followed by the
24-base pair FLAG coding sequence (5'-GACTACAAGGACGACGATGACAAG-3') and
nucleotides 80-98 of human G6Pase (9); the 3'-primer
contained nucleotides 625 to 602 (I-2) of human
G6Pase-DraIII (14). The amplified fragment was
ligated into the pSVLhG6Pase-DraIII 3'-fragment (14). The
5'-primer for C-terminal FLAG-tagged G6Pase (G6Pase-3'FLAG) contained
nucleotides 611-634 (I-1) of human G6Pase-DraIII
(14); the 3'-primer contained the last coding nucleotides (1150 to
1133) of human G6Pase (9) followed by the 24-base pair FLAG
coding sequence and a termination codon. The amplified fragments were ligated into the pSVLhG6Pase-DraIII 5'-fragment (14).
The 5'-primer for G6Pase-(5-357) and G6Pase-(5-357)-3'FLAG, with an
N-terminal deletion of residues 1-4, contained nucleotides 92-110 of
human G6Pase, and the ATG initiation codon was provided by
nucleotides 92-94 of human G6Pase. The 5'-primer for
G6Pase-(14-357) and G6Pase-(14-357)-3'FLAG, with an N-terminal
deletion of residues 1-13, contained an ATG initiation codon followed
by nucleotides 119-139 of human G6Pase. The 3'-primer for
G6Pase-(5-357) and G6Pase-(14-357) is I-2, and after polymerase chain
reaction, the amplified fragment was ligated into the
pSVLhG6Pase-DraIII 3'-fragment. The 3'-primer for
G6Pase-(5-357)-3'FLAG and G6Pase-(14-357)-3'FLAG contained the last
coding nucleotides (1150 to 1133) of human G6Pase followed
by the 24-base pair FLAG coding sequence and a termination codon. After
polymerase chain reaction, the amplified fragment was ligated into the
pSVL vector (Pharmacia Biotech Inc.).
The two outside polymerase chain reaction primers for codon 176 (nucleotides 605-607) mutants are nucleotides 77-96 (sense) and
nucleotides 1130-1156 (antisense) of human G6Pase (9). Codon 176 (CAT) mutant primers (nucleotides 598-616) are as follows: H176A, GCT; H176I, ATT; H176K,
AAG; H176M, ATG; H176N,
AAT, H176S, TCT; and H176R, CGT. The
amplified fragments were ligated into the pSVL vector. All constructs
were verified by DNA sequencing.
Expression in COS-1 Cells and Northern Blot and Western Blot
Hybridization Analyses--
COS-1 cells were grown at 37 °C in
HEPES-buffered Dulbecco's modified minimal essential medium
supplemented with streptomycin, penicillin, and 4% fetal bovine serum.
The G6Pase construct in a pSVL vector was transfected into
COS-1 cells by the DEAE-dextran/chloroquine method (21). Mock
transfections of COS-1 cells with the pSVL vector alone were used as
controls. After incubation at 37 °C for 3 days, the transfected
cultures were either harvested for G6Pase assays and Western blot
analysis or lysed for RNA isolation.
RNA was isolated by the guanidinium thiocyanate/CsCl method (22),
separated by electrophoresis on 1.2% agarose gels containing 2.2 M formaldehyde, and transferred to Nytran membranes
(Schleicher & Schuell). The filters were hybridized at 42 °C in the
presence of the phG6Pase-1 probe as described previously (9).
For Western blot analysis of tagged G6Pase, microsomal proteins were
separated by electrophoresis through a 10% SDS-polyacrylamide gel and
blotted onto polyvinylidene fluoride membranes (Millipore Corp.,
Bedford, MA). The filters were incubated with a monoclonal antibody
against the FLAG epitope (Kodak Scientific Imaging Systems). The
immunocomplex was then incubated with a second antibody conjugated to
alkaline phosphatase and visualized by 5-bromo-4-chloro-3-indolyl phosphate/nitro blue tetrazolium substrate (Kirkegarrd & Perry Laboratories, Inc., Gaithersburg, MD).
Phosphohydrolase Assay--
Microsomal preparations and
phosphohydrolase assays in intact or disrupted microsomes were
performed essentially as described (8). Appropriate amounts of
microsomal proteins were incubated at 30 °C for 10 min in reaction
mixtures (100 µl) containing 50 mM sodium cacodylate
buffer, pH 6.5, 10 mM Glu-6-P, and 2 mM EDTA. Sample absorbance was determined at 820 nm and is related to the amount
of phosphate released using a standard curve constructed by a stock of
inorganic phosphate solution. Disrupted microsomal membranes were
prepared by incubating intact membranes in 0.2% deoxycholate for 20 min at 0 °C. Nonspecific phosphatase activity in microsomes was
estimated by preincubating microsomal preparations at pH 5 for 10 min
at 37 °C, a condition that inactivates the thermolabile G6Pase (23).
The latency or intactness of microsomal preparations was assessed by
comparing mannose-6-phosphohydrolase activities in intact and
detergent-disrupted microsomes (1). Liver microsomes with latency
values of 93-95% were used in this study.
Protease Protection Assays--
Mouse liver microsomes or cell
homogenates from G6Pase-WT-, G6Pase-5'FLAG-, or
G6Pase-3'FLAG-transfected COS-1 cells were used for protease protection
assays. Microsomes or cell homogenates were treated with trypsin (Type
XIII, 500 µg/mg of protein) or proteinase K (50 µg/mg of protein)
for 30 min at room temperature. Phenylmethylsulfonyl fluoride (final
concentration of 5 mM) was then added to inactivate
proteinase K, and phenylmethylsulfonyl fluoride and trypsin inhibitor
(final concentration of 6 mg/mg of protein) were added to inactivate
trypsin. The reaction mixtures were diluted 100-fold to 10 ml with cold
buffer A (0.25 M sucrose and 5 mM HEPES, pH
7.4) and centrifuged at 100,000 × g for 1 h at
4 °C. The microsomal pellets were resuspended in buffer A and used
for either phosphohydrolase assays or Western blot analysis. Microsomes
or cell homogenates treated first with 0.5% deoxycholate and then with
trypsin or proteinase K were used as controls.
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RESULTS |
Characterization of N-terminal Deleted and/or Tagged G6Pase
Mutants--
G6Pase is tightly associated with the ER membrane (1).
Therefore, protease protection assays using N- and C-terminal tagged G6Pase constructs should allow us to assess whether G6Pase
possesses an even or odd number of transmembrane segments as well as
the location of its N and C termini with respect to the ER lumen. We
have previously shown that the eight C-terminal residues of human
G6Pase are not required for activity (14), suggesting that a small
C-terminal tag should not markedly disturb G6Pase activity. To
investigate whether N-terminal perturbation affects G6Pase, we examined
the effects of N-terminal deletions on enzymatic activity. While
deletion of residues 1-13 (G6Pase-(14-357)) abolished G6Pase
activity, deletion of residues 1-4 yielded mutant G6Pase (G6Pase-(5-357)) retaining ~60% of WT enzymatic activity (Fig. 1), suggesting that a small N-terminal
tag should also not greatly disturb G6Pase activity.

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Fig. 1.
Analysis of G6Pase mRNA
expression and phosphohydrolase activity after transient expression of
WT and mutant G6Pase cDNA constructs in COS-1
cells. Total RNA was separated by formaldehyde-agarose gel
electrophoresis, blotted onto a Nytran membrane, and hybridized to the
phG6Pase-1 probe (5) labeled by random priming. G6Pase activity was
determined in deoxycholate (0.2%)-disrupted microsomes as described
under "Materials and Methods" using two independent isolates of
each construct in three separate transfections. Data are presented as
the mean ± SD. Numbers in parentheses represent percent of WT enzymatic activity.
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To tag G6Pase, we used the eight-amino acid FLAG marker peptide
DYKDDDDK, which is small and hydrophilic and should offer minimal
disruption to the native protein configuration of the enzyme. As
expected, the N-terminal (G6Pase-5'FLAG) and C-terminal (G6Pase-3'FLAG)
tagged G6Pase constructs retained >70% of WT enzymatic activity when analyzed after transient transfection in COS-1 cells (Fig. 1). Moreover, the G6Pase-(5-357)-3'FLAG construct also retained >60% of WT activity. Northern blot analysis of G6Pase
transcripts showed that WT and N-terminal deleted and tagged
G6Pase constructs directed the expression of similar levels
of G6Pase mRNA in transfected cells (Fig. 1). Our data
show that G6Pase mRNA expression was not affected by the
addition of N- or C-terminal FLAG or by the deletion of up to 14 N-terminal residues. Therefore, the decrease in enzymatic activity of
the tagged or G6Pase-(5-357) constructs and the loss of activity of
the G6Pase-(14-357) and G6Pase-(14-357)-3'FLAG constructs were not
due to a decrease in transfection efficiency.
Transmembrane Topology of G6Pase--
The nine-transmembrane
helical model (19, 20) predicts that the N and C termini of G6Pase
would be situated at the opposite sides of the ER membrane. On the
other hand, the six-transmembrane helical model (10, 14) predicts that
both the N and C termini of G6Pase would be situated at the same side
of the ER membrane. Therefore, the FLAG tag at the luminal N or C
terminus should be resistant to proteolysis, whereas the FLAG tag at
the cytoplasmic N or C terminus should be sensitive to protease
digestion. Intact microsomes, isolated from G6Pase-5'FLAG- and
G6Pase-3'FLAG-transfected COS-1 cells, were subjected to digestion by
two serine proteases, proteinase K and trypsin, in the absence or
presence of deoxycholate. Proteinase K is a broad spectrum protease
exhibiting no pronounced cleavage specificity (24), whereas trypsin
cleaves peptide bonds between Lys or Arg and an unspecific amino acid
(25). The presence of the FLAG epitope was visualized by Western blot
analysis (Fig. 2). Regardless of the
transmembrane topology, the C-terminal domain of human G6Pase contains
a trypsin cleavage site (KKSL, amino acids 354-357), and there is none
at the N-terminal domain (9). However, tagging the N or C terminus of
G6Pase with the FLAG peptide DYKDDDDK generated an artificial trypsin
cleavage site at each terminus. In the absence of detergent, the
C-terminal FLAG tag in G6Pase was removed by both proteinase K and
trypsin, whereas the N-terminal FLAG tag was resistant to digestion by
both proteases (Fig. 2). This indicates that human G6Pase possesses an
odd number of transmembrane helices, with the N terminus localized in
the ER lumen and the C terminus in the cytoplasm. As expected, in detergent-permeabilized microsomes, both N- and C-terminal FLAG tags
were cleaved by proteinase K and trypsin (Fig. 2).

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Fig. 2.
Sensitivity of N and C termini of human
G6Pase to digestion by proteinase K or trypsin. Intact microsomes,
isolated from G6Pase-5'FLAG- and G6Pase-3'FLAG-transfected COS-1 cells, were subjected to digestion by proteinase K (50 µg/mg of microsomal protein) or trypsin (500 µg/mg of microsomal protein) for 30 min at
room temperature as described under "Materials and Methods." Mock-transfected cells were used as controls. The presence or absence
of the FLAG epitope was analyzed by Western blot hybridization and
visualized by an anti-FLAG monoclonal antibody. The G6Pase-FLAG proteins are indicated by arrows.
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To efficiently cleave the C-terminal FLAG tag in human G6Pase, we used
a trypsin concentration (500 µg/mg of microsomal protein) that was at
least 50-fold higher than previously used for intact liver microsomes
(4-7). We therefore examined the effects of proteinase K and of a
higher concentration of trypsin on hepatic microsomal G6Pase activity
in the absence or presence of deoxycholate (Table
I). In the absence of proteases, G6Pase
activity in intact hepatic microsomes was lower than the activity in
detergent-permeabilized microsomes (Table I), a characteristic of
mammalian microsomal G6Pase (1). Both proteinase K and trypsin
moderately inhibited G6Pase activity in intact liver microsomes.
However, enzymatic activity increased when detergent was added to
intact microsomes after prior inactivation of either protease (Table
I). On the other hand, both proteases markedly inhibited G6Pase
activity in detergent-disrupted microsomes.
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Table I
Effects of proteinase K and trypsin on microsomal G6Pase activity
Intact microsomes were subjected to digestion by proteinase K (50 µg/mg of microsomal protein) or trypsin (500 µg/mg of microsomal protein) as described under "Materials and Methods."
Phosphohydrolase activity was assayed in reactions containing 10 mM Glu-6-P, and the data are means ± S.D. of three
independent experiments. G6Pase activity from mock-transfected cells
has been subtracted from the activity in microsomes from G6Pase-WT-,
G6Pase-5'FLAG-, and G6Pase-3'FLAG-transfected cells.
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To demonstrate that in vitro expressed WT or tagged G6Pases
in intact microsomes are also resistant to limited proteolysis, we
examined microsomal G6Pase activity from G6Pase-WT-, G6Pase-5'FLAG-, and G6Pase-3'FLAG-transfected COS-1 cells after digestion by proteinase K or trypsin in the absence or presence of deoxycholate (Table I). In
the absence of proteases, G6Pase activity in intact microsomes isolated
from G6Pase-WT-, G6Pase-5'FLAG-, or G6Pase-3'FLAG-transfected cells was
lower than the activity in detergent-permeabilized microsomes, as was
observed with hepatic microsomes (Table I). Treatment of intact
microsomes isolated from G6Pase-WT-, G6Pase-5'FLAG-, or
G6Pase-3'FLAG-transfected cells with either proteinase K or trypsin
moderately decreased G6Pase activity, and enzymatic activity increased
when detergent was added to the microsomes after inactivation of either
protease (Table I). As expected, proteinase K or trypsin greatly
reduced G6Pase activity in detergent-permeabilized microsomes (Table
I). Therefore, protease sensitivity of in vitro expressed G6Pase is comparable to that of the endogenous liver enzyme, and catalytic activity of G6Pase was not affected by the addition of an N-
or C-terminal FLAG tag.
Role of His-176 in G6Pase Catalysis--
Our study demonstrates
that G6Pase possesses an odd number of transmembrane helices,
supporting the new nine-transmembrane helical model (19, 20). According
to this model, Arg-83, His-119, and His-176, which are predicted to
contribute to the active site of G6Pase, reside on the same side of the
ER membrane (Fig. 3). In an earlier
study, we have shown that Arg-83 is involved in positioning the
phosphoryl moiety and that His-119 is absolutely required for G6Pase
activity (14). Active-site alignment of vanadium-containing
chloroperoxidase and mammalian G6Pases suggests that Arg-83 in G6Pase
is a candidate for positioning the phosphate, His-119 is a proposed
acid-base group in catalysis, and His-176 is the residue that
covalently binds the phosphoryl moiety (17, 19). If His-176 is indeed
the phosphoryl acceptor, then this residue should be essential for
G6Pase activity. Therefore, we generated mutant G6Pase constructs with
seven different substitutions at codon 176 (Ala (H176A), Ile (H176I),
Lys (H76K), Met (H176M), Asn (H176N), Ser (H176S), and Arg (H176R)),
and the resulting G6Pase activity was analyzed after transient
expression in COS-1 cells (Table II).
Consistent with our hypothesis, none of these codon 176 mutants had
detectable G6Pase activity.

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Fig. 3.
The predicted nine-transmembrane helical
structure of human G6Pase. The amino acids predicted to contribute
to the active site of G6Pase are shaded. The locations of 16 missense mutations and one codon deletion mutation identified in the
G6Pase gene of GSD-1a patients are shown in
black. Arg-83 is one of the residues that contributes to the
active center and is mutated in some GSD-1a patients. Transmembrane
helices were identified by the algorithm of Hoffman and Stoffel
(20).
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Table II
G6Pase activity of codon 176 mutant constructs
Phosphohydrolase activity was assayed as described under "Materials
and Methods" using two independent isolates of each construct in
three separate transfections. Data are presented as means ± S.D.
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Northern blot hybridization analysis of G6Pase transcripts
from transfected cells showed that WT as well as the various codon 176 mutant G6Pase mRNAs were expressed at similar levels
(data not shown). This indicates that the reduction in enzymatic
activity was due to the defect in the G6Pase protein and not due to a
decrease in transfection efficiency.
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DISCUSSION |
In this study, we have characterized the orientation of human
G6Pase in the ER and demonstrated that this enzyme contains an odd
number of transmembrane helices, with the N terminus localized in the
ER lumen and the C terminus in the cytoplasm. Our data best support the
nine-transmembrane helical model for G6Pase (19, 20), rather than the
six-transmembrane helical model (14) previously predicted (10).
According to the nine-transmembrane helical topology, the residues
predicted to compose the active center in G6Pase, Arg-83, His-119, and
His-176, are all situated on the luminal side of the ER membrane (Fig.
3). Our data support the proposal that helices 2-5 are in close
contact and form the core of the catalytic center of G6Pase (19).
Sequence alignment of mammalian G6Pases, acid phosphatases, lipid
phosphatases, and vanadium-containing chloroperoxidase has identified a
conserved phosphatase sequence motif,
KXXXXXXRPX12-54PSGHSRXXXXXHXXXD (17, 18). This supports the results of our earlier study, which
demonstrated that Arg-83 and His-119 in G6Pase (the two boldface
residues in this motif) are essential for G6Pase activity (14). During
catalysis, a phosphoryl-enzyme intermediate is formed, and a His
residue in G6Pase is the phosphoryl acceptor (11-13). Comparison of
the known tertiary structure of vanadium-containing chloroperoxidase
suggested that the function of Arg-83 in G6Pase is to position the
phosphoryl moiety (19), as we originally proposed (14). This alignment
also suggests that His-119 provides the proton needed to liberate the
glucose moiety and that His-176 acts as the nucleophile forming the
phosphohistidine-enzyme intermediate (19). Mutagenesis studies
confirmed that His-119 is absolutely essential for G6Pase activity, and
it was suggested that His-119 could be the phosphoryl acceptor in
G6Pase (14). We now show that codon 176, proposed to covalently bind
the phosphoryl moiety, is another residue absolutely required for
G6Pase activity. Whether His-176 is indeed the phosphoryl acceptor in
G6Pase will be the focus of future studies. Alignment of the active
sites of vanadium-containing chloroperoxidase and mammalian G6Pases
suggests that, in addition to Arg-83, Lys-76 and Arg-170 in human
G6Pase also participate in positioning the phosphoryl moiety, and
Ser-117 and Gly-118 may participate in hydrogen bonding (Fig. 3). It
will be of interest to study the roles of these residues during G6Pase
catalysis.
Microsomes are closed vesicles with a defined cytoplasmic-side out
orientation (26). Therefore, protein domains exposed on the outside of
the ER can be selectively digested by proteases. G6Pase activity in
intact liver microsomes is resistant to limited proteolysis, suggesting
that the active site is not on the cytoplasmic side of the ER membrane
(4-7). In this study, we demonstrate that G6Pase activity in intact
microsomes isolated from mouse liver as well as from G6Pase-WT-,
G6Pase-5'FLAG-, and G6Pase-3'FLAG-transfected COS-1 cells is resistant
to limited proteolysis. Therefore, the orientation of the in
vitro expressed G6Pase in the ER is similar to that of liver
microsomal G6Pase. To date, at least 29 mutations have been identified
in the G6Pase gene of GSD-1a patients (9, 27-33). Seventeen
amino acids in human G6Pase were altered by the known missense and
codon deletion mutations. Thirteen missense mutations and the codon
deletion mutation (
F327) uncovered in this laboratory were shown to
abolish or greatly reduce G6Pase activity in transient expression
assays (9, 27, 28, 33). The 17 amino acids mutated in the
G6Pase gene of GSD-1a patients are illustrated in Fig. 3.
According to the nine-transmembrane helical topology of G6Pase, the
four loops facing the cytoplasm are relatively short, varying from 8 to
12 residues. The two large loops, which are situated between helices 2 and 3 (37 residues, loop 1L) and helices 6 and 7 (33 residues, loop
3L), are located on the luminal side of the ER. Among the 17 amino
acids mutated in GSD-1a patients, 14 are situated in transmembrane
helices 1-9, two are located in loop 1L, and one in loop 3L (Fig. 3).
No missense or codon deletion mutations have yet been identified in the
four cytoplasmic loops or N- and C-terminal domains of human G6Pase. Therefore, G6Pase activity depends on the structural integrity of the
transmembrane helices, and residues in the two large luminal loops also
play crucial roles. It is tempting to suggest that during catalysis,
the structural requirement of the cytoplasmic loops and N- and
C-terminal domains are less stringent than the other features of human
G6Pase.
It is worth noting that the apparent molecular mass of the G6Pase
protein in intact microsomes was not notably altered after digestion by
either proteinase K or trypsin. This was demonstrated by Western blot
analysis of the G6Pase-5'FLAG protein before and after proteolysis.
This is expected since trypsin is predicted to cleave off a small
peptide (KSL, amino acids 355-357 of human G6Pase) of 0.3 kDa, which
would not substantially reduce the apparent molecular mass of the
G6Pase protein. Additionally, G6Pase activity increased to the levels
of the permeabilized control microsomes when detergent was added to
intact microsomes, after inactivation of either protease. Our data
indicate that only a small number of C-terminal residues were removed
by either proteinase K or trypsin and that the tertiary structure of
G6Pase remained intact after proteolysis. It appears that the short
cytoplasmic loops are not readily accessible to proteolysis, suggesting
that they are closely associated with the ER membranes.