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Journal of Bacteriology, March 2005, p. 1859-1865, Vol. 187, No. 5
0021-9193/05/$08.00+0 doi:10.1128/JB.187.5.1859-1865.2005
Copyright © 2005, American Society for Microbiology. All Rights Reserved.
Polyphosphate:AMP Phosphotransferase as a Polyphosphate-Dependent Nucleoside Monophosphate Kinase in Acinetobacter johnsonii 210A
Toshikazu Shiba,1,2*
Hiromichi Itoh,1,2
Atsushi Kameda,3
Keiju Kobayashi,3
Yumi Kawazoe,1 and
Toshitada Noguchi4
Regenetiss Co., Ltd., Okaya,1
School of Dentistry, Matsumoto Dental University, Shiojiri, Nagano,2
Division of Molecular Chemistry, Graduate School of Engineering, Hokkaido University, Sapporo,3
Biochemical Division, YAMASA Corporation, Choshi, Chiba, Japan4
Received 1 July 2004/
Accepted 29 November 2004

ABSTRACT
We have cloned the gene for polyphosphate:AMP phosphotransferase
(PAP), the enzyme that catalyzes phosphorylation of AMP to ADP
at the expense of polyphosphate [poly(P)] in
Acinetobacter johnsonii 210A. A genomic DNA library was constructed in
Escherichia coli,
and crude lysates of about 6,000 clones were screened for PAP
activity. PAP activity was evaluated by measuring ATP produced
by the coupled reactions of PAP and purified
E. coli poly(P)
kinases (PPKs). In this coupled reaction, PAP produces ADP from
poly(P) and AMP, and the resulting ADP is converted to ATP by
PPK. The isolated
pap gene (1,428 bp) encodes a protein of 475
amino acids with a molecular mass of 55.8 kDa. The C-terminal
region of PAP is highly homologous with PPK2 homologs isolated
from
Pseudomonas aeruginosa PAO1. Two putative phosphate-binding
motifs (P-loops) were also identified. The purified PAP enzyme
had not only strong PAP activity but also poly(P)-dependent
nucleoside monophosphate kinase activity, by which it converted
ribonucleoside monophosphates and deoxyribonucleoside monophosphates
to ribonucleoside diphosphates and deoxyribonucleoside diphosphates,
respectively. The activity for AMP was about 10 times greater
than that for GMP and 770 and about 1,100 times greater than
that for UMP and CMP.

TEXT
Inorganic polyphosphate [poly(P)], a linear phosphate polymer
linked by high-energy phosphoanhydride bonds, is found in all
cells in nature (
12,
13). Many enzymes that phosphorylate nucleotides
using poly(P) as a phosphate donor (
12,
13) have been identified.
Identified enzymes include poly(P) kinases (PPKs) (PPK1 in
Escherichia coli and PPK2 in
Pseudomonas aeruginosa), which generate nucleoside
triphosphate (
1,
6,
8,
11,
14,
20), and polyphosphate:AMP phosphotransferase
(PAP), which generates ADP (
3,
4; T. Shiba and A. Kornberg,
unpublished data). Therefore, one of the main physiological
functions of poly(P) in microorganisms might be as an energy
or phosphate reservoir to supply a wide variety of phosphorylated
compounds in cells.
PAP was first identified in Corynebacterium xerosis (4) and was also found in Myxococcus xanthus (Shiba and Kornberg, unpublished) and Acinetobacter johnsonii 210A as a major poly(P)-utilizing enzyme (3). PAP catalyzes phosphoconversion of poly(P) to AMP, forming ADP. In A. johnsonii 210A, it has been reported that a large amount of poly(P) (300 mg of Pi per g of dry cells) is accumulated during growth and that poly(P) is degraded and Pi is released into the medium when energy generation is no longer possible (18, 19). PAP makes it possible to conserve the energy liberated from the cleavage of poly(P) coupling with adenylate kinase, which mainly catalyzes ATP and AMP formation from two molecules of ADP, to use poly(P) as a source of ATP generation (18, 19). In a previous study, PAP was purified more than 1,500 fold from a crude lysate of A. johnsonii 210A, and its basic properties were analyzed (3).
To further investigate biochemical characteristics of PAP, we cloned a gene encoding PAP from A. johnsonii 210A. This is the first study to reveal the primary structure of the pap gene as well as some novel characteristics of the PAP enzyme.
Nucleotide sequence accession number.
The DNA sequence determined in this study has been assigned GenBank accession number AB092983.
Molecular cloning of pap gene.
Genomic DNA extracted from A. johnsonii 210A was partially digested with Sau3AI, and 7- to 10-kb DNA fragments were purified by glycerol gradient centrifugation. The purified DNA fragments were ligated into the BamHI site of the pBluescript II SK(+) cloning vector (Toyobo, Osaka, Japan). The ligated DNA was introduced into E. coli DH5
, and recombinant E. coli was selected by ampicillin (100 µg/ml). Crude lysates of about 6,000 ampicillin-resistant colonies were screened by the coupled reaction of PAP and purified E. coli PPK1 (Fig. 1A) producing ATP from AMP in the presence of poly(P) (10). The amounts of ATP generated by the PAP-PPK coupling reaction were determined with the ATP Bioluminescence Assay kit CLSII (Roche Diagnostics, Mannheim, Germany). When a mixture of lysate that produced a relatively high level of ATP had been found, the PAP activity of each clone was directly measured using [32P]poly(P) as a substrate, with an average chain length of around 750 phosphate residues, synthesized with [
-32P]ATP and purified E. coli PPK1 as described previously (1). Products were separated on polyethyleneimine cellulose-thin-layer chromatography (TLC) plates (Merck) in 0.75 M KH2PO4 (pH 3.5), and the radioactive spots were analyzed by using a BAS 2000 radioimage analyzer (FUJIX, Kyoto, Japan).
A plasmid containing the
pap gene, designated pPAP2, was obtained
from the genomic library of
A. johnsonii 210A. A lysate prepared
from a clone harboring pPAP2 was found to have strong PAP activity
(Fig.
1B). The entire region of the inserted DNA fragment of
pPAP2 (9.6 kb) was sequenced. Three putative open reading frames
(ORFs) were found in the inserted DNA fragment. Each putative
ORF was subcloned, and the encoded protein was separately expressed.
A crude lysate prepared from
E. coli expressing one subcloned
ORF had PAP activity. The ORF that encodes the PAP encodes 475
amino acids with a calculated molecular mass of 55.8 kDa. A
putative Shine-Dalgarno sequence was found 6 bp upstream from
the start codon of PAP. A palindromic sequence, which is a putative
transcriptional termination signal, was also found 11 bp downstream
of the termination codon. A BLAST search (
2) of GenBank databases
with the amino acid sequence of PAP revealed significant similarities
with the PPK2 homolog 2 (41.3% identical). The C-terminal region
of PAP (200 amino acid residues) is homologous with both PPK2
(15.8% identical) and PPK2 homolog 1 (17.5% identical) (Fig.
2) (
20). PPK2 has been isolated as a poly(P)-dependent GDP kinase
from
P. aeruginosa PAO1 and has nearly 100-fold-greater poly(P)-utilizing
activity (GDP kinase activity) than it has poly(P) synthetic
activity (GTPase activity) (
7). The similarity between PAP and
these PPK2 homologs indicates that there is a common mechanism
by which these enzymes use poly(P) for phosphorylation of nucleosides.
The C-terminal region of PAP, including one P-loop (amino acids
286 to 292), is thought to be essential for poly(P)-dependent
nucleotide phosphorylation. An additional P-loop was found in
the N-terminal region (amino acids 45 to 51) of PAP, and this
region is also highly homologous with PPK2 homolog 2, suggesting
that this second P-loop may play a crucial role in binding with
nucleoside monophosphate. Since PPK2 homologs have been found
in many bacteria (
20), PAP could also be distributed in a wide
variety of bacteria. PAP homologs were also found in archaea
such as
Methanosarcina acetivorans and
Methanosarcina mazei.
No PAP homologs have been found in
Saccharomyces cerevisiae or other eukaryotes (
20).
Purification and complex formation of PAP.
The standard assay for PAP activity was performed by measuring
the amount of ADP produced from AMP and poly(P) by high-performance
liquid chromatography (HPLC) according to our previously published
paper (
9). The reaction mixture (50 µl) contained 50 mM
Tris-HCl (pH 8.0), 100 mM MgCl
2 (20 mM MgCl
2 for the assay in
purification), 10 mM poly(P) with an average chain length of
18 phosphate residues (Sigma), and 5 mM AMP. All poly(P) concentrations
are presented in terms of phosphate residues. One unit of enzyme
is defined as the amount of PAP that produces one micromole
of ADP per minute at 37°C. For overproduction of PAP,
E. coli JM109 containing pPAP2 was employed because this strain
produces a large amount of PAP with a high level of activity
(Fig.
3A, lane I). The cells were cultivated at 30°C for
20 h in 1 liter of Luria-Bertani medium containing 100 µg
of ampicillin/ml. Cells were harvested and washed twice with
40 ml of 50 mM Tris-HCl (pH 7.5) and then sonicated for 10 min
at 300 W of output with an ultrasonic homogenizer VP-30S (Taitec
Co., Tokyo, Japan) in 20 ml of the same buffer. The intact cells
and debris were removed by centrifugation (23,000
x g; 15 min),
and the supernatant was pooled as a crude extract. PAP was purified
from the crude extract by successive steps of the following
operations: ammonium sulfate precipitation, DEAE-Toyopearl anion-exchange
column (1.4 by 10 cm; Tosoh) chromatography, CHT-II column (1.35
by 3.5 cm; Bio-Rad) chromatography; and Hiload 16/60 Superdex
200 pg gel filtration chromatography (GFC) (Amersham Pharmacia
Biotech) (Fig.
3A). The scheme for purification of PAP is shown
in Table
1. Two protein peaks that had PAP activities were obtained
by GFC (Hiload 16/60, Superdex 200 pg) (Fig.
3B). They were
electrophoretically homogeneous in the presence of sodium dodecyl
sulfate (SDS) and appeared to be composed of only one polypeptide
with a molecular mass of 56 kDa (Fig.
3A), which is in good
agreement with the molecular mass deduced from the DNA sequence.
This indicates that the enzyme forms both a tetramer and a dimer.
Since the specific activity of the PAP dimer was 2.2-fold higher
than that of the tetramer, the dimer fraction was used as purified
PAP for the following experiments. The PAP dimer, purified 22.1
fold from crude extract with a 26.5% yield, gave an apparently
homogeneous protein (Table
1 and Fig.
3A). The N-terminal amino
acid sequences of the purified PAP dimer and tetramer were MDTETIASAV
and MDTETIAS, respectively. These sequences are identical to
those of the N-terminal peptides predicted by the nucleotide
sequence of the gene.
PAP as a poly(P)-dependent NMP kinase.
Since we found in a previous study that PAP in
M. xanthus has
poly(P)-dependent GDP kinase activity (
10), we carried out an
experiment to determine whether purified PAP also has poly(P)-dependent
nucleoside monophosphate kinase activity. Substrate specificities
of PAP on AMP, GMP, UMP, CMP, IMP, dAMP, dGMP, dCMP, and TMP
were examined by measuring phosphoconversion from [
32P]poly(P)
to ribonucleoside monophosphates (NMPs) or deoxyribonucleoside
monophosphates. As shown in Fig.
4, poly(P)-dependent phosphoconversion
was observed with all substrates, although the activity in pyrimidine
nucleotides such as UMP, CMP, dUMP, dCMP, and TMP was weak.
When an excess amount of the enzyme (500 ng) was added to the
reaction mixture, formation of labeled UDP, CDP, dUDP, dCDP,
and TDP was clearly observed. The migration positions of labeled
products in TLC are consistent with the positions of nucleoside
diphosphates. To compare the activities of PAP on NMPs and deoxyribonucleoside
monophosphates (dNMPs), relative activities on 5 mM substrates
were measured by HPLC (
9) with 10 mM poly(P) with an average
chain length of 18 phosphate residues (Table
2). The activity
was 10 times lower for GDP kination and around 1,000 times lower
for pyrimidine nucleotide (UMP and CMP) kination than the efficiency
of AMP kination. The activities on deoxyribonucleotides were
around four to six times less than that on ribonucleotides,
and the activity on deoxypyrimidine was also about 1,000 times
lower than that of dAMP.
Km and
Vmax values of the PAP dimer on these substrates were
determined by HPLC analysis (
9) of PAP activity in reaction
mixtures containing 100 mM MgCl
2 and 10 mM poly(P) with an average
chain length of 18 phosphate residues at 37°C. These values
were calculated by the method of least squares with Taylor expansion
(
17).
Km values for AMP and GMP were 0.27 ± 0.02 and
4.4 ± 0.3 mM, respectively (Table
3). The
Vmax,
kcat,
and
kcat/
Km values of PAP for AMP were 180 ± 6 µmol
mg
1 min
1, 10,000 ± 300 min
1, and
3.7
x 10
4 min
1 mM
1, respectively. These kinetic
parameters are also consistent with the substrate preference
of PAP. Since maximum PAP activity was obtained in the reaction
mixture containing 100 mM MgCl
2 and in that containing both
20 mM MgCl
2 and 50 mM (NH
4)
2SO
4, we employed both reaction conditions
to measure the kinetic parameters (Table
3). Other metal ions
such as Mn
2+, Fe
2+, Ca
2+, Cu
2+, Zn
2+, and Co
2+ could not enhance
the activity, even though the reaction mixture contained (NH
4)
2SO
4 (data not shown). Since a lower
Km value was observed with a
high concentration of MgCl
2, all other assays were performed
in the presence of 100 mM MgCl
2. In
A. johnsonii 210A, Mg
2+ is always released and taken up simultaneously with phosphate,
since Mg
2+ is necessary as a major counterion for poly(P) accumulated
in the cells (
19). Thus, cellular Mg
2+ concentration could be
high when a large amount of poly(P) accumulates. For this reason,
PAP may require relatively high Mg
2+ concentrations to obtain
its maximum activity.
Although the
Km value of PAP for AMP is almost the same as that
of
E. coli PPK1 for ADP, the
kcat value of PAP is about 16 times
higher than that of PPK1 (
14). This means that PAP is much more
efficient at phosphoconversion than is PPK1.
P. aeruginosa PPK2
also has a higher level of phosphoconversion activity than does
E. coli PPK1. The
Vmax value of PPK2 is of the same order as
that of PAP and is 51 times higher than that of PPK1.
To confirm that purified NMP kinase activity is not derived from contamination of other enzymes in the purified PAP fraction, the correlation between a protein peak of GFC and NMP kinase activity was examined. As shown in Fig. 5, all of the activities were found in the dimer and tetramer, and the elution profile of the protein obtained by GFC with a Hiload 16/60 Superdex 200 pg (Fig. 5A and B) completely matched between the elution profiles of PAP (Fig. 5C) and GMP kinase activity (Fig. 5D). Other NMP kinase activities were also matched with the elution profiles of PAP (data not shown). Thus, purified PAP is a poly(P)-dependent nucleoside monophosphate kinase.
Other basic characteristics of PAP.
The optimum pH of PAP was between 8.0 and 9.0, and the PAP was
stable between pH 7 and 11 (data not shown). Optimum temperature
for PAP was 50°C, and the level of activity at 50°C
was 3.2- and 1.8-fold higher than that at 30 and 37°C, respectively
(data not shown). PAP was stabilized in the presence of poly(P).
PAP retained about 40% of its original activity after being
heated at 45°C for 10 min without poly(P). However, in the
presence of poly(P) (10 mM), the remaining PAP activity was
85% of the original activity (data not shown).

ACKNOWLEDGMENTS
This work was supported by a Grant-in-Aid for Innovations through
Business-Academic-Public Sector Cooperation and a Grant-in-Aid
for Scientific Research on Priority Areas (B) from the Ministry
of Education, Culture, Sports, Science and Technology of Japan.

FOOTNOTES
* Corresponding author. Mailing address: Regenetiss Co., Ltd., 1-5-17, Akabane, Okaya, Nagano 394-0002, Japan. Phone and fax: 81-42-584-8176. E-mail:
shiba{at}regenetiss.com.


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Journal of Bacteriology, March 2005, p. 1859-1865, Vol. 187, No. 5
0021-9193/05/$08.00+0 doi:10.1128/JB.187.5.1859-1865.2005
Copyright © 2005, American Society for Microbiology. All Rights Reserved.
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