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Infection and Immunity, January 2007, p. 518-522, Vol. 75, No. 1
0019-9567/07/$08.00+0 doi:10.1128/IAI.01344-06
Copyright © 2007, American Society for Microbiology. All Rights Reserved.
Protein P200 Is Dispensable for Mycoplasma pneumoniae Hemadsorption but Not Gliding Motility or Colonization of Differentiated Bronchial Epithelium
Jarrat L. Jordan,1,
How-Yi Chang,1
Mitchell F. Balish,1,
Lynley S. Holt,1
Stephanie R. Bose,1
Benjamin M. Hasselbring,1
Robert H. Waldo III,1,
Thomas M. Krunkosky,2 and
Duncan C. Krause1*
Department of Microbiology,1
Department of Anatomy and Radiology, University of Georgia, Athens, Georgia2
Received 21 August 2006/
Returned for modification 19 September 2006/
Accepted 6 October 2006

ABSTRACT
Mycoplasma pneumoniae protein P200 was localized to the terminal
organelle, which functions in cytadherence and gliding motility.
The loss of P200 had no impact on binding to erythrocytes and
A549 cells but resulted in impaired gliding motility and colonization
of differentiated bronchial epithelium. Thus, gliding may be
necessary to overcome mucociliary clearance.

TEXT
Mycoplasma pneumoniae causes tracheobronchitis, bronchopneumonia,
and a variety of extrapulmonary manifestations in humans (
35).
A distinct polar structure (
5,
18) mediates host cell attachment
(cytadherence) by this novel cell wall-less prokaryote. Electron
micrographs of experimentally infected human and animal cells,
as well as cells collected from natural
M. pneumoniae infections,
reveal an intimate relationship between this terminal organelle
and the epithelial cell surface (
7). The terminal organelle
also mediates gliding motility (
16,
29), which may facilitate
mycoplasma traversal of the airway surface liquid to allow access
to host receptors on the apical surface of respiratory epithelium
and refuge from the turbulent mucociliary escalator.
M. pneumoniae binds erythrocytes (hemadsorption [HA]), and while not thought to encounter erythrocytes typically during infection, a limited correlation exists between HA and virulence (14, 25, 32). Characterization of HA mutants (Table 1) has identified a number of proteins associated with cytadherence, including HMW1, HMW2, HMW3, B, C, P65, and adhesins P1 and P30 (13, 14, 21, 22, 24). Proteins HMW1, HMW2, HMW3, and P65 are mostly or entirely insoluble in the detergent Triton X-100 (TX) (3), a parameter that serves as a working definition for cytoskeletal elements of eukaryotic cells (3, 21). HMW1, HMW3, and P65 also contain acidic proline-rich (APR) domains, which are highly repetitive sequences of variable length and uncertain function (2). The TX-insoluble protein P200 likewise contains an APR domain, as well as six imperfect repeats of a 31- to 33-amino-acid motif of undetermined function, designated the EAGR (for enriched in aromatic and glycine residues) box (2, 28) (Fig. 1A). This motif is also found in HMW1 and an uncharacterized APR-domain-containing protein in M. pneumoniae but is otherwise found in no other proteins to date except orthologs in other mycoplasma species (2). The deduced sequence and biochemical features of P200 suggest it might also be a terminal organelle protein with a role in cytadherence (3). Here we determined the subcellular location of P200 and characterized a mutant lacking P200.
We localized P200 by immunofluorescence microscopy as described
elsewhere (
19) except that cells incubated on coverslips for
3 h were fixed in 4% paraformaldehyde in phosphate-buffered
saline (PBS) and blocked using PBS-5% (wt/vol) powdered milk-0.02%
NaN
3-0.05% Tween 20 (pH 7.2), and primary antibody was used
at 1:500. Cells of the wild-type
M. pneumoniae M129 strain (
26)
generally exhibited a single polar fluorescent focus corresponding
to the terminal organelle (Fig.
2). In contrast, a patchy fluorescence
pattern was observed with the cytadherence mutants M6, I-2,
II-3, III-4, and IV-22; this patchy distribution appeared to
be more pronounced for mutant IV-22, suggesting a possible relationship
between P1 and P200, but additional studies are required to
examine this further. However, to our surprise, no fluorescence
was observed in the HMW3
mutant 201G or HMW3
+ revertant
201GR1, described by us previously (
37).
Western immunoblotting (
11) revealed that P200 was present at
wild-type levels in each mutant examined except 201G and 201GR1,
the latter hereafter referred to as the P200
mutant (Fig.
1B). The MPN567 allele encoding P200 from this mutant was sequenced
utilizing templates from multiple overlapping PCR encompassing
the entire gene from mycoplasma genomic DNA. Sequence analysis
revealed an IS
256 insertion at a site corresponding to residue
657 (of 1,036; Fig.
1C) and an adjacent 8-bp duplication (not
shown).
The terminal organelle in wild-type M. pneumoniae has a characteristic electron-dense core (5), which is atypical in mutant 201G, often separating at the proximal end to yield a chevron shape (37). Cells were examined by transmission electron microscopy as described previously (37), except that fixed mycoplasmas were enrobed in 3% noble agar at 58 to 60°C before postfixation, and samples were embedded in Epon-Araldite plastic by using flatbed molds and allowed to polymerize at 75°C overnight. The electron-dense cores in the P200 mutant were indistinguishable from the wild type and distinct from the atypical cores found in mutant 201G (Fig. 3), indicating that the loss of P200 had no obvious impact on the appearance of the core and confirming that the core defect in the HMW3 mutant (37) was indeed a result of the loss of HMW3.
M. pneumoniae cells glide intermittently in the direction of
the terminal organelle (
29). We examined cell gliding as described
previously (
15), with the P200
mutant exhibiting both
a slower velocity and a higher percentage of time resting than
the wild type (Table
2). Impaired gliding was not a function
of glass binding, which for the mutant was at least as high
as the wild type (data not shown).
View this table:
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TABLE 2. Characterization of cell gliding by wild-type M. pneumoniae, P200 mutant 201GR1, and 201GR1 complemented with the recombinant wild-type P200 allele
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In order to assess the requirement for P200 in cytadherence,
we measured the binding of radiolabeled mycoplasmas to erythrocytes
in suspension (
15) and to the A549 human lung adenocarcinoma
cell line in submerged culture. Briefly, A549 cells were plated
at 5
x 10
4 cells/cm
2 on Transwell-clear culture inserts (12
mm, 0.4-µm pore size; Costar, Cambridge, MA) thin coated
with rat-tail collagen type I (Collaborative Research, Bedford,
MA) and fed apically and basally in F12K medium (Gibco-BRL,
Rockville, MD) with 10% fetal bovine serum but without antibiotics
for 4 to 5 days until the monolayer was confluent. The apical
medium was removed, the apical surface was washed with 500 µl
of prewarmed Hanks balanced salt solution (HBSS; Sigma Chemical,
St. Louis, MO), and radiolabeled mycoplasmas in 150 µl
of Hayflick medium were added, followed by incubation for 4
h. The apical liquid was then gently aspirated, and the membranes
were washed four times with PBS, kept overnight at ambient temperature
to dry, separated from the plastic Transwell supports, added
to scintillation vials containing 300 µl of 1% sodium
dodecyl sulfate, incubated overnight at 37°C, and processed
for liquid scintillation counting. The P200
mutant was
indistinguishable from wild-type
M. pneumoniae with respect
to binding to erythrocytes or A549 cells (Fig.
4A and B).
We also examined the ability of the P200
mutant to colonize
NHBE cells in air-liquid interface culture (
23). When cultured
as described, NHBE cells have functional cilia, secrete mucins,
and establish tight junctions (
10), unlike A549 cells (
38).
NHBE cells were cultured submerged on Transwell inserts as described
above for 5 to 7 days, the apical medium was removed, and incubation
was continued with the cells exposed to medium only on the basal
surface. Prior to infection cells were washed on the apical
surface with 500 µl of prewarmed HBSS to remove excess
mucus and then infected immediately or after incubation for
the indicated times. Mucin produced by NHBE cells accumulates
in the Transwell inserts over time. To control for mucus depth,
we tested
M. pneumoniae colonization at different time points
after washing the NHBE cell apical surface. The P200
mutant colonized NHBE cells at substantially lower levels than
did wild-type
M. pneumoniae (Fig.
4C). Mycoplasma attachment
dropped with length of incubation after washing, corresponding
to increased mucus depth, but at similar rates for wild-type
and mutant
M. pneumoniae.
We cloned a 3.8-kbp FspI fragment containing the MPN567 gene encoding P200 from wild-type M. pneumoniae chromosomal DNA into the SmaI site in a Tn4001 derivative within pKV104 (12). The resulting construct was confirmed by PCR sequencing (Integrated Biotech Laboratories, University of Georgia) and electroporated into M. pneumoniae for transposon delivery; transformants were cultured in Hayflick medium plus 10 or 24 µg of chloramphenicol/ml (12). The recombinant MPN567 allele produced P200 at wild-type levels (data not shown) and restored gliding velocity and resting frequency in the P200 mutant (Table 2), and these transformants colonized NHBE cells at wild-type levels (Fig. 4).
Given its TX insolubility and similarity in structure to other M. pneumoniae cytadherence-associated proteins, it came as no surprise that P200 localized to the terminal organelle in wild-type M. pneumoniae and had an altered subcellular localization in several cytadherence mutants. For these reasons, it was therefore surprising that P200 was dispensable for HA and binding to A549 cells. Nevertheless, this finding is consistent with a recent report that the P200 ortholog in the closely related Mycoplasma genitalium is likewise not required for HA (27).
At least two models might account for the impaired colonization of NHBE cells by this mutant. In the first model, P200 might function as an adhesin that binds receptors absent from erythrocytes and A549 cells but present on NHBE cells. In support of this model, preliminary analysis of protease sensitivity suggests that P200 is surface accessible (data not shown), although P200, like P65 and HMW1, has no obvious signal or transmembrane sequences. Clearly, the presence of supplementary receptor-binding mechanisms and the complexity of mycoplasma-host cell interactions has been documented (1, 8, 20).
Alternatively, the mucociliary activity of differentiated NHBE cells cultured as described presents a formidable barrier to mycoplasma attachment not found with erythrocytes and A549 cells. Estimates of the thickness of the mucus layer over airway epithelium vary from 5 to 7 µm to perhaps as much as 10-fold higher, while the upward flow rate toward the trachea is approximately 4.2 to 7 mm/min (6, 31, 34). The NHBE cell cultures used here produced approximately 1.25 µl of mucus/h/cm2, and ciliary action swept fluorescent beads to the periphery in Transwell inserts (data not shown). Therefore, we favor a model that affords an explanation whereby impaired gliding by the P200 mutant limited its ability to access NHBE cell receptors. While attachment by both wild-type and mutant mycoplasmas decreased as mucus accumulated on the apical surface, the P200 mutant was not impacted more severely than the wild type; thus, the mucus barrier alone may not account for impaired colonization by the mutant. Analysis of other gliding-deficient mutants and the spatial and temporal aspects of NHBE cell colonization are required to elucidate further the relationship between gliding and colonization of the respiratory mucosa.

ACKNOWLEDGMENTS
We thank H.-K. Park, M. Ard, K. Lee, C. Page, and R. Krause
for technical assistance; R. Herrmann for anti-P200 serum; and
M. Willby for preliminary characterization of 201GR1.
This study was supported by the University of Georgia Biomedical and Health Sciences Institute Respiratory Disease Working Group to T.M.K. and D.C.K. and by Public Health Service research grants AI22362 and AI49194 from the National Institute of Allergy and Infectious Diseases to D.C.K.

FOOTNOTES
* Corresponding author. Mailing address: Department of Microbiology, 523 Biological Sciences Building, University of Georgia, Athens, GA 30602. Phone: (706) 542-2671. Fax: (706) 542-2674. E-mail:
dkrause{at}uga.edu.

Published ahead of print on 16 October 2006. 
Editor: D. L. Burns
Present address: Centocor Research and Development, Inc., Radnor, PA. 
Present address: Department of Microbiology, Miami University, Oxford, OH. 
Present address: University of Tennessee Health Science Center, Memphis, TN. 

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Infection and Immunity, January 2007, p. 518-522, Vol. 75, No. 1
0019-9567/07/$08.00+0 doi:10.1128/IAI.01344-06
Copyright © 2007, American Society for Microbiology. All Rights Reserved.
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