This study aimed to determine the channel size and oligomeric structure of the Borrelia burgdorferi porin P66. Nonelectrolytes of varying sizes were used to measure the channel diameter and identify potential constriction zones. P66 conductance was reduced by nonelectrolytes up to 0.60 nm in radius, indicating an entrance diameter of 1.9 nm with a 0.8 nm constriction. Blocking experiments and blue native PAGE analysis suggested that P66 forms a heptameric bundle of pores, which could explain its unusually high conductance. This challenges the view of P66 as a single large channel and provides new insights into its structure-function relationship.

1. Iván Bárcena Uribarri†1, Marcus Thein1, Elke Maier1, Mari Bonde2, Ignas Bunikis2, Sven Bergström2 and Roland Benz1
1 Biozentrum University of Würzburg, D- 97974 Würzburg, Germany.
2 Department of Molecular Biology, Umeå University, Sweden.
†Corresponding author: ivan.barcenauribarri@uni-wuerzburg.de
Channel size and oligomeric constitution
of the Borrelia burgdorferi P66 porin
Borrelia species are obligate parasites transmitted to mammals by ticks. Borrelia have limited biosynthetic capacity [1] and, therefore, they are dependent on nutrients provided by their hosts. The first step of
nutrient availability is accomplished by water-filled channels, so called porins, across the outer membrane of these bacteria. P66 is so far the best studied porin in Borrelia showing dual function, acting not only
as a porin but also as an adhesin [2,3]. P66 form pores in planar lipid bilayers with a conductance of 11 nS in 1 M KCl [2] which is an atypical and rare high single channel conductance for Gram-negative
bacterial porins [4]. Previous estimations of P66 channel diameter led to a diameter estimation of 2.6 nS [2]. This calculation of the P66 channel diameter was based on the assumption that the conductance of
the channel is equal to the conductivity of a simple cylinder of aqueous salt solution. Therefore, the calculated value of the P66 diameter appears to be somewhat preliminary and its apparent size and structure
remain unclear. An applied method using nonelectrolytes with known hydrodynamic radii [5] was used to calculate the real diameter of P66. This method should provide a more accurate estimate of the P66
channel diameter using a biophysical approach.
Planar Lipid Bilayer Assay Use of Nonelectrolytes to Determine the Channel Diameter
Results (1): P66 Pore Diameter
Results (2): Blockage of P66
and Oligomeric Structure
Nonelectrolyte
G
(nS)
r
(nm)
χ
(mS
cm-1)
F%
None 11 - 110.3 -
Ethylene glycol 6.5 0.26 57.2 96.8
Glycerol 5.5 0.31 49.1 106.7
Arabinose 7 0.34 63.7 100.6
Sorbitol 7.5 0.39 57.8 65.2
PEG 200 7.5 0.50 46.1 64.1
PEG 300 7.5 0.60 45.5 42.6
PEG 400 1 0.70 46.4 -
PEG 600 1 0.80 54.1 -
PEG 1000 12 0.94 49.5 9.0
PEG 3000 14 1.44 48.9 5.2
PEG 6000 10.5 2.50 50.5 5.2
0
20
40
60
80
100
120
0,0 0,5 1,0 1,5 2,0
Hydrodynamic radius (nm)
Conductance (%)
NE inside
the channel
NE too big to be
inside the channel
NE block the channel
What are nonelectrolytes?
-substances that are not charged when dissolved in
aqueous solutions. They should have a spherical
shape with a particular radius when dissolved in
water (Table 1).
Principles:
- NEs, when added to salt solutions, increase the
viscosity and thus decrease their conductivity (20%
NEs reduce the aqueous conductivity to 50-
60%)(Table 1).
- This decrease in the conductivity will only affect the
channel conductance when the NEs enter the
channel interior (Fig. 6).
K+
Cl-
Cl-
K+
K+
Cl-
Cl-
K+
Radius of the constriction zone:
should be equal to the radius of the smallest NE
that do not pass freely trough the channel and
therefore don’t fill it 100%.
Radius of the channel entrance:
should be equal to the radius of the smallest NEs that do not enter the pore.
Channel filling concept: Finding a inner constriction.
- The portion of a channel filled with an nonelectrolyte
can be determined using the following formulas:
F%=100
F%= 0
F%= 30-80
F = [(Go-Gi)/ Gi]/[(Χo -Χi)/ Χi]
F% = 2Fi/(F1+F2)*100%
Instrumentation:
-A Teflon chamber with two compartments containing a
1M KCl salt solution (Fig.1). The two compartments are
separated by a thin wall and connected by a 0.4 mm2
small circular hole. The membranes are formed
spreading a 1% (w/v) solution of diphytanoyl
phosphatidylcholine (PC) in n-decane over the hole
(Fig.2).
-Ag/AgCl electrodes (Fig.1)
-Voltage source (Fig.3)
-Amplifier (Fig.3)
-PC/Recorder (Fig.3)
F%
100%
60%
0%
Large nonpermeant NEs with hydrodynamic radii between 0.94
and 2.50 nm did not enter the P66 channel and showed no
effect on its conductance. However, in the presence of small
NEs with hydrodynamic radii up to 0.60 nm, the P66 single-channel
conductance decreased proportional to that of the bulk
solution conductivity (Fig.8).
Surprisingly, the presence of PEG 400 and PEG 600 resulted in
an exceptional low single-channel conductance of 0.9 nS that
was not proportional to the bulk aqueous conductivity (Fig.8).
To determine a possible constriction the results of the dependence
of F% on the hydrodynamic radii of the NEs are shown in figure 9.
The estimation of the P66 pore size based on our single-channel
measurements with different NEs indicated an entrance pore
diameter of approximately 1.9 nm with a 0.8 nm inner constriction.
References:
Membrane experiments in the presence of 20 % PEG
400 or PEG 600 resulted in drastically reduced single-channel
conductance during multi-channel
measurements which revealed that the P66
conductance could be blocked by 80-90% after the its
addition (Fig.10).
The size of the channel as derived from measurement with
NEs does not agree with its extremely high single-channel
conductance of about 11 nS in 1 M KCl. Furthermore, the
stepwise block of a single P66 unit with certain NEs
occurred in seven substates (Fig. 11). All these results
suggested that the P66 channel may be formed by a bundle
of pores.
To support this view, purified P66 was investigated by Blue
native PAGE, a method that allows the determination of native
protein masses and oligomeric states of protein complexes. A
460 kDa band agree with the oligomeric theory as a P66
heptamer would have a molecular mass of 462 kDa.
Fig. 1:Teflon chamber
and Ag/AgCl electrodes.
Fig. 3: Black Lipid assay set.
Fig. 2: Bilayer Formation. Multicolor multilayers lead to the
formation of only one bilayer (black)
Fig. 4: Porin inserted in a
lipid bilayer
Fig. 5: Step-like record from
a pore forming sample.
Principles:
- When added to a KCl solution,
protein samples with pore-forming
activity get inserted in the PC
membranes increasing its
conductance (Fig.4). Each
insertion is registered by the
recorder as a step (Fig.5).
Fig. 6: Effects of NEs in the
conductance of a porin.
Fig. 7: Channels filled by NEs to different degrees.
Fig. 8: Effects of different NEs in the conductance of P66.
Fig. 9: P66 channel filling with different NEs .
Table 1: Conductance of P66 in presence of
different NEs (G), NEs hydrodynamic radius (r),
solution conductance (χ) and channel filling in terms
of percentage (F%)
Fig. 10: Blockage of approximately hundred P66 channels with PEG600.
Fig. 11: Blockage of a single P66 unit with PEG400.
Fig. 12: BN Page and WB for native P66.
[1] Fraser et al. Genomic sequence of a Lyme disease spirochaete, Borrelia burgdorferi, Nature 390 (1997) 580-586.
[2] Skare et al. The Oms66 (p66) protein is a Borrelia burgdorferi porin, Infect Immun 65 (1997) 3654-3661.
[3] Coburn et al. Characterization of a candidate Borrelia burgdorferi beta3-chain integrin ligand identified using a phage display library, Mol Microbiol 34 (1999) 926-940.
[4] Benz, R. Porin from bacterial and mitochondrial outer membranes. CRC Crit Rev Biochem (1985) 19:145-190.
[5] Krasilnikov et al. A novel approach to study the geometry of the water lumen of ion channels: colicin Ia channels in planar lipid bilayers. J Membr Biol (1998)161:83-92.
Conclusions
The extremely high conductance of P66 gives an idea
of a big channel that would allow a free molecule
exchange between the environment and the periplasmic
space. This fact could impair the defense function of the
outer membrane and it makes it difficult to understand
why small pores like Oms38 are next to such big
channels.
Using the nonelectrolytes method, a constriction zone
could be determined with a diameter of 0.8 nm. Such
an estimation is much smaller than a previous one of
2.6 nm which was based on theoretical considerations.
Single P66 blockage experiments with some NEs lead
to the idea of P66 being an association of smaller
channels and not a big hole in the outer membrane.
This fact could explain the high conductance of P66.
If that is the case, P66 could be the first known example
of a porin constituted by a bundle of seven independent
channels in a protein complex. Such a structure is until
today only observed in Borrelia, but not in any other
bacterium or any other living organisms