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Porins


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Gram-negative bacteria are characterized by the construction of their cell wall: the cytoplasmic or inner membrane engulfes the cytoplasm, a murein layer lends mechanical stability and shape. A second, outer membrane surrounds the cell with few contacts to the inner membrane. In between is the periplasm; in this compartment there are some metabolic activities which would disturb the metabolism within the cell proper - e.g. reactions dealing with toxic substances. The exchange of substances between cytoplasm and periplasm is regulated by highly specific transport systems. The exchange between periplasm and environment occurs via porins, which may be unspecific or specific for groups of substances.

The expression of porins is regulated (the total protein content of the outer membrane is constant). In Escherichia coli grown in media with low osmotic pressure predominates the nonspecific porin (outer membrane protein) OmpF, at higher osmolarity more OmpC is inserted into the membrane. Lack of phosphate induces PhoE, group specific porins are e.g. LamB (sugar) and Tsx (nucleosides).

OmpF-Pore from Escherichia coli
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General properties:

The construction principle of porins is the same irrespective of their type: a chain of 300 - 420 amino acids folds to an antiparallel beta-barrel of 16 or 18 strands (exemplified here by the unspecific porin OmpF) . The wall of the pore has a thickness of one amino acid only. On the side of the barrel facing the periplasm the beta strands are connected by short loops or turns . On the other side the loops directed to the environment are larger and variable. The loop connecting beta strands 5 and 6 is of special importance: it is folded into the barrel and constricts the cross section . At the narrowest point there are some ionizable amino acids . The filter properties of the pore are defined at this point.
This principle is found in other nonspecific porins too, although there is no sequence homology. In the frames below the starting view is from the periplasm through the pores to the outside. Use the mouse to see the details!
   
Porin from Rhodobacter capsulatus
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Porin from Rhodpseudomonas blastica
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PhoE from Escherichia coli
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OmpF porin in the membrane of Escherichia coli
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Quarternary structure:

Porins are inserted in the outer membrane as trimers. Amino and carboxy termini of the single molecules face the threefold symmetry axis of the complex. As found in other transmembrane proteins there are two belts of aromatic amino acids pointed to the surfaces of the membrane (amino acids at the outer face of the outer membrane are marked green, those facing the periplasmic side are in blue). Between the belts the surface of the barrels is composed mainly from hydrophobic amino acids. The belts are placed in a distance of ~ 25 Å corresponding to the thickness of the outer membrane.
The loops at the outer face narrow down the entrance opening of the pores. One loop is positioned in a way to line the opening of a neighboring pore . This 'domain swapping' stabilizes the quarternary structure.

Function:

Porins are tubes with a diameter of about 1 nm which are filled with water. Nonspecific porins allow the diffusion of ions and molecules up to a molecular weight of 600. The diffusion speed depends on both the difference of concentration in the periplasm and outside and the molecular weight of the solute.

The passing of ions may be regulated electrically. If (in vitro) a voltage of + or - 100mV is applied, the channel is closed for ions (voltage gating). This phenomenon is also found in another class of beta-barrel pores, the toxins. From mutation experiments it was concluded that there is no movement of the loop constricting the inside of the channel. Probably the applied voltage changes the electrostatic properties of the interior wall. A physiological function of voltage gating is not obvious.


Sugar selective porins

The enzymatic degradation of starch results in maltodextrines (alpha-1-4-connected polyglucose molecules). The entry system into the periplasm for maltodextrines up to seven residues in length is the maltoporin. Previously this protein was recognized as the receptor for phage lambda and therefore termed LamB. It is induced together with other proteins neccessary for maltose metabolism, among them a system for transport through the cytoplasmic membrane. Besides maltoporines LamB allows the passage of some monosaccharides (in some cases with a larger diffusion speed) and disaccharides (trehalose; lactose and sucrose pass much slower).

LamB pore from Escherichia coli
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The barrel of sugar selective porins is composed of 18 antiparallel beta strands . In LamB three loops extend into the barrel . From the spacefilling view it is obvious that this channel is tighter at the narrowest spot than the general porins (the opening is 7 Å x 10 Å). This shows in the ion conductivity: LamB has a conductivity of 0.15 nS vs. 0.8 nS in OmpF. The sugar (here maltotriose) fills up the opening completely.

What is the reason for the sugar selectivity of these pores? The X-ray structure determination of the protein crystal with bound sugar gives a snapshot of the diffusion process in the moment when the maltodextrin (here trimeric glucose) is stuck at the narrow point. So we can read the interchange of sugar and protein in the recognition moment. In the inner Wall of the channel there is a helical belt of aromatic amino acids, a "greasy slide" . Opposite to this there is a tyrosine in the constricting loop . This restricts the way in a manner, that only sugar with a flat geometry (like maltodextrin composed of glucose) may pass the channel. The 'uppermost' tryptophan of the slide is part of a loop of a neighboring pore. There are apolar van der Waals contacts between the sugar and the greasy slide . Besides the slide there are a couple of amino acids detecting the sugar via hydrogen bonds (if you lost your orientation now, use the mouse to turn the molecule). In a total view you may see: the greasy slide (blue), the hydrophilic narrow spot (yellow), the amino acids involved in phage binding (green) and the sugar in one of the pores (red).


Saccharose: example of rotational movement between glucose and fructoseSaccharose in LamB: contacts with amino acids
keep the sugar in a fixed position,
rotations along single bonds are not possible

The disaccharide saccharose is more bulky in structure than maltose. The space between the slide and Tyr118 is too narrow to let saccharose pass, in addition one asparagine and arginine each obstruct the way more than in the case with maltose. So the diffusion speed of saccharose is only 2.5% of that of maltose.

However, in enteric bacteria there is a saccharose specific transport system too. A plasmid codes a regulon containing a PTS situated in the cytoplasmic membrane and a porin (ScrY) in the outer membrane. This porin exhibits only 20% amino acid homology to maltoporin, but there is an identical topology in the structure of the beta-barrel.

ScrY-Pore (Plasmid pUR400)
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The functional difference is caused by the arrangement of the narrow spot. The aromatic amino acids of the slide are positioned as in LamB, with the last one facing the periplasm missing. Instead of the limiting tyrosine and asparagine (LamB) there are differently positioned asparagine and phenylalanine. The hydrophilic amino acids are spaced somewhat further apart, so there is an opening of 8,5 Å x 11 Å. The ion conductivity of this channel is 1.4 nS.

As in LamB there is a loop in ScrY that hooks on the rim of a neighboring pore lending one aromatic amino acid to the slide and tightening the contacts in the trimer. On the periplasmic face another connecting device is found: each monomer contains two phenylalanines parallel to each other (stacking of pi-electrons of the aromatic rings). These stacks are adjacent in the trimer giving strong hydrophobic contacts around the trifold axis .


Relationships

Structural relationships of proteins may be found in special databanks, e.g. CAMPASS (Cambridge University). For each protein family there are alignment data (primary structure) together with structural data (secondary structure). Additionally atomic coordinates are given with spatial superpositioning of the protein chains. The porin family in CAMPASS contains OmpF, the porin from Rhodobacter capsulatus (both 16-stranded) and maltoporin (LamB, 18-stranded). Here please find an excerpt of the data bank:


porins
1mal-0
maltoporin (escherichia coli) 3.10:21.70
    0    1   1  421   (DDBASE:
2omf-0
matrix porin outer membrane protein f (escherichia coli) 2.40:16.70
    0    1   1  340   (DDBASE:
2por-0
porin (crystal form b) ((rhodobacter $capsulatus) (strain 37b4) (formerly) 1.80: 9.99
    0    1   1  301   (DDBASE:

Percentage identity matrix
2por-0 2omf-0 1mal-0 2por-0 100.0 16.0 10.2 2omf-0 16.0 100.0 7.6 1mal-0 10.2 7.6 100.0 2por-0 2omf-0 1mal-0 high highest percentage identity low lowest percentage identity
Alignment based on similarities in structural features (COMPARER alignment)

2por-0( 1 ) evklsgdarmgvmy----------nGd----dw--nfssrs

2omf-0( 1 ) aeiynkdgnkvdlyGkaVgLhyfSkg-------nGenSyggn--gdmtya

1mal-0( 1 ) vdfhGyarsgiGwTGsggeqqçFqTtgAqskYRLgNEçet

bbbbbbbbbbbb b



2por-0( 26 ) rvlftmsgttd---sgl-efgasfk---ahes--vgaetg---edgtvfl

2omf-0( 42 ) rlGfkgetqin---sdltgygqweyNfqgnnsegadaqtgnktrlafagl

1mal-0( 41 ) yaelklGqevwkegdksfyfdtNvAysvaqqn----dweatdpafrEaNv

bbbbbbbbb bbbbbbb bbbb



2por-0( 64 ) sga-------fgkiemgdAlGASEalFgdLyeVGYtdLddrgGNdIpYLT

2omf-0( 89 ) kyad------vgsfdygrnyGVVydAlg-yTdmLpef-----GgdtAysd

1mal-0( 87 ) qGknliewlpgStiWagkrfYq-----rh--dVhMId------FyYWdIs

bb bbbbb



2por-0( 107 ) GderlTaedNpVlLytysa-----gafsvAasmSdgkvge----------

2omf-0( 127 ) DFFVGrVg--gVaTyrNsnffglvdglnfAvQyLgknerd----------

1mal-0( 124 ) ---------gpgaglenidvg----fgklslAaTrsseaGGSSsfasnni

bbbbbb bbbbbb



2por-0( 142 ) ts-eddaqEmAvAaaytf----gnytvglGyEkIdSp--dta-lma----

2omf-0( 165 ) tarrSNGdgvggSisyey----egfGiVGAyGaAdRTnlQeaqpLGn--g

1mal-0( 161 ) ydYtneTaNdVfDvRlaqmeinpgGtlelGvDyGrAnlr-dnyrLvdgAS

bbbbbbbbbb bbbbbbbbbbb



2por-0( 180 ) -dMeQlElAaiakfgatnvkaYyAdge-----lDrdfAravfdlt---pv

2omf-0( 209 ) kkAeQwAtGlkydanniylAaNygetrnATpItnkft-------------

1mal-0( 210 ) kdgwLfTaEhtqsvlkgfnkfVvQyAtdSMTsqGkGlSqGSgVafDnekf

bbbbbbbbbbbb bbbbbbbbbbb



2por-0( 221 ) aaaAtavdHkAyglSvdstf----gattvggYvQvldId-tIddvt-yyG

2omf-0( 246 ) -ntsGFAnkTqdvLlVaQyqfdfglrpSiAyTkSkAkdVegigdVdlvnY

1mal-0( 260 ) AyninNnghMlRiLdhGAismgdnwDmMyVgMyQdinwd-ndnGtk-WwT

bbbbbbbbbbb bbbbbbbbbbbb bb bbb



2por-0( 265 ) lgasydlg--ggasivggiAdndlp-----nSdmVaDlgvkfkf

2omf-0( 295 ) fEvGatyyfnknmstyvDyIiNqIdsdnkLgvGsddTVAvGivyqf

1mal-0( 308 ) vgiRpMykwtpimStVmEiGyDnVeSqrtgdkNnQyKiTlAqQwQagdsI

bbbbbbbb bbbbbbbbbb bbbbbbbb



2por-0( )

2omf-0( )

1mal-0( 358 ) wsRpAiRvFaTyAkwdEkWGyDytgnAdnnanfGkAVpadfnggsfgrgd



2por-0( )

2omf-0( )

1mal-0( 408 ) sdewTfgaqmEiww


Key

solvent inaccessible                    UPPER CASE      X
solvent accesible lower case x
alpha helix red x
beta strand blue x
3 - 10 helix maroon x
hydrogen bond to main chain amide bold x
hydrogen bond to mainchain carbonyl underline x
disulphide bond cedilla ç
positive phi italic x


Mind the low amino acid homology, which is 16% only for the two 16-stranded pores. Nevertheless the Calpha atoms of the beta-barrel structures may be superimposed with small deviations only (the frame to the left shows the backbone structures of the proteins, e.i. lines connecting the Calpha atoms). Even the 18-stranded pore has homology in the part where the subunits contact each other (switch LamB on in the table below).

 

onporeoff
OmpF
R. caps. pore
LamB

 



Literature:
R Benz & K Bauer, Permeation of hydrophilic molecules through the outer membrane of gram-negative bacteria, Eur. J. Biochem. 176 (1988) 1-19
T Schirmer, General and specific porins from bacterial outer membranes, J. Struct. Biol. 121 (1998) 101-109
G Bainbridge et al, Voltage gating is a fundamental feature of porin and toxin beta-barrel membrane channels, FEBS Lett. 431 (1998) 305-308
R Dutzler et al, Crystal structures of various maltooligosaccharides bound to maltoporin reveal specific sugar translocation pathway, Structure 4 (1996) 127-134
D Forst et al, Structure of the sucrose-specific porin ScrY from Salmonella typhimurium and its complex with sucrose, Nature Structural Biology 5 (1998) 37-46
R Sowdhamini et al, CAMPASS: A database of structurally aligned protein superfamilies, Structure 6 (1998) 1087-1094





6-99 © Rolf Bergmann