II. Porin Structure and Function: The General Diffusion Pores Matrix Porin
III. Maltoporin Monomer Structure
IV. Structural and Funcitonal Relationships
Note: This exhibit is best viewed if the cue buttons ( ) are pressed in sequence and if the viewer does not independently manipulate the molecule on the left.
Porins allow bacterial cells to interact with their environment through the passive diffusion of small (<600 Da) hydrophillic solutes across bacterial membranes. Most porins form general, non-specific channels that are regulated by environmental changes.
A few porins display substrate-specificity
in addition to general diffusion properties. Maltoporin, also known as the LamB
porin, is among the best studied examples of the substrate-specific porins.
It is responsible for the guided diffusion of maltose and maltodextrins into
E. coli cells. Maltoporin is also one of the many porins that contains
a bacteriophage recognition sequence. It was identified as the E. coli
receptor for phage lambda before its role in the translocation of sugar was
The closed nature of the barrel keeps polar main chain atoms away from the membrane core by occupying them in interstrand H-bonds. Monomer barrels are further stabilized by internal loop structures and a hydrogen bonding brace created by Tyr residues on the barrel walls .
The profound stability of porin trimers is a product of tight monomer interactions. The monomer interface contains the amino-carboxy salt bridge and provides a hydrophobic core through extensive (mainly hydrophobic) residue interactions over 35% of the molecule. The C termini and strand 16 are essential to trimer formation. Points of contact include the barrel walls and peripheral contacts between L1 and L5. Cell surface loop 2 increases trimer stability by folding into the channel of an adjacent monomer and hydrogen bonding with loops 2, 3, and 4. Salt bridges are also formed with arginines in L3.
Many of the channel's functional properties stem from its loops. Some loops pack together to form a hydrophilic umbrella structure over the channel opening . It is inferred that these loops protect the channel and screen solutes based upon size and charge.
folds into the channel and packs against the channel wall, forming a 9 Å
constriction zone halfway through the barrel . Contributors to the constriction with the barrel wall are highly conserved
Pro, Glu, Phe, Gly residues at the tip of loop
3 as well as Asp113 and Glu117
. The constriction zone determines a pore's selectivity and absolute
solute size limitation. The channel diameter widens to 15 X 22 Å below
the constriction .
Successive stands are connected though periplasmic b-hairpin turns and irregular cell surface loops . The loops found in maltoporin are considerably longer than those found in OMPF. Parts of loop 6, together with loop 4, loop 5, and loop 9 form the protective compact structure over the channel entrance .
In addition to loop
2 from an adjacent subunit , three monomer surface loops fold into the channel . As in OMPF, aromatic and ionizable residues in loop
3 form a mid-channel constriction zone with residues on the barrel wall
. Additionally, residues from loop 1 and
loop 6 narrow the channel entrance .
Another series of resistance mutations occurs in sites 151, 152, 163, 245, 247, and 250, all buried by loop compaction . These mutations may affect umbrella behavior or surface structure and have variable effects on sugar translocation. An additional mutation site (residue18) abolishes translocation and is located on loop 1 . The precise nature and structure of the lambda binding site is unclear.
This aromatic helical path, or "greasy slide", is surrounded by a number of ionizable residues from the channel lining that are assumed to replace the hydration shells of diffusing molecules and convey sugar specificity to the channel .
Although the precise molecular mechanisms
underlying maltoporin function are still unknown, translocation is currently
modeled as follows. Residues from L1 and L6 serve to orient sugar molecules
which are then guided through the constriction zone along the greasy slide.
Channel specificity is determined by residues in the channel lining.
Cowan, S. W., T. Schirmer, G. Rummel, M. Steiert, R. Ghosh, R. A. Pauptit, J. N. Jansonius, and J. P. Rosenbusch. (1992). Crystal structures explain functional properties of two E. coli porins. Nature 358: 727-733.
Hofnug, M (1995). An Intelligent Channel (and More). Science 267: 473-474.
Schirmer, Tilman, Thomas A. Keller,
Yan-Fei Wang, and Jurg P. Rosenbusch. (1995). Structural Basis for Sugar Translocation
Through Maltoporin Channels at 3.1 A Resolution. Science 267: 512-514.