II. Porin Structure and Function: The General Diffusion Pores Matrix Porin
III. Maltoporin Monomer Structure
IV. Structural and Functional Relationships in maltoporin
The porin superfamily contains a number of homotrimeric (transmembrane) proteins that form water-filled pores across the outer cell membranes of gram-negative bacteria. It has been possible to isolate a number porins in their active forms and generate monomer crystal structures.
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.
Although there is little to no sequence homology among porins, they often share a strong structural resemblance. OMPF (outer membrane protein f) is among the most general E. coli porins and is, therefore, used as a reference point in discussing the structures of more specific porins such as maltoporin. The active form of the OMPF protein is formed by the joining of three identical monomers. A major component of OMPF monomer is a 16-stranded antiparallel beta sheet barrel that encloses a channel 7 X 11 Å in diameter <
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 <
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 <
Loop 3 folds into the channel and packs against the channel wall, forming a 9 Å constriction zone halfway through the barrel <III. Maltoporin Monomer Structure
Although there is no recognized sequence homology between maltoporin and OMPF, there are a number of folding similarities. Maltoporin monomers are 80 residues longer and take the form of 18-stranded beta barrels <
Successive stands are connected though periplasmic beta hairpin turns and irregular cell surface loops <
In addition to loop 2 from an adjacent subunit <IV. Structure/Function Relationships in Maltoporin
Another series of resistance mutations occurs in sites 151, 152, 163, 245, 247, and 250, all buried by loop compaction <
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
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