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.

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 which contains a bacteriophage recognition sequence. It was recognized as the E. coli receptor for phage lambda before its role in the translocation of sugar was elucidated.   

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 channel is slightly cation selective. Successive beta strands are connected through periplasmic beta hairpin turns and long, tightly packed cell surface loops that are variable in structure <>. The barrel is completed by a salt bridge-mediated joining of the amino Ala and highly conserved Phe carboxy termini in the last (16th) strand <>.

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.

Loop 3 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 <>.

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 <> that enclose smaller channels, 5-6 Å in diameter. The barrel is completed by a linking of the amino (Val¹) and carboxy (Trp⁴²¹) termini in strand 18 <>, and barrel stability and active trimer formation are influenced by many of the same types of residue interactions described for OMPF.

Successive stands are connected though periplasmic beta 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 <>. 

A. Lambda Phage Receptor

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.

B. Sugar Translocation

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. 

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