Mycobacterial membrane protein, main porin (MspA)
Mycobacterium smegmatis (bacteria)
Biological membranes are made up of a lipid bilayer which is highly hydrophobic. These membranes hold a host of proteins spanning the membrane, called transmembrane proteins. Due to their highly hydrophobic nature, membranes prevent the direct transfer of charged, hydrophilic molecules across them. Hence special transmembrane proteins are required to facilitate the transport of hydrophilic molecules across the membranes. Porins are proteins found in the outer membrane of bacteria. They are transmembrane channel proteins that allow hydrophilic nutrients to pass through the hydrophobic outer membrane.
They normally have cylindrically shaped structures composed entirely of beta strands. Their inner surfaces are polar and hydrophilic allowing the passage of hydrophilic molecules while their outer surfaces are hydrophobic allowing them to interact with the lipid bilayer. Loops of polypeptide chains lining the lumen of the channel often protrude into it, narrowing the channel and allowing only certain molecules to pass through. The structure here is that of MspA, the main porin from Mycobacterium smegmatis. MspA is consists of 8 identical subunits which are tightly connected together to form a central channel. The channel has a goblet like structure with a thick rim of anti-parallel beta sheets at the top, a beta barrel stem and a narrow beta barrel base at the bottom. The structure shown here is a homodimer. And four of it align annularly and make up homooctamer explained above.
The narrow base, also called the pore eyelet, constricts the channel and gives it a small diameter of 28 Angstroms. The outer surface of the rim has polar residues and that of the stem and the base have non-polar residues indicating that the stem and the base are the major parts of the porin fraction embedded in the outer membrane.
Protein Data Bank (PDB)
author: Ashwini Patil