Structural analysis of outer membrane beta-stranded porins using B-factor

Porins are integral membrane proteins that are found in the outer membrane of Gram-negative bacteria, eukaryotic mitochondria and chloroplasts. They act as molecular sieves to ensure that the unhindered diffusion of nutrients and waste materials of size <600 Da into the periplasmic space while protecting the cell from hostile substances like degrading enzymes, bile salts, antibiotics, toxins, phages and abrupt changes in osmotic pressure. Porins are composed of beta barrel monomers and each monomer has 250-450 residue. Typically for 16-stranded porins, –strands β 15 and 1516 forms the inner wall facing towards the trimer interface while 713 β β β β β β forms the outer wall that interacts with lipids or detergents while 6 and 14 forms the boundary β β between inner and outer walls. There are loops at extracellular ends and turns at periplasmic ends. The loop L3 forms the eyelet that restricts the pore size and he loop L2 latch out with neighboring monomers. There are some other porins reported according to no of strands like 8, 18 stranded porins whose crystal structures are known. Crystallographic B-factor can be used as indicator of conformational mobility or flexibility of proteins. B-factor distribution analysis has been used earlier for analyzing structural and functional characteristics of protein structures. We have investigated the use of B-factor for analysis of mobility in some typical porin structures from the Protein Data Bank which have resolution better than 3.0A and R-value < 0.23. Results