Temperature factor analysis of outer membrane β-stranded porin crystal structures suggests the pore is not uniformly rigid

Outer membrane beta-stranded porins form a diverse and complex set of proteins which allow passage of molecules across the membrane interface have been analyzed here from a biophysical and structural perspective using atomic temperature factors or B-factors. Generally atomic temperature factors of molecules from crystal structures indicate the degree of mobility or disorder seen in the crystal structure. Structures of six porins (four 16 stranded beta barrel porins and two 8 stranded beta barrel porins) were taken from the PDB for the analysis based on resolution (better than 3.0 A) and R-factor (< 0.23). The residue distribution and mobility distribution was found to be characteristic of each of the porins. The mobility and residue distribution amongst the secondary structural elements were found to follow the level of homology at the sequence and structural level. The loops (L2 and L3) that had defined functional roles in structural terms were found to have lower temperature factors than the other loops. The turn regions that are thought to face the periplasmic region in the cell, showed higher temperature factors. For both the 16 stranded and the 8 stranded barrels it was found one part of the barrel (the lower wall or 'inner' wall comprising the trimer interface in the case of the 16 stranded barrels) was more rigid and the other half of the barrel (the higher or 'outer' wall) showed more mobility as seen from the temperature factors. This seems to be an intrinsic structural component of the beta barrels.