Proteins of cytosol and amniotic fluid increase the voltage dependence of human type-1 porin.

Heat-stable proteins from human and porcine cytosol and human amniotic fluid were found to increase the voltage dependence of human type-1 porin reconstituted in planar phospholipid bilayers. Purification processes revealed that these regulatory molecules were characterized by anionic charge and apparent molecular weights of between 23 and 64 kDa. The human cytosol proteins exerted inhibitory activity only when added to the compartment with applied negative potential. The observed increase in voltage dependence of porin was due to the presence of specific proteins in cytosol and amniotic fluid, since human cerebral spinal fluid in comparable amounts had no significant effect on the channel properties. Furthermore, other anionic proteins and polypeptides investigated demonstrated no inhibitory activity, indicating that anionic charge alone could not mimic the molecular properties of the regulatory proteins. With respect to the well-documented expression of porin in the plasma membrane of various cells and species, the presented data give first clues for a biochemical regulation of the channel in this compartment.