CALCIUM ION-MEDIATED OPENING OF THE CHANNEL GATE IN THE PSEUDOMONAS-AERUGINOSA PORIN

The gate-forming domain of protein D2 (OprD2) in the outer membrane of Pseudomonas aeruginosa contains an amino acid sequence homologous to the calcium-binding site of the myosin light chain. This observation lets us test the effect of Ca2+ on the channel function of OprD2. The diffusion rate of p-nitrophenyl phosphate (PNPP) through OprD2 was 2.3 times higher in the presence of mM order of Ca2+, but not Mg2+ or Mn2+. Concomitantly, the intrinsic fluorescence emission of OprD2 became about 10% lower in the presence of Ca2+. As the proteolytic cleavage of the gate-forming domain of OprD2 results the channel activity about 7 times higher, we tested the effect of Ca2+ on the solute permeability through the trypsin-treated OprD2. The diffusion rate of PNPP in the trypsin-treated OprD2 appeared to be nearly identical in the presence and absence of Ca2+. The result suggests that Ca2+ activates the OprD2 channel exerting its effect on the gate-forming domain. © 1993 Academic Press, Inc.