TOPOLOGY OF THE OUTER-MEMBRANE PHOSPHOLIPASE-A OF SALMONELLA-TYPHIMURIUM

The outer membrane phospholipase A (OMPLA) of Enterobacteriaceae has b een proposed to span the membrane 14 times as antiparallel amphipathic beta-strands, thereby exposing seven loops to the cell surface. We ha ve employed the epitope insertion method to probe the topology of OMPL A of Salmonella typhimurium. First, missense mutations were introduced at various positions in the pldA gene, encoding OMPLA, to create uniq ue BamHI sites. These BamHI sites were subsequently used to insert lin kers, encoding a 16-amino-acid B-cell epitope. Proper assembly of an m utant proteins was revealed by their heat modifiability in sodium dode cyl sulfate-polyacrylamide gel electrophoresis. The accessibility of t he inserted epitopes was assessed. Immunofluorescence analysis of inta ct cells with antibodies against the inserted epitope showed that thre e of seven predicted loops are indeed cell surface exposed. Trypsin ac cessibility experiments verified the cell surface exposure of two addi tional loops and provided support for the proposed periplasmic localiz ation of three predicted turns. For two other predicted exposed loops, the results were not conclusive. These results support to a large ext ent the proposed topology model of OMPLA. Furthermore, the observation that the substitutions Glu66Pro and Glu247Gly virtually abolished enz ymatic activity indicates that these residues might play a major role in catalysis.