THE CRYSTAL STRUCTURE OF OUTER MEMBRANE PHOSPHOLIPASE, AN INTEGRAL MEMBRANE ENZYME.

H.J. Snijder, N. Dekker, I. Ubarretxena, M. Blaauw, K.H. Kalk, H.M. Verheij and B.W. Dijkstra.

University of Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands
University of Utrecht, Padualaan 8, 3584 CH Utrecht, The Netherlands

Keywords: crystal structure, membrane enzyme, phospholipase, dimerisation

The gene encoding for outer membrane phospholipase A (OMPLA) is wide spread among enterobacteriaceae, of which many are pathogenic. OMPLA is one of the few enzymes in the outer membrane of these bacteria. This 31 kDa protein hydrolyses phospholipids, displays broad substrate specificity, is strictly calcium dependent and shows no sequence homology with water soluble (phospho)lipases or with the structurally related porins. This phospholipase is surrounded by its own substrate but is dormant in normally growing cells. Perturbations of the membrane trigger the hydrolysis of phospholipids by forming active dimers.

We report the crystal structure of this membrane enzyme to 2.4 A resolution. The fold is similar to the unrelated porins, revealing a 12 stranded anti-parallel ß-barrel. The active site is located at the end of a ß-strand and it harbours a unique catalytic triad comprising an asparagine, histidine and a serine. Furthermore, the calcium binding site has been identified as well as the dimerisation interface.