Crystal and Electron Microscopy Structures of Sticholysin II Actinoporin Reveal Insights into the Mechanism of Membrane Pore Formation

Sticholysin II (StnII) is a pore-forming protein (PFP) produced by the sea anemone Stichodactyla helianthus. We found out that StnII exists in a monomeric soluble state but forms tetramers in the presence of a lipidic interface. Both structures have been independently determined at 1.7 A˚ and 18 A˚ resolution, respectively, by using X-ray crystallography and electron microscopy of two-dimensional crystals. Besides, the structure of soluble StnII complexed with phosphocholine, determined at 2.4 A˚ resolution, reveals a phospholipid headgroup binding site, which is located in a region with an unusually high abundance of aromatic residues. Fitting of the atomic model into the electron microscopy density envelope suggests that while the β sandwich structure of the protein remains intact upon oligomerization, the N-terminal region and a flexible and highly basic loop undergo significant conformational changes. These results provide the structural basis for the membrane recognition step of actinoporins and unexpected insights into the oligomerization step. [Copyright &y& Elsevier]