3-D structure of fragaceatoxin c reveals that actinoporins form alfa-helical barrel pores

MECHALY AE; BELLOMIO A; GIL-CARTÓN D; MORANTE K; VALLE M; GONZALEZ MAÑAS JM; GUÉRIN DM

San Miguel de Tucumán

Congreso; XLV Reunión Anual de SAIB; 2009

Sociedad Argentina de Investigación en Bioquímica y Biología Molecular

Sea anemones are soft-bodied invertebrates that live attached to rocky substrates. Actinoporins are ~20 kDa soluble pore-forming proteins components of the anemones´s poison. After binding of monomers to the target membrane, and in a process that depends on the coexistence of lipidic phases, they undergo a conformational change that leads to oligomerization and insertion of the N-Terminal α-helices into the lipid bilayer to form an active pore. The accepted pore model is the so-called toroidal pore, which is composed of 4 protein subunits and the walls are formed by the N-Terminal α-helices and lipids. In this work, we present the 3-D crystallographic structure at 1.8 Å resolution of an actinoporin recently isolated from Actinia fragacea: fragaceatocin C (FraC). In the presence of detergent, FraC displays a nonameric crown-shaped structure where the N-terminal helices are still attached to the main body of the molecule, thus suggesting a putative pre-pore state. In contrast, the cryo-electron microscopy density map at ~30 Å resolution of FraC bound to PC-SM liposomes reveals a 8-fold radial symmetry pore in the membrane inserted state. Our results suggest that actinoporin pores are formed by a cluster of 8-9 a-helices without lipids lining the walls. The stability of this pore is sustained by the remainder of the proteins, which are tightly assembled at the membrane-water interface.