Many plant aspartic proteases contain an additional sequence of ϳ100 amino acids termed the plant-specific insert, which is involved in host defense and vacuolar targeting. Similar to all saposin-like proteins, the plant-specific insert functions via protein-membrane interactions; however, the structural basis for such interactions has not been studied, and the nature of plantspecific insert-mediated membrane disruption has not been characterized. In the present study, the crystal structure of the saposin-like domain of potato aspartic protease was resolved at a resolution of 1.9 Å , revealing an open V-shaped configuration similar to the open structure of human saposin C. Notably, vesicle disruption activity followed Michaelis-Menten-like kinetics, a finding not previously reported for saposin-like proteins including plant-specific inserts. Circular dichroism data suggested that secondary structure was pH-dependent in a fashion similar to influenza A hemagglutinin fusion peptide. Membrane effects characterized by atomic force microscopy and light scattering indicated bilayer solubilization as well as fusogenic activity. Taken together, the present study is the first report to elucidate the membrane interaction mechanism of plant saposin-like domains whereby pH-dependent membrane interactions resulted in bilayer fusogenic activity that probably arose from a viral type pH-dependent helix-kink-helix motif at the plant-specific insert N terminus. Aspartic proteases (APs)2 are characterized by a common bilobal tertiary structure containing two catalytic aspartic acid residues (Asp 32 and Asp 215 in pepsin) within an active site cleft(1, 2). They are found in all higher organisms, and their respective roles are well established, although structural and functional characteristics of APs in plants are least understood. Of practical interest among plant APs are their roles in plant pathogen resistance (3) as well as in senescence and postharvest physiology (4, 5). Plant APs share the common AP bilobal structure; however, some contain an additional sequence of ϳ100 residues inserted within the C-terminal primary structure. These additional amino acids unique to plant APs (6 -8) create an extra domain protruding from the canonical AP molecule (9 -11). This structural oddity among APs is called the plantspecific insert (PSI), also known as the plant-specific sequence, which belongs to the saposin-like protein (SAPLIP) family (12, 13). Plant APs are found in either monomeric or heterodimeric forms (9, 14); the latter result from post-translational proteolysis, which includes the removal of part or all of the PSI, whereas the PSI is retained in monomeric plant APs (6,8).In general, members of the SAPLIP family have various physiological functions, all of which entail membrane interaction (14 -16) manifested in three principal ways: membrane binding, membrane perturbation without permeabilization, and membrane permeabilization (15). Examples of SAPLIP functions include roles in exohydrolase degradation of sphingolipids in the...

Interleukin-19 (IL-19) is a novel cytokine that was ini-

Interferon-λ 1 (IFN-λ1, also known as IL-29) belongs to the recently discovered group of type III IFNs. All type III IFNs initiate signaling processes through formation of specific hetero-dimeric receptor complexes consisting of IFN-λR1 and IL-10R2. We have determined the structure of human IFN-λ1 complexed with human IFN-λR1, a receptor unique to type III IFNs. The overall structure of IFN-λ1 is topologically similar to the structure of IL-10 and other members of the IL-10 family of cytokines. IFN-λR1 consists of two distinct domains having fibronectin type III topology. The ligand/receptor interface includes helix A, loop AB and helix F on the IFN site, and loops primarily from the N-terminal domain and inter-domain hinge region of the IFN-λR1. Composition and architecture of the interface that includes only a few direct hydrogen bonds supports an idea that long-range ionic interactions between ligand and receptor govern the process of initial recognition of the molecules while hydrophobic interactions finalize it.