Defensins are a class of ubiquitously expressed cationic antimicrobial peptides (CAPs) that play an important role in innate defense. Plant defensins are active against a broad range of microbial pathogens and act via multiple mechanisms, including cell membrane permeabilization. The cytolytic activity of defensins has been proposed to involve interaction with specific lipid components in the target cell wall or membrane and defensin oligomerization. Indeed, the defensin Nicotiana alata defensin 1 (NaD1) binds to a broad range of membrane phosphatidylinositol phosphates and forms an oligomeric complex with phosphatidylinositol (4,5)-bisphosphate (PIP2) that facilitates membrane lysis of both mammalian tumor and fungal cells. Here, we report that the tomato defensin TPP3 has a unique lipid binding profile that is specific for PIP2 with which it forms an oligomeric complex that is critical for cytolytic activity. Structural characterization of TPP3 by X-ray crystallography and site-directed mutagenesis demonstrated that it forms a dimer in a "cationic grip" conformation that specifically accommodates the head group of PIP2 to mediate cooperative higher-order oligomerization and subsequent membrane permeabilization. These findings suggest that certain plant defensins are innate immune receptors for phospholipids and adopt conserved dimeric configurations to mediate PIP2 binding and membrane permeabilization. This mechanism of innate defense may be conserved across defensins from different species. P lant defensins are small (ϳ5 kDa), cysteine-rich, cationic peptides that belong to the broad class of innate defense molecules known as cationic antimicrobial peptides (CAPs). Plant defensins play a major role in plant innate immunity and have been identified in all analyzed plant species to date, as either constitutively expressed or induced defense molecules that are produced in response to pathogenic attack or environmental stress (1). The tertiary structure of all plant defensins is highly conserved, comprising a triple-stranded antiparallel ␤-sheet and a single ␣-helix stabilized by four disulfide bridges, known as the "cysteine-stabilized alpha-beta" or "CS␣␤" motif (2). Despite this conserved three-dimensional structure, there is a high degree of variation in the primary sequence of plant defensins, particularly at intervening loop regions, which are typically important for activity (3).Many plant defensins have antifungal activity, but other functions have been reported, including antibacterial activity, ion channel blocking, protein synthesis inhibition, and trypsin and ␣-amylase inhibition as well as roles in heavy metal tolerance, plant development, and pollen tube guidance (3-5). They can be divided into two classes based on whether or not a C-terminal propeptide (CTPP) (of ϳ33 amino acids) is present (2, 6). This domain is involved in vacuolar targeting and protects the plant cells from phytotoxicity during transit through the secretory pathway (7). Defensins expressed with the additional CTPP domain are kn...

Structure–function analyses driven by a crystal structure of the cytosolic domain of the Drp1 receptor MiD51 reveals a nucleotidyltransferase fold and nucleotide binding activity that is independent of its Drp1 binding activity.

Mitochondria are dynamic organelles whose shape is regulated by the opposing processes of fission and fusion, operating in conjunction with organelle distribution along the cytoskeleton. The importance of fission and fusion homeostasis has been highlighted by a number of disease states linked to mutations in proteins involved in regulating mitochondrial morphology, in addition to changes in mitochondrial dynamics in Alzheimer's, Huntington's and Parkinson's diseases. While a number of mitochondrial morphology proteins have been identified, how they co-ordinate to assemble the fission apparatus is not clear. In addition, while the master mediator of mitochondrial fission, dynamin-related protein 1, is conserved throughout evolution, the adaptor proteins involved in its mitochondrial recruitment are not. This review focuses on our current understanding of mitochondrial fission and the proteins that regulate this process in cell homeostasis, with a particular focus on the recent mechanistic insights based on protein structures.

Crystallization of macromolecules is famously difficult. By knowing what has worked for others, researchers can ease the process, both in the case where the protein has already been crystallized and in the situation where more general guidelines are needed. The 264 crystallization communications published in Acta Crystallographica Section F in 2012 have been reviewed, and from this analysis some information about trends in crystallization has been gleaned. More importantly, it was found that there are several ways in which the utility of these communications could be increased: to make each individual paper a more complete crystallization record; and to provide a means for taking a snapshot of what the current `best practices' are in the field.

A search for the source of mysterious signals at the Parkes Observatory had puzzled CSIRO astrophysicists for years, until the answer came in a flash, writes Viviane Richter.Parkes Observatory hosts one of the largest single-dish telescopes in the southern hemisphere, upgraded many times since it started operating in 1961.

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