The inhibitor of apoptosis protein DIAP1 suppresses apoptosis in Drosophila, with the second BIR domain (BIR2) playing an important role. Three proteins, Hid, Grim, and Reaper, promote apoptosis, in part by binding to DIAP1 through their conserved N-terminal sequences. The crystal structures of DIAP1-BIR2 by itself and in complex with the N-terminal peptides from Hid and Grim reveal that these peptides bind a surface groove on DIAP1, with the first four amino acids mimicking the binding of the Smac tetrapeptide to XIAP. The next 3 residues also contribute to binding through hydrophobic interactions. Interestingly, peptide binding induces the formation of an additional alpha helix in DIAP1. Our study reveals the structural conservation and diversity necessary for the binding of IAPs by the Drosophila Hid/Grim/Reaper and the mammalian Smac proteins.

In the eukaryotic secretory and endocytic pathways, transport vesicles shuttle cargo among intracellular organelles and to and from the plasma membrane. Cargo delivery entails fusion of the transport vesicle with its target, a process thought to be mediated by membrane bridging SNARE protein complexes. Temporal and spatial control of intracellular trafficking depends in part on regulating the assembly of these complexes. In vitro, SNARE assembly is inhibited by the closed conformation adopted by the syntaxin family of SNAREs. To visualize this closed conformation directly, the X-ray crystal structure of a yeast syntaxin, Sso1p, has been determined and refined to 2.1 A resolution. Mutants designed to destabilize the closed conformation exhibit accelerated rates of SNARE assembly. Our results provide insight into the mechanism of SNARE assembly and its intramolecular and intermolecular regulation.

Astrephomene gubernaculifera Pocock is capable of growth on sodium acetate, even in the absence of light. From an evolutionary standpoint, it would be beneficial for this motile heterotrophic organism to be able to sense and swim to such an energy source. We used three assays to test for chemoattraction of A. gubernaculifera. In each case, there was strong chemoattraction to acetate. Propionate, a molecule structurally similar to acetate, also elicited a strong chemoattraction response, even though it was incapable of supporting growth in the dark in this strain. This implies that chemoattraction in A. gubernaculifera is based on a receptor‐mediated signaling response. Video analysis of colonies exhibiting the attraction response indicated that most of them remained motile in the vicinity of the acetate. This rules out the simplest possible model for accumulation in which colonies stop when they encounter increased acetate concentrations. We also tested a variety of other organic molecules for their ability to chemoattract A. gubernaculifera. Of the 11 sugars, 18 amino acids, and three other organic molecules tested, only the amino acid phenylalanine elicited chemoattraction.

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Astrephomene gubernaculifera is a colonial green alga capable of growth on sodium acetate in the dark. Because heterotrophic organisms often have mechanisms to detect and respond to food sources, we tested the response of A. gubernaculifera to a number of potential chemoeffectors. Acetate elicits strong chemoaccumulation in three different behavioral assays. However, there was no chemoaccumulation in response to the carbohydrates xylose, ribose, glucose, galactose, mannose, arabinose, fructose, maltose, lactose, and sucrose; the amino acids asparagine, arginine, cysteine, glutamine, glycine, histidine, isoleucine, lysine, phenylalanine, serine, threonine, and hydroxyproline; or glycerol. Interestingly, A. gubernaculifera did respond to propionate. This compound is structurally related to acetate, but is incapable of supporting growth in the dark. This implies that the chemosensory response in A. gubernaculifera is not the direct result of metabolic changes but rather involves a specific chemical receptor and subsequent internal signaling pathway. We are presently analyzing swimming patterns in colonies exposed to changes in acetate concentration to test several alternate models that could lead to chemoaccumulation.