Summary Potential predators emit uncharacterized chemosignals that warn receiving species of danger. Neurons that sense these stimuli remain unknown. Here we show that detection and processing of fear-evoking odors emitted from cat, rat, and snake require the function of sensory neurons in the vomeronasal organ. To investigate the molecular nature of the sensory cues emitted by predators, we isolated the salient ligands from two species using a combination of innate behavioral assays in naïve receiving animals, calcium imaging, and cFos induction. Surprisingly, the defensive behavior-promoting activity released by other animals is encoded by species-specific ligands belonging to the major urinary protein (Mup) family, homologs of aggression-promoting mouse pheromones. We show that recombinant Mup proteins are sufficient to activate sensory neurons and initiate defensive behavior similar to native odors. This co-option of existing sensory mechanisms provides a molecular solution to the difficult problem of evolving a variety of species-specific molecular detectors.

The vomeronasal organ (VNO) of the mouse has two neuronal compartments expressing distinct families of pheromone receptors, the V1Rs and the V2Rs. We report here that two families of major histocompatibility complex (MHC) class Ib molecules, the M10 and the M1 families, show restricted expression in V2R-expressing neurons. Our data suggest that neurons expressing a given V2R specifically co-express one or a few members of the M10 family. Biochemical and immunocytochemical analysis demonstrates that in VNO sensory dendrites M10s belong to large multi-molecular complexes that include pheromone receptors and beta2-microglobulin (beta2m). In cultured cells, M10s appear to function as escort molecules in transport of V2Rs to the cell surface. Accordingly, beta2m-deficient mice exhibit mislocalization of V2Rs in the VNO and a specific defect in male-male aggressive behavior. The functional characterization of M10 highlights an unexpected role for MHC molecules in pheromone detection by mammalian VNO neurons.

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We have isolated a cDNA clone, designated ZLKRSDH , encoding the bifunctional enzyme lysine-ketoglutarate reductase/saccharopine dehydrogenase (LKR/SDH) from maize. The predicted polypeptide has an N-terminal LKR domain and a C-terminal SDH domain that are similar to the yeast LYS1 and LYS9 monofunctional proteins, respectively. The maize LKR/SDH protein is located in the cytoplasm of subaleurone endosperm cell layers. Transcripts and polypeptides as well as enzyme activities showed an upregulation and downregulation during endosperm development. The developmental expression of ZLKRSDH was examined in normal and opaque2 seeds. In the mutant endosperm, mRNA levels were reduced by Ͼ 90%, with concomitant reductions in polypeptide levels and LKR/SDH activity. These results suggest that lysine levels in the endosperm are likely to be controlled at the transcriptional level by the Opaque2 transcription factor. INTRODUCTIONSince the discovery of the high-lysine opaque2 ( o2 ) maize mutant (Mertz et al., 1964), research efforts have been directed toward understanding the biochemical and molecular mechanisms leading to the increase in lysine content in the endosperm. Studies conducted during the past 30 years revealed that the homozygous o2 mutation causes an ‫ف‬ 70% reduction in zein content and affects the content of a number of proteins and enzymes related to nitrogen and sugar metabolism in the maize endosperm (Habben et al., 1993; Giroux et al., 1994; Gallusci et al., 1996). The cloning of the O2 gene revealed that it encodes a basic leucine zipper protein transcription factor involved in the transcriptional control of the zein genes, the b-32 ribosome-inactivating protein, and cytoplasmic pyruvate orthophosphate dikinase (cyPPDK), an enzyme involved in carbon partitioning (Lohmer et al., 1991; Bass et al., 1992;Schmidt et al., 1992; Cord Neto et al., 1995; Gallusci et al., 1996). Lysine catabolism also is affected in the o2 mutant. 14 C-lysine feeding experiments revealed that lysine was converted to glutamic acid and proline but to a much lesser extent in o2 than in normal endosperm (Sodek and Wilson, 1970).Despite the observation of extensive lysine degradation in maize seeds (Sodek and Wilson, 1970;da Silva and Arruda, 1979), there have been several attempts to establish positive selection strategies to isolate mutants with aspartate kinase (AK) insensitivity to feedback inhibition caused by lysine. AK is the first regulatory enzyme of the aspartate pathway, which leads to the biosynthesis of lysine, methionine, threonine, and isoleucine. Thus, the derepression of AK would result in an overproduction of these amino acids (reviewed in Azevedo et al., 1997). AK-insensitive mutants that overproduce threonine have been described in barley (Bright et al., 1982), maize (Diedrick et al., 1990), and tobacco (Frankard et al., 1992), but they exhibit only marginal effects on lysine accumulation. The absence of lysine overproduction in these mutants was attributed to the second regulatory enzyme, dihydrodipicolinate syn...