Catálogo de publicaciones - revistas

<jats:p> The sequencing of euryarchaeal genomes has suggested that the essential protein lysyl–transfer RNA (tRNA) synthetase (LysRS) is absent from such organisms. However, a single 62-kilodalton protein with canonical LysRS activity was purified from <jats:italic>Methanococcus maripaludis</jats:italic> , and the gene that encodes this protein was cloned. The predicted amino acid sequence of <jats:italic>M. maripaludis</jats:italic> LysRS is similar to open reading frames of unassigned function in both <jats:italic>Methanobacterium thermoautotrophicum</jats:italic> and <jats:italic>Methanococcus jannaschii</jats:italic> but is unrelated to canonical LysRS proteins reported in eubacteria, eukaryotes, and the crenarchaeote <jats:italic>Sulfolobus solfataricus</jats:italic> . The presence of amino acid motifs characteristic of the Rossmann dinucleotide-binding domain identifies <jats:italic>M. maripaludis</jats:italic> LysRS as a class I aminoacyl–tRNA synthetase, in contrast to the known examples of this enzyme, which are class II synthetases. These data question the concept that the classification of aminoacyl–tRNA synthetases does not vary throughout living systems. </jats:p>

<jats:p> Reticulospinal (RS) neurons constitute the main descending motor system of lampreys. This study reports on natural conditions whereby <jats:italic>N</jats:italic> -methyl- <jats:sc>d</jats:sc> -aspartate (NMDA)–mediated plateau potentials were elicited and associated with the onset of locomotion. Reticulospinal neurons responded in a linear fashion to mild skin stimulation. With stronger stimuli, large depolarizing plateaus with spiking activity were elicited and were accompanied by swimming movements. Calcium imaging revealed sustained intracellular calcium rise upon sensory stimulation. Blocking NMDA receptors on RS neurons prevented the plateau potentials as well as the associated rise in intracellular calcium. Thus, the activation of NMDA receptors mediates a switch from sensory-reception mode to a motor command mode in RS neurons. </jats:p>

<jats:p>Remodeling of the interface between human growth hormone (hGH) and the extracellular domain of its receptor was studied by deleting a critical tryptophan residue (at position 104) in the receptor, creating a large cavity, and selecting a pentamutant of hGH by phage display that fills the cavity and largely restores binding affinity. A 2.1 â„« resolution x-ray structure of the mutant complex showed that the receptor cavity was filled by selected hydrophobic mutations of hGH. Large structural rearrangements occurred in the interface at sites that were distant from the mutations. Such plasticity may be a means for protein-protein interfaces to adapt to mutations as they coevolve.</jats:p>

<jats:p> Cleland and Kreevoy recently advanced the idea that a special type of hydrogen bond (H-bond), termed a low-barrier hydrogen bond (LBHB), may account for the “missing” transition state stabilization underlying the catalytic power of many enzymes, and Frey <jats:italic>et al.</jats:italic> have proposed that the H-bond between aspartic acid 102 and histidine 57 in the catalytic triad of serine proteases is an example of a catalytically important LBHB. Experimental facts are here considered regarding the aspartic acid–histidine and <jats:italic>cis–</jats:italic> urocanic H-bonds that are inconsistent with fundamental tenets of the LBHB hypothesis. The inconsistencies between theory and experiment in these paradigm systems cast doubt on the existence of LBHBs, as currently defined, within enzyme active sites. </jats:p>

<jats:p> Palmitoylation of the α subunit of the guanine nucleotide-binding protein G <jats:sub>z</jats:sub> inhibited by more than 90 percent its response to the guanosine triphosphatase (GTPase)–accelerating activity of G <jats:sub>z</jats:sub> GAP, a G <jats:sub>z</jats:sub> -selective member of the regulators of G-protein signaling (RGS) protein family of GTPase-activating proteins (GAPs). Palmitoylation both decreased the affinity of G <jats:sub>z</jats:sub> GAP for the GTP-bound form of Gα <jats:sub>z</jats:sub> by at least 90 percent and decreased the maximum rate of GTP hydrolysis. Inhibition was reversed by removal of the palmitoyl group by dithiothreitol. Palmitoylation of Gα <jats:sub>z</jats:sub> also inhibited its response to the GAP activity of Gα-interacting protein (GAIP), another RGS protein, and palmitoylation of Gα <jats:sub>i1</jats:sub> inhibited its response to RGS4. The extent of inhibition of G <jats:sub>z</jats:sub> GAP, GAIP, RGS4, and RGS10 correlated roughly with their intrinsic GAP activities for the Gα target used in the assay. Reversible palmitoylation is thus a major determinant of G <jats:sub>z</jats:sub> deactivation after its stimulation by receptors, and may be a general mechanism for prolonging or potentiating G-protein signaling. </jats:p>

<jats:p>A central issue in cognitive neuroscience concerns the functional architecture of the prefrontal cortex and the degree to which it is organized by sensory domain. To examine this issue, multiple areas of the macaque monkey prefrontal cortex were mapped for selective responses to visual stimuli that are prototypical of the brain's object vision pathway—pictorial representations of faces. Prefrontal neurons not only selectively process information related to the identity of faces but, importantly, such neurons are localized to a remarkably restricted area. These findings suggest that the prefrontal cortex is functionally compartmentalized with respect to the nature of its inputs.</jats:p>

<jats:p> Vascular structures for heat conservation in the tongue of the gray whale ( <jats:italic>Eschrichtius robustus</jats:italic> ) are reported here. Numerous individual countercurrent heat exchangers are found throughout the massive tongue. These converge at the base of the tongue to form a bilateral pair of retia. Temperature measurements from the oral cavity of a live gray whale indicate that more heat may be lost through the blubber layer over the body than through the tongue, despite the fact that the tongue is far more vascularized and has much less insulation. These heat exchangers substantially reduce heat loss when these whales feed in cold waters. </jats:p>

<jats:p> Pathogenic <jats:italic>Yersinia</jats:italic> species have a specialized secretion system (type III) to target cytotoxic Yop proteins during infection. The signals of YopE and YopN sufficient for the secretion of translational reporter fusions were mapped to the first 15 codons. No common amino acid or peptide sequence could be identified among the secretion signals. Systematic mutagenesis of the secretion signal yielded mutants defective in Yop translation; however, no point mutants could be identified that specifically abolished secretion. Frameshift mutations that completely altered the peptide sequences of these signals also failed to prevent secretion. Thus, the signal that leads to the type III secretion of Yop proteins appears to be encoded in their messenger RNA rather than the peptide sequence. </jats:p>