Catálogo de publicaciones - revistas

<jats:p>During the early history of the solar system, it is likely that the outer planets changed their distance from the sun, and hence, their influence on the asteroid belt evolved with time. The gravitational influence of Jupiter and Saturn on the orbital evolution of asteroids in the outer asteroid belt was calculated. The results show that the sweeping of mean motion resonances associated with planetary migration efficiently destabilizes orbits in the outer asteroid belt on a time scale of 10 million years. This mechanism provides an explanation for the observed depletion of asteroids in that region.</jats:p>

<jats:p> The kinetic mechanism by which the DNA repair helicase UvrD of <jats:italic>Escherichia coli</jats:italic> unwinds duplex DNA was examined with the use of a series of oligodeoxynucleotides with duplex regions ranging from 10 to 40 base pairs. Single-turnover unwinding experiments showed distinct lag phases that increased with duplex length because partially unwound DNA intermediate states are highly populated during unwinding. Analysis of these kinetics indicates that UvrD unwinds duplex DNA in discrete steps, with an average “step size” of 4 to 5 base pairs (approximately one-half turn of the DNA helix). This suggests an unwinding mechanism in which alternating subunits of the dimeric helicase interact directly with duplex DNA. </jats:p>

<jats:p>Transmembrane receptors for hormones, neurotransmitters, light, and odorants mediate their cellular effects by activating heterotrimeric guanine nucleotide-binding proteins (G proteins). Crystal structures have revealed contact surfaces between G protein subunits, but not the surfaces or molecular mechanism through which Gαβγ responds to activation by transmembrane receptors. Such a surface was identified from the results of testing 100 mutant α subunits of the retinal G protein transducin for their ability to interact with rhodopsin. Sites at which alanine substitutions impaired this interaction mapped to two distinct Gα surfaces: a βγ-binding surface and a putative receptor-interacting surface. On the basis of these results a mechanism for receptor-catalyzed exchange of guanosine diphosphate for guanosine triphosphate is proposed.</jats:p>

<jats:p>Models predict that food-web structure is regulated by both consumers and resources, and the strength of this control is dependent on trophic position and food-web length. To test these hypotheses, a meta-analysis was conducted of 11 fish (consumer)-by-nutrient (resource) factorial plankton community experiments. As predicted, zooplankton biomass was under strong consumer control but was weakly stimulated by nutrient additions; phytoplankton biomass was under strong resource control with moderate control by fish. However, the phytoplankton and zooplankton responses to nutrient additions did not follow theoretical predictions based on the number of trophic levels in the food web.</jats:p>

<jats:p>Molecular chaperones of the 70-kilodalton heat shock protein (Hsp70) class bind to partially unfolded polypeptide substrates and participate in a wide variety of cellular processes. Differences in peptide-binding specificity among Hsp70s have led to the hypothesis that peptide binding determines specific Hsp70 functions. Protein domains were identified that were required for two separate functions of a yeast Hsp70 family. The peptide-binding domain was not required for either of these specific Hsp70 functions, which suggests that peptide-binding specificity plays little or no role in determining Hsp70 functions in vivo.</jats:p>

<jats:p> A nonlinear demographic model was used to predict the population dynamics of the flour beetle <jats:italic>Tribolium</jats:italic> under laboratory conditions and to establish the experimental protocol that would reveal chaotic behavior. With the adult mortality rate experimentally set high, the dynamics of animal abundance changed from equilibrium to quasiperiodic cycles to chaos as adult-stage recruitment rates were experimentally manipulated. These transitions in dynamics corresponded to those predicted by the mathematical model. Phase-space graphs of the data together with the deterministic model attractors provide convincing evidence of transitions to chaos. </jats:p>

<jats:p> Familial combined hyperlipidemia (FCHL) is a common inherited lipid disorder, affecting 1 to 2 percent of the population in Westernized societies. Individuals with FCHL have large quantities of very low density lipoprotein (VLDL) and low density lipoprotein (LDL) and develop premature coronary heart disease. A mouse model displaying some of the features of FCHL was created by crossing mice carrying the human apolipoprotein C-III ( <jats:italic>APOC3</jats:italic> ) transgene with mice deficient in the LDL receptor. A synergistic interaction between the apolipoprotein C-III and the LDL receptor defects produced large quantities of VLDL and LDL and enhanced the development of atherosclerosis. This mouse model may provide clues to the origin of human FCHL. </jats:p>

<jats:p> The tyrosine kinase class of receptors induces mitogen-activated protein kinase (MAPK) activation through the sequential interaction of the signaling proteins Grb2, Sos, Ras, Raf, and MEK. Receptors coupled to heterotrimeric guanine triphosphate-binding protein (G protein) stimulate MAPK through G <jats:sub>β</jats:sub> <jats:sub>γ</jats:sub> subunits, but the subsequent intervening molecules are still poorly defined. Overexpression of phosphoinositide 3-kinase γ (PI3Kγ) in COS-7 cells activated MAPK in a G <jats:sub>β</jats:sub> <jats:sub>γ</jats:sub> -dependent fashion, and expression of a catalytically inactive mutant of PI3Kγ abolished the stimulation of MAPK by G <jats:sub>β</jats:sub> <jats:sub>γ</jats:sub> or in response to stimulation of muscarinic (m2) G protein-coupled receptors. Signaling from PI3Kγ to MAPK appears to require a tyrosine kinase, Shc, Grb2, Sos, Ras, and Raf. These findings indicate that PI3Kγ mediates G <jats:sub>β</jats:sub> <jats:sub>γ</jats:sub> -dependent regulation of the MAPK signaling pathway. </jats:p>

<jats:p>Two patterns in the distribution of species have become firmly but independently established in ecology: the species-area curve, which describes how rapidly the number of species increases with area, and the positive relation between species' geographical distribution and average local abundance. There is no generally agreed explanation of either pattern, but for both the two main hypotheses are essentially the same: divergence of species along the ecological specialist-generalist continuum and colonization- extinction dynamics. A model is described that merges the two mechanisms, predicts both patterns, and thereby shows how the two general, but formerly disconnected, patterns are interrelated.</jats:p>

<jats:p>The proto-oncogene-encoded transcription factor c-Jun activates genes in response to a number of inducers that act through mitogen-activated protein kinase (MAPK) signal transduction pathways. The activation of c-Jun after phosphorylation by MAPK is accompanied by a reduction in c-Jun ubiquitination and consequent stabilization of the protein. These results illustrate the relevance of regulated protein degradation in the signal-dependent control of gene expression.</jats:p>