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211.
212.
Jaeyong Lee Joanne Johnson Ziwei Ding Mark Paetzel Rosemary B. Cornell 《The Journal of biological chemistry》2009,284(48):33535-33548
CTP:phosphocholine cytidylyltransferase (CCT) is the key regulatory enzyme in the synthesis of phosphatidylcholine, the most abundant phospholipid in eukaryotic cell membranes. The CCT-catalyzed transfer of a cytidylyl group from CTP to phosphocholine to form CDP-choline is regulated by a membrane lipid-dependent mechanism imparted by its C-terminal membrane binding domain. We present the first analysis of a crystal structure of a eukaryotic CCT. A deletion construct of rat CCTα spanning residues 1–236 (CCT236) lacks the regulatory domain and as a result displays constitutive activity. The 2.2-Å structure reveals a CCT236 homodimer in complex with the reaction product, CDP-choline. Each chain is composed of a complete catalytic domain with an intimately associated N-terminal extension, which together with the catalytic domain contributes to the dimer interface. Although the CCT236 structure reveals elements involved in binding cytidine that are conserved with other members of the cytidylyltransferase superfamily, it also features nonconserved active site residues, His-168 and Tyr-173, that make key interactions with the β-phosphate of CDP-choline. Mutagenesis and kinetic analyses confirmed their role in phosphocholine binding and catalysis. These results demonstrate structural and mechanistic differences in a broadly conserved protein fold across the cytidylyltransferase family. Comparison of the CCT236 structure with those of other nucleotidyltransferases provides evidence for substrate-induced active site loop movements and a disorder-to-order transition of a loop element in the catalytic mechanism. 相似文献
213.
If local communities are saturated with species, the relationship between local and regional species richness [the local species
richness (LSR)–regional species richness (RSR) relationship] is predicted to become increasingly curvilinear at more local
spatial scales. This study tested whether the LSR–RSR relationship for coral species was linear or curvilinear at three local
scales across the west-central Pacific Ocean, along a regional biodiversity gradient that includes the world’s most diverse
coral assemblages. The local scales comprised transects 100–2 m apart, sites 103–4 m apart and islands 104–6 m apart. The LSR–RSR relationship was never significantly different from linear at any scale. When the Chao1 estimator was
used to predict true RSR and LSR, all relationships were also strongly linear. We conclude that local assemblages are open
to regional influences even when the local scale is very small relative to the regional scale, and even in extraordinarily
rich regions. 相似文献
214.
215.
Summary. Computer modelling studies of two groups of biologically-active peptides derived from A-gliadin indicated that the most likely
structures were α-helical ones, in the case of serine-containing peptides, and random peptides coil types featuring β-turns, in the case of proline-rich, tyrosine-containing peptides.
The serine-containing group of peptides appear to be essentially cytotoxic in animal models of coeliac disease, whilst the
tyrosine-containing group have the capacity to initiate damaging immunological reactions in patients with coeliac disease.
Both types of activity in coeliac disease are only possible if there is defective digestion of the active peptides, as mucosal
digestion studies indicate. In the case of the serine-containing peptides, activity of the peptides is linked to the presence
of PSQQ and also probably QQQP motifs. With the tyrosine-containing peptides, sequences such as QQPY and/or QPYP are associated
with immunological activity and hence toxicity.
Received November 8, 2000 Accepted December 11, 2000 相似文献
216.
Cornell SJ Isham VS Grenfell BT 《Proceedings. Biological sciences / The Royal Society》2004,271(1545):1243-1250
Host-parasite systems provide powerful opportunities for the study of spatial and stochastic effects in ecology; this has been particularly so for directly transmitted microparasites. Here, we construct a fully stochastic model of the population dynamics of a macroparasite system: trichostrongylid gastrointestinal nematode parasites of farmed ruminants. The model subsumes two implicit spatial effects: the host population size (the spatial extent of the interaction between hosts) and spatial heterogeneity ('clumping') in the infection process. This enables us to investigate the roles of several different processes in generating aggregated parasite distributions. The necessity for female worms to find a mate in order to reproduce leads to an Allee effect, which interacts nonlinearly with the stochastic population dynamics and leads to the counter-intuitive result that, when rare, epidemics can be more likely and more severe in small host populations. Clumping in the infection process reduces the strength of this Allee effect, but can hamper the spread of an epidemic by making infection events too rare. Heterogeneity in the hosts' response to infection has to be included in the model to generate aggregation at the level observed empirically. 相似文献
217.
218.
Jamison CS Cornell LL Jamison PL Nakazato H 《American journal of physical anthropology》2002,119(1):67-76
An unresolved question arising from human evolutionary research relates to the function of the postreproductive period in human females. If menopause is not merely an artifact resulting from the benefits of civilization, there must be an adaptive mechanism favoring the offspring of women who continue to thrive well past the time of their last ovulation. The "grandmother hypothesis" was developed on the basis of the original suggestion by Williams (1957 Evolution 11:32-39) that "stopping early" would benefit already-born children. This idea, combined with the concepts of kin selection (Hamilton 1964 J Theor Biol 7:1-52) and parental investment (Trivers 1972 Sexual Selection and the Descent of Man, Chicago: Aldine, p. 136-179), was expanded to suggest that postreproductive women (in contrast to males) contribute to their inclusive fitness by extending support to their grandchildren. We used discrete time event history analysis (Allison [1984] Event History Analysis, Newbury Park: Sage; Allison [1995] Survival Analysis, Cary, NC: SAS Institute) and logistic regression on data provided in population registers (Shūmon Aratame Chō, or SAC) from a village in central Japan, covering the period from 1671-1871, in a preliminary investigation of the effects of household grandparental presence on the probability of a child's death. We found that after accounting for the presence of other household members, the only grandparent whose presence exerted a consistent negative effect on the likelihood of a child's death was the mother's mother. Due to the small sample size of households that contained maternal grandmothers, these results failed to achieve statistical significance. Their importance, however, is in what they suggest about future research, i.e., census data from preindustrial societies can provide a basis for testing evolutionary proposals, including the "grandmother hypothesis." 相似文献
219.
A critical and integrated view of the yeast interactome 总被引:3,自引:0,他引:3
220.
Johnson JE Xie M Singh LM Edge R Cornell RB 《The Journal of biological chemistry》2003,278(1):514-522
Amphitropic proteins are regulated by reversible membrane interaction. Anionic phospholipids generally promote membrane binding of such proteins via electrostatics between the negatively charged lipid headgroups and clusters of basic groups on the proteins. In this study of one amphitropic protein, a cytidylyltransferase (CT) that regulates phosphatidylcholine synthesis, we found that substitution of lysines to glutamine along both interfacial strips of the membrane-binding amphipathic helix eliminated electrostatic binding. Unexpectedly, three glutamates also participate in the selectivity for anionic membrane surfaces. These glutamates become protonated in the low pH milieu at the surface of anionic, but not zwitterionic membranes, increasing protein positive charge and hydrophobicity. The binding and insertion into lipid vesicles of a synthetic peptide containing the three glutamates was pH-dependent with an apparent pK(a) that varied with anionic lipid content. Glutamate to glutamine substitution eliminated the pH dependence of the membrane interaction, and reduced anionic membrane selectivity of both the peptide and the whole CT enzyme examined in cells. Thus anionic lipids, working via surface-localized pH effects, can promote membrane binding by modifying protein charge and hydrophobicity, and this novel mechanism contributes to the membrane selectivity of CT in vivo. 相似文献