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S Hubbell

Publications and source records attributed to S Hubbell.

9 recordsLinked to original sources
PubMed2005

Spatial genetic structure of Simarouba amara Aubl. (Simaroubaceae), a dioecious, animal-dispersed Neotropical tree, on Barro Colorado Island, Panama.

Simarouba amara (Simaroubaceae) is a vertebrate-dispersed, insect-pollinated Neotropical tree found in lowland moist forest from upper Mesoamerica to the Amazon basin. We assessed the spatial genetic structure of S. amara within the 50-ha Forest Dynamics Plot on Barro Colorado Island in the Republic of Panama. A total of 300 individuals were genotyped using five microsatellite loci, representing 100 individuals with a dbh>or=10 cm, 100 individuals of 1-10 cm dbh, and 100 individuals of <1 cm dbh. The 200 individuals in the two larger size classes were also genotyped with 155 AFLP loci. Spatial autocorrelation analysis using Moran's Index detected significant genotypic association at the smallest distance classes for 1-10 cm dbh (0-20 m) and >10 cm dbh (0-40 m) size categories. Significant spatial autocorrelations were detected over larger scales (0-140 m) in <1 cm dbh individuals. The relatively weak genetic structure of S. amara, in comparison to other recent studies, may be explained by pollen and seed dispersal over the 50 ha plot, overlapping seed shadows, and postrecruitment mortality.

Demography↗
PubMed2000

Predicting species diversity in tropical forests.

A fundamental question in ecology is how many species occur within a given area. Despite the complexity and diversity of different ecosystems, there exists a surprisingly simple, approximate answer: the number of species is proportional to the size of the area raised to some exponent. The exponent often turns out to be roughly 1/4. This power law can be derived from assumptions about the relative abundances of species or from notions of self-similarity. Here we analyze the largest existing data set of location-mapped species: over one million, individually identified trees from five tropical forests on three continents. Although the power law is a reasonable, zeroth-order approximation of our data, we find consistent deviations from it on all spatial scales. Furthermore, tropical forests are not self-similar at areas </=50 hectares. We develop an extended model of the species-area relationship, which enables us to predict large-scale species diversity from small-scale data samples more accurately than any other available method.

Journal Article↗
PubMed1998

A role for the MEK/MAPK pathway in PMA-induced cell cycle arrest: modulation of megakaryocytic differentiation of K562 cells.

In vitro megakaryocytic differentiation of the pluripotent K562 human leukemia cell line is induced by PMA. Treatment of K562 cells with PMA results in growth arrest, polyploidy, morphological changes, and increased cell-cell and cell-substrate adhesion. These PMA-induced changes in K562 cells are preceded by a rapid rise in the activity of MEK (MAP kinase/extracellular regulated kinases) that leads to a sustained activation of ERK2 (extracellular regulated kinase; MAPK). Blockade of MEK1 activation by PD098059, a recently described specific MEK inhibitor [D. T. Dudley et al. (1995). Proc. Natl. Acad. Sci. USA 92, 7686-7689], reverses both the growth arrest and the morphological changes of K562 cells induced by PMA treatment. These changes are not associated with a disruption of PMA-induced down-regulation of BCR-ABL kinase or early integrin signaling events but are associated with a block of the cell-surface expression of the gpIIb/IIIa (CD41) integrin, a cell marker of megakaryocytic differentiation. These results demonstrate that the PMA-induced signaling cascade initiated by protein kinase C activation requires the activity of the MEK/ERK signaling complex to regulate cell cycle arrest, thus regulating the program that leads to the cell-surface expression of markers associated with megakaryocytic differentiation.

Calcium-Calmodulin-Dependent Protein Kinases↗
PubMed1996

The product of the cbl oncogene forms stable complexes in vivo with endogenous Crk in a tyrosine phosphorylation-dependent manner.

The cellular homologs of the v-Crk oncogene product are composed exclusively of Src homology region 2 (SH2) and SH3 domains. v-Crk overexpression in fibroblasts causes cell transformation and elevated tyrosine phosphorylation of specific cellular proteins. Among these proteins is a 130-kDa protein, identified as p130cas, that forms a stable complex in vivo with v-Crk. We have explored the role of endogenous Crk proteins in Bcr-Abl-transformed cells. In the K562 human chronic myelogenous leukemia cell line, p130cas is not tyrosine phosphorylated or bound to Crk. Instead, Crk proteins predominantly associate with the tyrosine-phosphorylated proto-oncogene product of Cbl. In vitro analysis showed that this interaction is mediated by the SH2 domain of Crk and can be inhibited with a phosphopeptide containing the Crk-SH2 binding motif. In NIH 3T3 cells transformed by Bcr-Abl, c-Cbl becomes strongly tyrosine phosphorylated and associates with c-Crk. The complex between c-Crk and c-Cbl is also seen upon T-cell receptor cross-linking or with the transforming, tyrosine-phosphorylated c-Cbl. These results indicate that Crk binds to c-Cbl in a tyrosine phosphorylation-dependent manner, suggesting a physiological role for the Crk-c-Cbl complex in Bcr-Abl tyrosine phosphorylation-mediated transformation.

3T3 Cells↗
PubMed1991

Regulation of phosphatidylglycerolphosphate synthase in Saccharomyces cerevisiae by factors affecting mitochondrial development.

Phosphatidylglycerolphosphate synthase (PGPS; CDP-diacylglycerol glycerol 3-phosphate 3-phosphatidyltransferase; EC 2.7.8.5) catalyzes the first step in the synthesis of cardiolipin, an acidic phospholipid found in the mitochondrial inner membrane. In the yeast Saccharomyces cerevisiae, PGPS expression is coordinately regulated with general phospholipid synthesis and is repressed when cells are grown in the presence of the phospholipid precursor inositol (M. L. Greenberg, S. Hubbell, and C. Lam, Mol. Cell. Biol. 8:4773-4779, 1988). In this study, we examined the regulation of PGPS in growth conditions affecting mitochondrial development (carbon source, growth stage, and oxygen availability) and in strains with genetic lesions affecting mitochondrial function. PGPS derepressed two- to threefold when cells were grown in a nonfermentable carbon source (glycerol-ethanol), and this derepression was independent of the presence of inositol. PGPS derepressed two- to fourfold as cells entered the stationary phase of growth. Stationary-phase derepression occurred in both glucose- and glycerol-ethanol-grown cells and was slightly greater in cells grown in the presence of inositol and choline. PGPS expression in mitochondria was not affected when cells were grown in the absence of oxygen. In mutants lacking mitochondrial DNA [( rho0] mutants), PGPS activity was 30 to 70% less than in isogenic [rho+] strains. PGPS activity in [rho0] strains was subject to inositol-mediated repression. PGPS activity in [rho0] cell extracts was derepressed twofold as the [rho0] cells entered the stationary phase of growth. No growth phase derepression was observed in mitochondrial extracts of the [rho0] cells. Relative cardiolipin content increased in glycerol-ethanol-grown cells but was not affected by growth stage or by growth in the presence of the phospholipid precursors inositol and choline. These results demonstrate that (i) PGPS expression is regulated by factors affecting mitochondrial development; (ii) regulation of PGPS by these factors is independent of cross-pathway control; and (iii) PGPS expression is never fully repressed, even during anaerobic growth.

Anaerobiosis↗
PubMed1988

Inositol regulates phosphatidylglycerolphosphate synthase expression in Saccharomyces cerevisiae.

The enzyme phosphatidylglycerolphosphate synthase (PGPS; CDPdiacylglycerol-glycerol-3-phosphate 3-phosphatidyltransferase; EC 2.7.8.5) catalyzes the committed step in the synthesis of cardiolipin, a phospholipid found predominantly in the mitochondrial inner membrane. To determine whether PGPS is regulated by cross-pathway control, we analyzed PGPS expression under conditions in which the regulation of general phospholipid synthesis could be examined. The addition of inositol resulted in a three- to fivefold reduction in PGPS expression in wild-type cells in the presence or absence of exogenous choline. The reduction in enzyme activity in response to inositol was seen in minutes, suggesting that inactivation or degradation of the enzyme plays an important role in inositol-mediated repression of PGPS. In cho2 and opi3 mutants, which are blocked in phosphatidylcholine synthesis, inositol-mediated repression of PGPS did not occur unless choline was added to the media. Three previously identified genes that regulate general phospholipid synthesis, INO2, INO4, and OP11, did not affect PGPS expression. Thus, ino2 and ino4 mutants, which are unable to derepress biosynthetic enzymes involved in general phospholipid synthesis, expressed wild-type levels of PGPS activity under derepressing conditions. PGPS expression in the opi1 mutant, which exhibits constitutive synthesis of general phospholipid biosynthetic enzymes, was fully repressed in the presence of inositol and partially repressed even in the absence of inositol. These results demonstrate for the first time that an enzymatic step in cardiolipin synthesis is coordinately controlled with general phospholipid synthesis but that this control is not mediated by the same genetic regulatory circuit.

Gene Expression Regulation↗
PubMed1986

Functional antagonism of D-1 and D-2 dopaminergic mechanisms affecting striatal acetylcholine release.

Rat striatal slices prelabelled with [3H]choline were superfused with dopamine D-1 and D-2 agonists and antagonists, separately and in combination, during measurement of [3H]acetylcholine (ACh) release. SKF38393 (D-1 agonist), 10(-7)-10(-4) M, and SCH23390 (D-1 antagonist), 10(-7)-10(-5) M, produced a dose-dependent increase in [3H]ACh release when given separately. The increased [3H]ACh release induced by either drug could not be attenuated by sufficient L-sulpiride to block D-2 receptors. Yet both SKF38393, 10(-6)-10(-5) M, and SCH23390, 10(-6)-10(-5) M, were able to partially or fully overcome the [3H]ACh release-depressant effect of cosuperfused LY171555 (D-2 agonist), 10(-6) M. This suggests that a functional antagonism regarding striatal ACh release exists between D-1 and D-2 dopaminergic receptor-mediated mechanisms, but that D-1 modulation of local ACh release does not occur at the level of the recognition site of the striatal D-2 receptor. Finally, although attenuation of the increased release of striatal [3H]ACh induced by 10(-5) M SCH23390 by SKF38393 was seen, it is possible that such functional antagonism is not mediated by exclusively D-1 dopaminergic means.

2,3,4,5-Tetrahydro-7,8-dihydroxy-1-phenyl-1H-3-ben↗