Does Auxin Binding Protein 1 Control Both Cell Division and Cell Expansion?

The Auxin-Binding Protein 1 (ABP1) was identified over 30 years ago thanks to it''s high affinity for active auxins. ABP1 plays an essential role in plant life yet to this day, its function remains ‘enigmatic.’ A recent study by our laboratory shows that ABP1 is critical for regulation of the cell cycle, acting both in G₁ and at the G₂/M transition. We showed that ABP1 is likely to mediate the permissive auxin signal for entry into the cell cycle. These data were obtained by studying a conditional functional knock-out of ABP1 generated by cellular immunization in the model tobacco cell line, Bright Yellow 2.Key Words: auxin responses, auxin-binding protein 1, immunomodulation, cellular immunisation     

Molecular systematics and paleobiogeography of the South American sigmodontine rodents [published erratum appears in Mol Biol Evol 1998 Feb;15(2):224]

The murid rodent subfamily Sigmodontinae contains 79 genera which are distributed throughout the New World. The time of arrival of the first sigmodontines in South America and the estimated divergence time(s) of the different lineages of South American sigmodontines have been controversial due to the lack of a good fossil record and the immense number of extant species. The "early-arrival hypothesis" states that the sigmodontines must have arrived in South America no later than the early Miocene, at least 20 MYA, in order to account for their vast present-day diversity, whereas the "late-arrival hypothesis" includes the sigmodontines as part of the Plio-Pleistocene Great American Interchange, which occurred approximately 3.5 MYA. The phylogenetic relationships among 33 of these genera were reconstructed using mitochondrial DNA (mtDNA) sequence data from the ND3, ND4L, arginine tRNA, and ND4 genes, which we show to be evolving at the same rate. A molecular clock was calibrated for these genes using published fossil dates, and the genetic distances were estimated from the DNA sequences in this study. The molecular clock was used to estimate the dates of the South American sigmodontine origin and the main sigmodontine radiation in order to evaluate the "early-" and "late-arrival" scenarios. We estimate the time of the sigmodontine invasion of South America as between approximately 5 and 9 MYA, supporting neither of the scenarios but suggesting two possible models in which the invading lineage was either (1) ancestral to the oryzomyines, akodonts, and phyllotines or (2) ancestral to the akodonts and phyllotines and accompanied by the oryzomyines. The sigmodontine invasion of South America provides an example of the advantage afforded to a lineage by the fortuitous invasion of a previously unexploited habitat, in this case an entire continent.      

Multiple duplications of yeast hexose transport genes in response to selection in a glucose-limited environment

When microbes evolve in a continuous, nutrient-limited environment, natural selection can be predicted to favor genetic changes that give cells greater access to limiting substrate. We analyzed a population of baker's yeast that underwent 450 generations of glucose-limited growth. Relative to the strain used as the inoculum, the predominant cell type at the end of this experiment sustains growth at significantly lower steady-state glucose concentrations and demonstrates markedly enhanced cell yield per mole glucose, significantly enhanced high-affinity glucose transport, and greater relative fitness in pairwise competition. These changes are correlated with increased levels of mRNA hybridizing to probe generated from the hexose transport locus HXT6. Further analysis of the evolved strain reveals the existence of multiple tandem duplications involving two highly similar, high- affinity hexose transport loci, HXT6 and HXT7. Selection appears to have favored changes that result in the formation of more than three chimeric genes derived from the upstream promoter of the HXT7 gene and the coding sequence of HXT6. We propose a genetic mechanism to account for these changes and speculate as to their adaptive significance in the context of gene duplication as a common response of microorganisms to nutrient limitation.      

Purification and characterization of two forms of a high-molecular-weight cysteine proteinase (porphypain) from Porphyromonas gingivalis.

Porphyromonas gingivalis, and organism implicated in the etiology and pathogenesis of human periodontal diseases, produces a variety of potent proteolytic enzymes, and it has been suggested that these enzymes play a direct role in the destruction of periodontal tissues. We now report that two cell-associated cysteine proteinases of P. gingivalis W12, with molecular masses of approximately 150 kDa (porphypain-1) and 120 kDa (porphypain-2), as determined by sodium dodecyl sulfate (SDS)-polyacrylamide gel electrophoresis, have been separated and purified to apparent homogeneity. These proteinases appear to be SDS-stable conformational variants of a 180-kDa enzyme, and they are the largest cysteine proteinases yet purified from P. gingivalis. The purified proteinases hydrolyze fibrinogen, tosyl-Gly-L-Pro-L-Arg p-nitroanilide, and tosyl-Gly-L-Pro-L-Lys p-nitroanilide. While hydrolysis of both synthetic substrates by porphypain-1 and -2 requires activation by reducing agents, is inhibited by EDTA, and is stimulated in the presence of derivatives of glycine, the Arg-amidolytic activity is sensitive to leupeptin and H-D-tyrosyl-L-prolyl-L-arginyl chloromethyl ketone, whereas the Lys-amidolytic activity is sensitive to tosyl-L-lysyl chloromethyl ketone and insensitive to leupeptin. These data suggest that porphypains contain two types of active sites. These cell-associated P. gingivalis proteinases may contribute significantly and directly to periodontal tissue destruction.     

Multiple cis-acting targeting sequences are required for orb mRNA localization during Drosophila oogenesis.

The targeting of positional information to specific regions of the oocyte or early embryo is one of the key processes in establishing anterior-posterior and dorsal-ventral polarity. In many developmental systems, this is accomplished by localization of mRNAs. The germ line-specific Drosophila orb gene plays a critical role in defining both axes of the developing oocyte, and its mRNA is localized in a complex pattern during oogenesis. We have identified a 280-bp sequence from the orb 3' untranslated region capable of reproducing this complex localization pattern. Furthermore, we have found that multiple cis-acting elements appear to be required for proper targeting of orb mRNA.     

Mucosal-associated invariant T (MAIT) cells: an evolutionarily conserved T cell subset

Besides mainstream TCRalphabeta T cells harboring a very diverse repertoire, two subsets display an evolutionarily conserved invariant repertoire. This striking conservation indicates important and unique functions. CD1d-restricted NK-T cells expressing an invariant Valpha14 TCRalpha chain have been implicated in microbial and tumor responses as well as in auto-immunity. In this review, we describe the other subset, which bears the canonical hValpha7.2/mValpha19-Jalpha33 TCRalpha chain paired with a restricted set of Vbeta segments. These invariant T cells are present in mice, humans and cattle. They are preferentially located in the gut lamina propria (LP) of humans and mice and are therefore called mucosal-associated invariant T (MAIT) cells. Selection/expansion of this population requires B lymphocytes expressing MR1, a monomorphic major histocompatibility complex class I-related molecule that is also strikingly conserved in diverse mammalian species. MAIT cells are not present in germ-free mice, indicating that commensal flora is required for their expansion in the gut LP. The nature of the ligand and the putative functions of these MAIT cells are discussed.     

The Toll-like receptor 7 (TLR7) agonist, imiquimod, and the TLR9 agonist, CpG ODN, induce antiviral cytokines and chemokines but do not prevent vaginal transmission of simian immunodeficiency virus when applied intravaginally to rhesus macaques

The initial host response to viral infection occurs after Toll-like receptors (TLRs) on dendritic cells (DC) are stimulated by viral nucleic acids (double-stranded RNA, single-stranded RNA) and alpha interferon (IFN-alpha) and IFN-beta are produced. We hypothesized that pharmacologic induction of innate antiviral responses in the cervicovaginal mucosa by topical application of TLR agonists prior to viral exposure could prevent or blunt vaginal transmission of simian immunodeficiency virus (SIV). To test this hypothesis, we treated rhesus monkeys intravaginally with either the TLR9 agonist, CpG oligodeoxynucleotides (ODN), or the TLR7 agonist, imiquimod. Both immune modifiers rapidly induced IFN-alpha and other antiviral effector molecules in the cervicovaginal mucosa of treated animals. However, both CpG ODN and imiquimod also induced proinflammatory cytokine expression in the cervicovaginal mucosa. In the vaginal mucosa of imiquimod-treated monkeys, we documented a massive mononuclear cell infiltrate consisting of activated CD4(+) T cells, DC, and beta-chemokine-secreting cells. After vaginal SIV inoculation, all TLR agonist-treated animals became infected and had plasma vRNA levels that were higher than those of control monkeys. We conclude that induction of mucosal innate immunity including an IFN-alpha response is not sufficient to prevent sexual transmission of human immunodeficiency virus.