Characterization of a pollen-expressed gene encoding a putative pectin esterase ofPetunia inflata

From a pollen tube cDNA library ofPetunia inflata, we isolated cDNA clones encoding a protein, PPE1, which exhibits sequence similarity with plant, bacterial, and fungal pectin esterases. Genomic clones containing thePPE1 gene were isolated using cDNA for PPE1 as a probe, and comparison of the cDNA and genomic sequences revealed the presence of a single intron in thePPE1 gene. During pollen development,PPE1 mRNA was first detected in anthers containing uninucleate microspores; it reached the highest level in mature pollen and persisted at a high level inin vitro germinated pollen tubes. The observed expression pattern of thePPE1 gene suggests that its product may play a role in pollen germination and/or tube growth.     

Electrophysiological Actions of Oxytocin on the Rabbit Myometrium

The electrical activities of myometrial cells of the pregnant rabbit uterus have been studied by means of sucrose-gap and intracellular micro-electrode recording techniques. The resting potential of the myometrial cell was about -50 mv, and it is unaffected by the duration of pregnancy or placental attachment. Action potentials of the myometrium, although dependent on external Na⁺, were not always of the regenerative type; preparations from nonparturient uteri often produce mainly small spikes. The mean spike amplitude was 35 mv, rising at a mean maximum rate of 3 v/sec. Oxytocin, in concentrations less than 500 µU/ml, increased the mean spike amplitude to 48 mv and the mean maximum rate of rise to 7 v/sec, without affecting the resting potential. The relation between membrane potential and dV/dt of the spike was steepened by oxytocin, suggesting that oxytocin increased the number of normally sparse sodium gates in the myometrial membrane. By this action, oxytocin is believed to increase the probability of successful regenerative spikes and thereby initiate electrical activity in quiescent preparations, increase the frequency of burst discharges, the number of spikes in each burst, and the amplitude of spikes in individual cells.     

Association of NMT2 with the acyl-CoA carrier ACBD6 protects the N-myristoyltransferase reaction from palmitoyl-CoA

The covalent attachment of a 14-carbon aliphatic tail on a glycine residue of nascent translated peptide chains is catalyzed in human cells by two N-myristoyltransferase (NMT) enzymes using the rare myristoyl-CoA (C₁₄-CoA) molecule as fatty acid donor. Although, NMT enzymes can only transfer a myristate group, they lack specificity for C₁₄-CoA and can also bind the far more abundant palmitoyl-CoA (C₁₆-CoA) molecule. We determined that the acyl-CoA binding protein, acyl-CoA binding domain (ACBD)6, stimulated the NMT reaction of NMT2. This stimulatory effect required interaction between ACBD6 and NMT2, and was enhanced by binding of ACBD6 to its ligand, C_18:2-CoA. ACBD6 also interacted with the second human NMT enzyme, NMT1. The presence of ACBD6 prevented competition of the NMT reaction by C₁₆-CoA. Mutants of ACBD6 that were either deficient in ligand binding to the N-terminal ACBD or unable to interact with NMT2 did not stimulate activity of NMT2, nor could they protect the enzyme from utilizing the competitor C₁₆-CoA. These results indicate that ACBD6 can locally sequester C₁₆-CoA and prevent its access to the enzyme binding site via interaction with NMT2. Thus, the ligand binding properties of the NMT/ACBD6 complex can explain how the NMT reaction can proceed in the presence of the very abundant competitive substrate, C₁₆-CoA.     

Summer profundal hypoxia determines the coupling of methanotrophic production and the pelagic food web in a subtropical reservoir

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Receptor-mediated vectorial transcytosis of epidermal growth factor by Madin-Darby canine kidney cells

Transcellular transport of a variety of ligands may be an important mechanism by which regulatory substances reach their site of action. We have studied the transcellular transport of two 6,000-mol-wt proteins, epidermal growth factor (EGF) and insulin, across polarized Madin-Darby canine kidney (MDCK) cells grown on dual-sided chambers on a nitrocellulose filter substrate. When grown on these chambers, MDCK cells are polarized and express distinct basal and apical surfaces. MDCK cells are capable of unidirectional transport of EGF from the basal-to-apical direction, 50% of bound EGF transported in 2 h. Transport was inhibited by the addition of unlabeled EGF in a dose-dependent manner. Anti-EGF receptor Ab, which inhibited binding, also inhibited transport. No transport in the apical-to-basal direction is noted. Insulin transport is not observed in either direction. Transport correlates with the presence of ligand-specific receptors on the cell surface. Hence, EGF receptors (Ro = 48,000, Kd = 3.5 X 10(-10) M) are found only on the basal surface of the MDCK cells and neither surface expresses insulin receptors. Characterization of the EGF receptors on MDCK cells, as assessed by affinity, molecular mass, and anti-receptor antibody binding reveals that this receptor has similar characteristics to EGF receptors previously described on a variety of cells. Hence, the EGF receptor can function as a transporter of EGF across an epithelial cell barrier.     

Humans express natural cytotoxic (NC) cell activity that is similar to murine NC cell activity

The expression of natural cytotoxic (NC) activity is well defined in mice, but poorly defined in humans. In this paper we report that humans express naturally occurring cytotoxic cell activity that recognizes and lyses murine targets that are sensitive to lysis mediated by murine NC cells, but not murine targets that are resistant to lysis by murine NC cells. We present data showing that these naturally occurring human cytotoxic cells and murine NC cells have similar lytic mechanisms. Both the human cytotoxic cells described here, and murine NC cells, use tumor necrosis factor (TNF) to mediate the lysis of sensitive targets. Moreover, targets that resist murine NC-mediated lysis by a protein synthesis-dependent post-recognitive mechanism use a similar mechanism to prevent lysis mediated by naturally occurring human cytotoxic cells. In addition to the similarity of naturally occurring human cytotoxic cells and murine NC cells in their specificity and lytic mechanism, naturally occurring human cytotoxic cells and murine NC cells are also similar in that their activity is both associated with a monocyte lineage and age independent. Taken together, these data indicate that humans express NC activity.