Processive bidirectional motion of dynein-dynactin complexes in vitro

Cytoplasmic dynein is the primary molecular motor responsible for transport of vesicles, organelles, proteins and RNA cargoes from the periphery of the cell towards the nucleus along the microtubule cytoskeleton of eukaryotic cells. Dynactin, a large multi-subunit activator of dynein, docks cargo to the motor and may enhance dynein processivity. Here, we show that individual fluorescently labelled dynein-dynactin complexes exhibit bidirectional and processive motility towards both the plus and minus ends of microtubules. The dependence of this activity on substrate ATP concentration, nucleotide analogues and inhibitors suggests that bidirectional motility is an active energy-transduction property of dynein-dynactin motor mechano-chemistry. The unique motility characteristics observed may reflect the flexibility of the dynein structure that leads to an enhanced ability to navigate around obstacles in the cell.     

EFFECT OF SESAME LEAF DIET ON DETOXIFICATION ACTIVITIES OF INSECTS WITH DIFFERENT FEEDING BEHAVIOR

Three diverse insects, a polyphagous “leaf chewer” (Atractomorpha lata), a polyphagous “sap feeder” (Myzus persicae), and a “restrictive feeder” (Plutella xylostella) responded differently when fed with eight cultivars of sesame either as whole leaf or via artificial diet. There was limited or no correlation in induction between detoxifying enzyme substrates (esterase, glutathione s‐transferase [GST], and mixed function oxidase [MFO] activities) when activity toward various substrates α‐naphthyl acetate, 1‐chloro‐2,4‐dinitrobenzene, 1,2‐dichloro‐4‐nitrobenzene, and p‐nitroanisole (pNA), were compared although they were generally elevated in the tissues from insects on sesame than a reference fed with radish seedlings. In A. lata, esterase activity for the cultivar 11Pusan and 45Laos were three‐fold higher compared to the reference, while other cultivars, 24Nanbu‐twasaki and 56S‐radiatum were—two‐ to three‐fold lower than the reference. In M. persicae, the esterase activity was as much as five‐fold higher than the reference in one test cultivar. GST activities of the sesame cultivars were generally higher (≈two‐fold) than the reference in all insects and at variable ratios among the cultivars. The MFO activity toward pNA in grasshoppers feeding on these sesame cultivars was either highly expressed or nonexistent. These results indicate that although the cultivars belong to the same species, they might have undergone changes in secondary phytochemicals in response to varying biogeographical distribution. Each insect species is suspected to target a specific plant chemical burden that it tries to overcome in each cultivar through enzyme activation.     

Characterization of follistatin-type domains and their contribution to myostatin and activin a antagonism

Follistatin (FST)-type proteins are important antagonists of some members of the large TGF-β family of cytokines. These include myostatin, an important negative regulator of muscle growth, and the closely related activin A, which is involved in many physiological functions, including maintenance of a normal reproductive axis. FST-type proteins, including FST and FST-like 3 (FSTL3), differentially inhibit various TGF-β family ligands by binding each ligand with two FST-type molecules. In this study, we sought to examine features that are important for ligand antagonism by FST-type proteins. Previous work has shown that a modified construct consisting of the FST N-terminal domain (ND) followed by two repeating follistatin domains (FSD), herein called FST ND-FSD1-FSD1, exhibits strong specificity for myostatin over activin A. Using cell-based assays, we show that FST ND-FSD1-FSD1 is unique in its specificity for myostatin as compared with similar constructs containing domains from FSTL3 and that the ND is critical to its activity. Furthermore, we demonstrate that FSD3 of FST provides affinity to ligand inhibition and confers resistance to perturbations in the ND and FSD2, likely through the interaction of FSD3 of one FST molecule with the ND of the other FST molecule. Additionally, our data suggest that this contact provides cooperativity to ligand antagonism. Cross-linking studies show that this interaction also potentiates formation of 1:2 ligand-FST complexes, whereas lack of FSD3 allows formation of 1:1 complexes. Altogether, these studies support that domain differences generate FST-type molecules that are each uniquely suited ligand antagonists.     

Rescue of misrouted GnRHR mutants reveals its constitutive activity

G protein-coupled receptors (GPCR) play central roles in almost all physiological functions, and mutations in GPCR are responsible for over 30 hereditary diseases associated with loss or gain of receptor function. Gain of function mutants are frequently described as having constitutive activity (CA), that is, they activate effectors in the absence of agonist occupancy. Although many GPCR have mutants with CA, the GnRH receptor (GnRHR) was not, until 2010, associated with any CA mutants. The explanation for the failure to observe CA appears to be that the quality control system of the cell recognizes CA mutants of GnRHR as misfolded and retains them in the endoplasmic reticulum. In the present study, we identified several human (h)GnRHR mutants with substitutions in transmembrane helix 6 (F(272)K, F(272)Q, Y(284)F, C(279)A, and C(279)S) that demonstrate varying levels of CA after being rescued by pharmacoperones from different chemical classes and/or deletion of residue K(191), a modification that increases trafficking to the plasma membrane. The movement of the mutants from the endoplasmic reticulum (unrescued) to the plasma membrane (after rescue) is supported by confocal microscopy. Judging from the receptor-stimulated inositol phosphate production, mutants F(272)K and F(272)Q, after rescue, display the largest level of CA, an amount that is comparable with agonist-stimulated activation. Because mutations in other GPCR are, like the hGnRHR, scrutinized by the quality control system, this general approach may reveal CA in receptor mutants from other systems. A computer model of the hGnRHR and these mutants was used to evaluate the conformation associated with CA.     

Mammalian HCA66 protein is required for both ribosome synthesis and centriole duplication

Ribosome production, one of the most energy-consuming biosynthetic activities in living cells, is adjusted to growth conditions and coordinated with the cell cycle. Connections between ribosome synthesis and cell cycle progression have been described, but the underlying mechanisms remain only partially understood. The human HCA66 protein was recently characterized as a component of the centrosome, the major microtubule-organizing center (MTOC) in mammalian cells, and was shown to be required for centriole duplication and assembly of the mitotic spindle. We show here that HCA66 is also required for nucleolar steps of the maturation of the 40S ribosomal subunit and therefore displays a dual function. Overexpression of a dominant negative version of HCA66, accumulating at the centrosome but absent from the nucleoli, alters centrosome function but has no effect on pre-rRNA processing, suggesting that HCA66 acts independently in each process. In yeast and HeLa cells, depletion of MTOC components does not impair ribosome synthesis. Hence our results suggest that both in yeast and human cells, assembly of a functional MTOC and ribosome synthesis are not closely connected processes.