The nerve hemoglobin of the bivalve mollusc Spisula solidissima: molecular cloning, ligand binding studies, and phylogenetic analysis.
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Members of the hemoglobin (Hb) superfamily are present in nerve tissue of several vertebrate and invertebrate species. In vertebrates they display hexacoordinate heme iron atoms and are typically expressed at low levels (microM). Their function is still a matter of debate. In invertebrates they have a hexa- or pentacoordinate heme iron, are mostly expressed at high levels (mM), and have been suggested to have a myoglobin-like function. The native Hb of the surf clam, Spisula solidissima, composed of 162 amino acids, does not show specific deviations from the globin templates. UV-visible and resonance Raman spectroscopy demonstrate a hexacoordinate heme iron. Based on the sequence analogy, the histidine E7 is proposed as a sixth ligand. Kinetic and equilibrium measurements show a moderate oxygen affinity (P(50) approximately 0.6 torr) and no cooperativity. The histidine binding affinity is 100-fold lower than in neuroglobin. Phylogenetic analysis demonstrates a clustering of the S. solidissima nerve Hb with mollusc Hbs and myoglobins, but not with the vertebrate neuroglobins. We conclude that invertebrate nerve Hbs expressed at high levels are, despite the hexacoordinate nature of their heme iron, not essentially different from other intracellular Hbs. They most likely fulfill a myoglobin-like function and enhance oxygen supply to the neurons. (+info)
Aurora B kinase maintains chromatin organization during the MI to MII transition in surf clam oocytes.
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Meiosis represents a specialized cell cycle whereby cells undergo two reductive divisions without an intervening S phase. In oocytes, the transition from meiosis I to II is brief, with paired sister chromatids remaining condensed throughout the interkinesis period. This stands in contrast to mitotic divisions where cytokinesis and the return to interphase is always accompanied by chromatin decondensation and nuclear envelope reformation. Because other aspects of M phase exit are normal, we probed the mechanisms that allow for polar body extrusion while retaining chromatin condensation in Spisula solidissima oocytes. If oocytes were activated in the presence of protein synthesis inhibitors, oocytes progressed normally through MI, but arrested in interkinesis with condensed chromatin, phosphorylated histone H3 and a disorganized MII spindle. Neither inhibition of CDK1- nor MAPK activity in arrested oocytes was sufficient to drive chromatin decondensation or nuclear envelope reformation, suggesting that these kinases were not responsible for the maintenance of chromatin condensation. However, inhibition of Aurora B kinase activity resulted in chromatin decondensation, loss of histone H3 phosphorylation and reformation of the nuclear envelope. Inhibition of Aurora B activity following MI also resulted in chromosome segregation defects during MII and blocked polar body formation, consistent with Aurora B's well-established role in cytokinesis. Together, these results suggest that extended Aurora B activity between meiotic divisions maintains chromatin condensation, thus allowing for the rapid reassembly of the MII spindle and progression through meiosis. (+info)
Centrosomal RNA correlates with intron-poor nuclear genes in Spisula oocytes.
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The small organic compound HMN-176 delays satisfaction of the spindle assembly checkpoint by inhibiting centrosome-dependent microtubule nucleation.
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