The Xenopus Cell Cycle: An Overview

Oocytes, eggs and embryos from the frog Xenopus laevis have been an important model system for studying cell-cycle regulation for several decades. First, progression through meiosis in the oocyte has been extensively investigated. Oocyte maturation has been shown to involve complex networks of signal transduction pathways, culminating in the cyclic activation and inactivation of Maturation Promoting Factor (MPF), composed of cyclin B and cdc2. After fertilisation, the early embryo undergoes rapid simplified cell cycles which have been recapitulated in cell-free extracts of Xenopus eggs. Experimental manipulation of these extracts has given a wealth of biochemical information about the cell cycle, particularly concerning DNA replication and mitosis. Finally, cells of older embryos adopt a more somatic-type cell cycle and have been used to study the balance between cell cycle and differentiation during development.     

The nucleobase-ascorbate transporter (NAT) signature motif in UapA defines the function of the purine translocation pathway

UapA, a member of the NAT/NCS2 family, is a high affinity, high capacity, uric acid-xanthine/H+ symporter of Aspergillus nidulans. We have previously presented evidence showing that a highly conserved signature motif ([Q/E/P]408-N-X-G-X-X-X-X-T-[R/K/G])417 is involved in UapA function. Here, we present a systematic mutational analysis of conserved residues in or close to the signature motif of UapA. We show that even the most conservative substitutions of residues Q408, N409 and G411 modify the kinetics and specificity of UapA, without affecting targeting in the plasma membrane. Q408 substitutions show that this residue determines both substrate binding and transport catalysis, possibly via interactions with position N9 of the imidazole ring of purines. Residue N409 is an irreplaceable residue necessary for transport catalysis, but is not involved in substrate binding. Residue G411 determines, indirectly, both the kinetics (K(m), V) and specificity of UapA, probably due to its particular property to confer local flexibility in the binding site of UapA. In silico predictions and a search in structural databases strongly suggest that the first part of the NAT signature motif of UapA (Q(408)NNG(411)) should form a loop, the structure of which is mostly affected by mutations in G411. Finally, substitutions of residues T416 and R417, despite being much better tolerated, can also affect the kinetics or the specificity of UapA. Our results show that the NAT signature motif defines the function of the UapA purine translocation pathway and strongly suggest that this might occur by determining the interactions of UapA with the imidazole part of purines.     

Lysosomal carboxypeptidase A

Lysosomal carboxypeptidase A (cathepsin A) is synthetized in the form of preproenzyme, which undergoes to active enzyme as a result of post-translational modification. It splits off C-terminal amino acid residues from peptides and proteins and synergizes with other proteases in degradation of cellular proteins in lysosomes. Lysosomal carboxypeptidase A has an effect on peptide hormones and peptides of biological activity of tissues and body fluids as well. It forms complexes with some glycosidases that protects them against proteolytic degradation. Deficiency of this enzyme induces storage diseases. Lysosomal carboxypeptidase A as multifunctional enzyme plays an important regulatory role in organismal metabolism.     

Overexpression of <Emphasis Type="Italic">AP1-</Emphasis>like genes from <Emphasis Type="Italic">Asteraceae</Emphasis> induces early-flowering in transgenic <Emphasis Type="Italic">Chrysanthemum</Emphasis> plants

Chrysanthemum is one of the most important commercial cut flowers in the world. Early-flowering cultivars are required to produce quality chrysanthemum flowers with a lower cost of production. To shorten the vegetative growth phase of chrysanthemum, three AP1-like genes from Asteraceae were constitutively overexpressed in 80 independent transgenic chrysanthemum lines. All lines were characterized by PCR and RT-PCR and demonstrated that overexpression of compositae AP1-homologs in transgenic chrysanthemum under long-day conditions had no effect on plant development compared to non-transgenic controls. Conversely, under short-day conditions, transgenic plants commenced bud initiation 2 wk earlier than non-transgenic chrysanthemum plants. Subsequently, transgenic chrysanthemum flowers showed color earlier and resulted in full opening of inflorescences 3 wk prior to non-transgenic control plants. These results open new possibilities for genetic improvement and breeding of chrysanthemum cultivars.