B23 interacts with PES1 and is involved in nucleolar localization of PES1

PES1, the human homolog of zebrafish pescadillo, is a nucleolar protein that is essential for cell proliferation. We report herein that a nucleolar marker protein B23 physically interacts with PES1 and is involved in the nucleolar localization of PES1. In vivo interaction between B23 and PES1 was verified by co-immunopreci- pitation of endogenous B23 and PES1 proteins, and they showed cellular co-localizations under both normal and actinomycin D-induced stress conditions. Furthermore, we mapped their interaction domains via in vitro pull- down assays. When B23 was knocked down by RNA interference, there appeared an increased nucleoplasmic distribution of PES1. Our results support a previous hypothesis that B23 might be a nucleolar hub protein for protein targeting to the nucleolus, and shed light on the nucleolar localization mechanism of PESI. The physical interaction between B23 and PES1 implies that they may participate in ribosome biogenesis in a protein complex.     

The GATA-binding protein CGF-1 is closely related to GT-1

Many light-regulated genes contain a conserved GATA motif in their 5-upstream region. We have characterized in detail the GATA-binding factor, CGF-1, which binds within a 73 bp TATA-proximal light/circadian regulatory element in the Arabidopsis cab2 promoter and to two more sites farther upstream. CGF-1 was found to be distinct from other metal-dependent GATA-binding factors, but to have the same sequence requirements for binding and similar physical and chemical properties as GT-1, a factor required for light regulation of the tobacco rbcS-3A gene. CGF-1 was found to be constitutively present in extracts and was shown to be immunologically related to GT-1. The close similarity between CGF-1 and GT-1 suggests that a GT-1-like factor is involved in the phytochrome/circadian regulation of the cab2 gene. CGF-1 and GT-1 were also found to have similar sequence specificities to another constitutively-regulated GATA factor, IBF-2b, which binds the I box region of the tomato nitrate reductase gene. Of three complexes detected using an IBF-2b-specific probe, only one was identical to CGF-1/GT-1. The other two were similar to IBF-2b, demonstrating that CGF-1/GT-1, although very similar, are actually distinct from IBF-2b. These data indicate that more than one factor can bind to the same short sequence and may indicate how constitutively present factors like GT-1 can play a role in light regulation.     

42Sp48 in previtellogenic Xenopus oocytes is structurally homologous to EF-1 alpha and may be a stage-specific elongation factor

We have isolated the cDNA for 42Sp48 and EF-1 alpha from mixed stage oocytes and tailbud (stage 22) Xenopus laevis cDNA libraries by use of the cDNA for human elongation factor-1 alpha (EF-1 alpha) as probe. The nucleotide and deduced amino acid sequences of the entire coding region of 42Sp48 and EF-1 alpha cDNA were established. The proposed functional homology of the proteins is reflected in highly conserved amino acid sequences (91% identity), while the large number of silent mutations at the gene level may serve to prevent recombination at their loci. 42Sp48 is apparently encoded by two genes in Xenopus, while no sequences corresponding to 42Sp48 could be found in murine or human genomic DNA. 42Sp48 has been proposed to act as a stage-specific elongation factor in Xenopus. Comparison of the deduced amino acid sequences of 42Sp48 and EF-1 alpha with that of elongation factor Tu from E. coli, for which the three-dimensional structure including that of the GTP binding sites have been determined, supports this hypothesis.