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We have studied the intracellular localization of poly(A)-binding protein 1 (PABP1) by indirect immunofluorescence as well as by tagging with the green fluorescent protein (GFP) in living cells. We show that PABP1 is able to enter the nucleus. Accumulation of PABP1 in the nuclei was observed upon transcription inhibition, suggesting that active transcription is required for PABP1 export. The nuclear import of PABP1 is an energy-dependent process since PABP1 fails to enter the nucleus upon ATP depletion and at low temperature. Transfection of PABP1 or PABP1-GFP resulted in heterogeneity of intracellular distribution of the protein. In the low expressing cells, PABP1 was localized in the cytoplasm, whereas in the high expressors, we observed accumulation of the protein in the nucleus. Nuclear PABP1 observed either after overexpression or after transcription inhibition was found in speckles and colocalized with splicing factor SC35. The ability of PABP1 to shuttle between nucleus and cytoplasm was also shown by heterokaryon formation upon cell fusion. Deletion mutagenesis showed that the minimal part of PABP1 retaining the ability to shuttle consists of the first two RNA-binding domains. This mutant interacted with poly(A) RNA with high affinity and accumulated in the nucleus. Deletion mutants exhibiting reduced RNA binding affinity did not accumulate in the nucleus. PABP1 has been proposed to participate at various steps of mRNA utilization. Our results suggest involvement of PABP1 in nuclear events associated with the formation and transport of mRNP to the cytoplasm and identify a new trafficking pattern for RNA-binding proteins.Eukaryotic mRNAs are organized in ribonucleoprotein complexes (1, 2). One predominant protein of these complexes is the poly(A)-binding protein 1 (PABP1) 1 , which associates with the 3Ј poly(A) tail of mRNA (3-6). PABP1 is an essential protein in yeast (7) and is highly conserved among eukaryotic organisms (3, 4). PABP1 is clearly a multifunctional protein, proposed to participate in 3Ј end formation of mRNA, translation initiation, mRNA stabilization, protection of poly(A) from nuclease activity, mRNA deadenylation, inhibition of mRNA decapping, and mRNP maturation (8, 9). PABP1 may be a key factor in mediating regulation of mRNA turnover through the inhibition of mRNA decapping by the poly(A) tail or by influencing the rate of deadenylation (10 -13). Several lines of evidence argue that PABP1 plays a role in stimulating translation initiation (14 -16), suggesting that the interaction of this protein with the 3Ј poly(A) sequence can influence events at the 5Ј end of mRNA. PABP1 availability for interaction with processed mRNA is very important. Recent studies have shown that PABP1 is involved in the processing of 3Ј end of premessenger RNA (9). These results imply that interaction of PABP1 with poly(A) tail occurs in the nucleus. Mature mRNA is exported from the nucleus accompanied by several hnRNP proteins (17-19). Presence of PABP1 on the poly(A) tails of exported mRNA could be advant...

Plasmepsin II is one of the four catalytically active plasmepsins found in the food vacuole of Plasmodium falciparum. These enzymes initiate hemoglobin degradation by cleavage at the -chain between Phe33 and Leu34. The crystal structures of Ser205 mutant plasmepsin II from P. falciparum in complex with two inhibitors have been re®ned at a resolution of 1.8 A Ê in the space group I222 and to R factors of 19.9 and 19.5%. Each crystal contains one monomer in the asymmetric unit. Both inhibitors have a Phe± Leu core and incorporate tetrahedral transition-state mimetic hydroxypropylamine. The inhibitor rs367 possesses a 2,6-dimethylphenyloxyacetyl group at the P2 position and 3-aminobenzamide at the P2 H position, while rs370 has the same P2 group but 4-aminobenzamide in the P2 H position. These complexes reveal key conserved hydrogen bonds between the inhibitor and the binding-cavity residues, notably with the¯ap residues Val78 and Ser79, the catalytic dyad Asp34 and Asp214 and the residues Ser218 and Gly36 that are in proximity to the catalytic dyad. The structures also show unexpected conformational variability of the binding cavity of plasmepsin II and may re¯ect the mode of binding of the hemoglobin -chain for cleavage.