Structural and functional similarities and differences in nucleolar Pumilio RNA-binding proteins between Arabidopsis and the charophyte Chara corallina.

backgroundPumilio RNA-binding proteins are evolutionarily conserved throughout eukaryotes and are involved in RNA decay, transport, and translation repression in the cytoplasm. Although a majority of Pumilio proteins function in the cytoplasm, two nucleolar forms have been reported to have a function in rRNA processing in Arabidopsis. The species of the genus Chara have been known to be most closely related to land plants, as they share several characteristics with modern Embryophyta.

resultsIn this study, we identified two putative nucleolar Pumilio protein genes, namely, ChPUM2 and ChPUM3, from the transcriptome of Chara corallina. Of the two ChPUM proteins, ChPUM2 was most similar in amino acid sequence (27% identity and 45% homology) and predicted protein structure to Arabidopsis APUM23, while ChPUM3 was similar to APUM24 (35% identity and 54% homology). The transient expression of 35S:ChPUM2-RFP and 35S:ChPUM3-RFP showed nucleolar localization of fusion proteins in tobacco leaf cells, similar to the expression of 35S:APUM23-GFP and 35S:APUM24-GFP. Moreover, 35S:ChPUM2 complemented the morphological defects of the apum23 phenotypes but not those of apum24, while 35S:ChPUM3 could not complement the apum23 and apum24 mutants. Similarly, the 35S:ChPUM2/apum23 plants rescued the pre-rRNA processing defect of apum23, but 35S:ChPUM3/apum24

conclusionsChPUM2 of C. corallina was functional in Arabidopsis, similar to APUM23, but ChPUM3 did not substitute for APUM24 in Arabidopsis. Protein homology modeling showed high coverage between APUM23 and ChPUM2, but displayed structural differences between APUM24 and ChPUM3. Together with the protein structure of ChPUM3 itself, a short ITS2 of Arabidopsis pre-rRNA may interrupt the binding of ChPUM3 to 3'-extended 5.8S pre-rRNA.