Functionally conserved RNA-binding and protein-protein interaction properties of LINE-ORF1p in an ancient clade of non-LTR retrotransposons of Entamoeba histolytica.

Retrotransposons are mobile genetic elements found in most organisms. Their origin and evolution is not very well understood. Retrotransposons that lack long terminal repeats (non-LTR) have been classified based on their reverse transcriptase (RT) and endonuclease sequences into groups, of which R2 is the most ancient. Its members contain a single open reading frame (ORF) while there are two ORFs in the other groups, of which ORF2 contains the RT and endonuclease sequences. It is thought that ORF1 was added later to the single-ORF-containing elements, and codes for a protein with nucleic acid binding activity. We have examined the non-LTR retrotransposons in Entamoeba histolytica, an early-branching parasitic protist, which belongs to the R2 group. However, unlike other members of R2, E. histolytica contains two ORFs. Here we show that EhLINE1-ORF1p is functionally related to the ORF1p found in the non-R2 groups. Its N-terminal region has RNA-binding activity and its C-terminal has a coiled coil domain which participates in protein-protein interaction. It lacks sequence-specificity of RNA-binding and binds to EhLINE1-RNA fragment and ribosomal RNA with comparable affinities. Our study suggests that ORF1p could have evolved independently to maintain functional conservation.