Proteins Binding to and Regulating Human Telomeric Transcripts

Telomere are the protective terminal structures of linear chromosomes. The repetitive TTAGGG sequences are bound by the dedicated shelterin complex, as well as by histones carrying modifications characteristic of silent chromatin. Like the centromeric heterochromatin, telomeres are transcribed, resulting in a heterogenous population of RNAs, containing the unique subtelomeric as well as the common telomeric sequences. The telomeric transcripts, TERRAs, are capped, and a fraction of them is polyadenylated. The presence of the poly(A) tail confers increased stability to the RNA, otherwise relatively short-lived. HnRNPA1 is an abundant, mostly nuclear protein, committed to multiple steps of mRNA biogenesis and trafficking.In vitro, hnRNPA1 shows a striking affinity for UUAGGG repeats. It associates with telomeres, and contributes to positive regulation of their length. In this work, we performed a screen for factors binding to UUAGGG repeats in vitro. We obtained a list of proteins specifically or preferentially associating with a telomeric bait oligonucleotide. We found hnRNPA1 among the most abundant proteins selected on UUAGGG repeats. Based on prelimary results in our lab, hnRNPA1 associates with endogenous TERRA. We set out to further characterize this interaction. We observed preferential binding of hnRNPA1 to nonadenylated telomeric transcripts, which was preserved in the cytoplasmic and nucleoplasmic cell fractions. Despite this bias, a knockdown of hnRNPA1 failed to disturb the ratio between poly(A)- and poly(A)+ TERRA. Our laboratory has previously observed disappearance of TERRA foci in interphase cells depleted for hnRNPA1. We asked, whether this effect could be explained by a change in subcellular localization of the telomeric RNA. In our hands, however, hnRNPA1 knockdown did not affect TERRA fractionation behaviour. We conclude that hnRNPA1 association with TERRA is affected by the presence of the poly(A) tail, and possibly, proteins binding to it. Based on the results obtained in cells knocked down for hnRNPA1, we infer that this protein is not linked to TERRA biogenesis nor to its cellular localization.