Heterogeneous nuclear ribonucleoprotein A3 binds single-stranded telomeric DNA and inhibits telomerase extension in vitro.

Telomeres are dynamic DNA-protein complexes at the end of linear chromosomes. Maintenance of functional telomeres is required for chromosome stability, and to avoid the activation of DNA damage response pathway and cell cycle arrest. Telomere-binding proteins play crucial roles in the maintenance of functional telomeres. In this study, we employed affinity pull-down and proteomic approach to search for novel proteins that interact with the single-stranded telomeric DNA. The proteins identified by two-dimensional gel electrophoresis were further characterized by matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF MS) and MALDI-TOF-TOF tandem MS. Among the five identified proteins, we report here the biochemical properties of a novel protein, hnRNP A3. The purified hnRNP A3 bound specifically to G-rich strand, but not to C-rich strand or double-stranded telomeric DNA. The RRM1 (RNA recognition motif 1) domain, but not RRM2, of hnRNP A3 is sufficient to confer specific binding to the telomeric sequence. In addition, we present evidence that hnRNP A3 can inhibit telomerase extension in vitro. These biochemical properties of hnRNP A3 suggest that hnRNP A3 can participate in telomere regulation in vivo.