In Vitro Properties of the Conserved Mammalian Protein hnRNP D Suggest a Role in Telomere Maintenance

Abstract

Mammalian chromosomes terminate with a 3′ tail which consists of reiterations of the G-rich repeat, d(TTAGGG). The telomeric tail is the primer for replication by telomerase, and it may also invade telomeric duplex DNA to form terminal lariat structures, or T loops. Here we show that the ubiquitous and highly conserved mammalian protein hnRNP D interacts specifically with the G-rich strand of the telomeric repeat. A single gene encodes multiple isoforms of hnRNP D. All isoforms bind comparably to the G-rich strand, and certain isoforms can also bind tightly and specifically to the C-rich telomeric strand. G-rich telomeric sequences readily form structures stabilized by G-G pairing, which can interfere with telomere replication by telomerase. We show that hnRNP D binding to the G-rich strand destabilizes intrastrand G-G pairing and that hnRNP D interacts specifically with telomerase in human cell extracts. This biochemical analysis suggest that hnRNP D could function in vivo to destabilize structures formed by telomeric G-rich tails and facilitate their extension by telomerase.

ACKNOWLEDGMENTS

We thank Cynthia Fan and Joan Steitz for providing recombinant HuR, and we thank Kenneth R. Williams for providing recombinant hnRNP A1. We are grateful to George Miller, David Schatz, and Joan Steitz for helpful suggestions and to Laurie Dempsey, David Hesslein, Michelle Duquette, and Yilun Liu for valuable experimental advice.

This research was supported by NCI P01 16038 to N.M. A.E. was supported by NIH predoctoral training grant T32 GM07223.