RPA and POT1: friends or foes at telomeres?

Abstract

Telomere maintenance in cycling cells relies on both DNA replication and capping by the protein complex shelterin. Two single-stranded DNA (ssDNA)-binding proteins, replication protein A (RPA) and protection of telomere 1 (POT1) play critical roles in DNA replication and telomere capping, respectively. While RPA binds to ssDNA in a non-sequence-specific manner, POT1 specifically recognizes singlestranded TTAGGG telomeric repeats. Loss of POT1 leads to aberrant accumulation of RPA at telomeres and activation of the ataxia telangiectasia and Rad3-related kinase (ATR)-mediated checkpoint response, suggesting that POT1 antagonizes RPA binding to telomeric ssDNA. The requirement for both POT1 and RPA in telomere maintenance and the antagonism between the two proteins raises the important question of how they function in concert on telomeric ssDNA. Two interesting models were proposed by recent studies to explain the regulation of POT1 and RPA at telomeres. Here, we discuss how these models help unravel the coordination, and also the antagonism, between POT1 and RPA during the cell cycle.

Figure 1

Expression of the TPP1^ΔRD mutant leads to RPA accumulation at telomeres. HeLa cells were transfected with plasmids expressing wild-type TPP1, the TPP1^ΔRD mutant or mock treated. Cells were synchronized in S phase with thymidine and subsequently released from the thymidine block. As the majority of cells entering G₂ (8 h after thymidine release), they were analyzed by immunofluorescence staining with RPA32 and TRF2 antibodies.

Figure 2

A model for coordinated functions of RPA and POT1 at telomeres. Tethering of POT1-TPP1 by TI N2 to the shelterin bound to telomeric dsDNA is required for POT1 to reach a critical concentration at telomeres. The cell cycle-regulated RPA-to-POT1 switch on telomeric ssDNA orchestrated by TERRA and hnRNPs enables RPA to support DNA replication of telomeres without compromising telomere capping.