Reciprocal telomerase inhibition by human and Xenopus PinX1

Abstract

The human TRF1/Pin2 interacting protein, hPinX1, has been shown to inhibit telomerase both in vitro and in vivo. Telomerase regulation is particularly important in the frog Xenopus, which expresses abundant telomerase in all adult somatic tissues. We have cloned and characterized the ortholog of PinX1 from X. laevis (xPinX1). The N‐terminal half of xPinX1 is 76% identical to hPinX1, while the C‐terminal half, containing hPinX1ˈs telomerase inhibition domain, is far more diverged. Quantitative RT‐PCR demonstrated that the xPinX1 transcript is relatively weakly expressed in high‐telomerase spleen, although there was no clear inverse correlation between telomerase activity and xPinX1 transcription in tissues with more modest levels of telomerase. To assess function in vitro, both xPinX1 and hPinX1 were purified from E. coli as fusions to His8‐GFP. Both fusion proteins inhibited telomerase in a PCR‐based assay, whereas His8‐GFP did not. Furthermore, each PinX1 was able to inhibit telomerase activity from both species. This suggests structural constraints on the inhibition mechanism, despite the rapid divergence in primary sequence between the two proteins. Further characterization of in vivo and in vitro functions are ongoing. Supported by grant 0642104 to JS from the National Science Foundation, a Betty Liu Research Fellowship to DAC, and a grant to Reed College by the James F. and Marion L. Miller Foundation.