La1: an evolutionarily conserved player in the Arabidopsis telomerase complex

Quantitative mass spectrometry of Arabidopsis telomerase uncovered AtLa1, a homolog of ciliate and yeast proteins that promotes telomerase maturation. AtLa1 is essential for telomerase function in vivo, and in vitro it engages the same region of AtTR bound by AtNAP57, homologous to a telomerase accessory from mammals. Striking divergence is evident in the biogenesis pathways and protein complements of telomerase from mammals and single-cell organisms. However, little is known about plant telomerase. Along with catalytic subunit TERT and templating RNA TR, we previously showed Arabidopsis thaliana telomerase is also associated with AtNAP57, a dyskerin homolog essential for mammalian telomerase biogenesis. Here we employ quantitative mass spectrometry (qMS) to uncover additional Arabidopsis telomerase constituents. We report AtLa1 as a new telomerase-associated protein. RNA-IP assays confirmed AtLa1 association with AtTR, while transient RNAi-mediated knockdown of AtLa1 strongly diminished telomerase activity, supporting a functional role for AtLa1 in telomere maintenance in vivo. In vitro binding studies revealed AtLa1 contacts AtTR via the UUU-3'OH and a plant-specific P1a-P1b-P4 three-way junction (TWJ). Notably, the TWJ is also required for AtNAP57 association with AtTR. However, this protein was not detected in our qMS experiment using overexpressed AtTERT and AtTR, perhaps because the purification scheme enriched for RNP assembly intermediates. La-related proteins serve as RNA chaperones and are associated with a wide variety of telomerase complexes. Therefore, we postulate that AtNAP57 and AtLa1 compete for AtTR or bind sequentially during telomerase biogenesis. Further exploration of Arabidopsis telomerase may offer novel insights into telomerase evolution and mechanisms of biogenesis.