La protein binding to telomerase RNA supports an evolutionary relationship between plant and ciliate telomerase pathways

The Arabidopsis thaliana La1 (AtLa1) protein is a member of the genuine La family of RNA biogenesis proteins, which are structurally similar to the La-related protein 7 (LARP7) family. LARP7 proteins participate in the biogenesis of the telomerase ribonucleoprotein complex in model systems, but are absent in plants. We show that AtLa1 binds to telomerase RNA in a manner reminiscent of the Tetrahymena LARP7 protein p65. Classical in vitro methods and microscale thermophoresis (MST) were used to specify the molecular structures involved in this multi-surface interaction. AtLa1 also enhances the binding of TR to the telomerase reverse transcriptase RNA binding domain. We therefore propose that biogenesis of telomerase RNA in plants and ciliates is achieved by a similar pathway, differing in the employment of genuine La or LARP7-like proteins, respectively. We also report that the domain of unknown function (DUF3223, DeCL) found in the AtLa1 protein binding partner, Domino, is an RNA binding domain with modest TR-binding capacity. This domain is also found in plant and ciliate proteins, including plant polymerases IV/V and the Tetrahymena La protein Mlp1. Together, these suggest that RNA biogenesis pathways in plants and ciliates have a conserved evolutionary relationship, with parallels between their La proteins.