A short C-terminal domain of Yku70p is essential for telomere maintenance

The Yku heterodimer from Saccharomyces cerevisiae, comprising Yku70p and Yk u80p, is involved in the maintenance of a normal telomeric DNA end structur e and is an essential component of nonhomologous end joining (NHEJ). To inv estigate the role of the Yku70p subunit in these two different pathways, we generated C-terminal deletions of the Yku70 protein and examined their abi lity to complement the phenotypes of a yku70(-) strain. Deleting only the 3 0 C-terminal amino acids of Yku70p abolishes Yku DNA binding activity and c auses a ykuphenotype; telomeres are shortened, and NHEJ is impaired. Using conditions in which at least as much mutant protein as full-length protein is normally detectable in cell extracts, deleting only 25 C-terminal amino acids of Yku70p results in no measurable effect on DNA binding of the Yku p rotein, and the cells are fully proficient for NHEJ. Nevertheless, these ce lls display considerably shortened telomeres, and significant amounts of si ngle-stranded overhangs of the telomeric guanosine-rich strands are observe d. Co-overexpression of this protein with Yku80p could rescue some but not all of the telomere-related phenotypes. Therefore, the C-terminal domain in Yku70p defines at least one domain that is especially involved in telomere maintenance but not in NHEJ.