Structurally distinct telomere-binding proteins in Ustilago maydis execute non-overlapping functions in telomere replication, recombination, and protection

Abstract

Duplex telomere binding proteins exhibit considerable structural and functional diversity in fungi. Herein we interrogate the activities and functions of two Myb-containing, duplex telomere repeat-binding factors in Ustilago maydis, a basidiomycete that is evolutionarily distant from the standard fungi. These two telomere-binding proteins, UmTay1 and UmTrf2, despite having distinct domain structures, exhibit comparable affinities and sequence specificity for the canonical telomere repeats. UmTay1 specializes in promoting telomere replication and an ALT-like pathway, most likely by modulating the helicase activity of Blm. UmTrf2, in contrast, is critical for telomere protection; transcriptional repression of Umtrf2 leads to severe growth defects and profound telomere aberrations. Comparative analysis of UmTay1 homologs in different phyla reveals broad functional diversity for this protein family and provides a case study for how DNA-binding proteins can acquire and lose functions at various chromosomal locations. Our findings also point to stimulatory effect of telomere protein on ALT in Ustilago maydis that may be conserved in other systems.

Duplex Telomere-Binding Proteins in Fungi With Canonical Telomere Repeats: New Lessons in the Rapid Evolution of Telomere Proteins

“…Experimental interrogation of UmTay1 and UmTrf2 confirmed the roles of both proteins in telomere regulation and revealed an interesting division of labor that is different from other systems (Yu et al, 2020). UmTay1, on the one hand, plays minimal roles in telomere protection -deletion of the gene triggers neither growth defects nor telomere structural abnormalities.…”

“…Previous studies have shown that the UmBlm helicase positively stimulates both telomere replication and telomere recombination (Yu et al, 2015(Yu et al, , 2018. Indeed, UmTay1 physically interacts with Blm and modulates Blm helicase activity in a substrate sequence-dependent manner (Yu et al, 2020). Thus, by regulating the Blm helicase activity, UmTay1 appears to perform similar functions in these two pathways.…”

“…These initial conjectures notwithstanding, a TRF/TBF-like gene (named UmTbf1 or UmTrf2) was subsequently uncovered in U. maydis, suggesting that this fungus may harbor two structurally distinct duplex telomere binding proteins (Sepsiova et al, 2016;Yu et al, 2020). To simplify the discussion, I will henceforth refer to these two proteins as UmTay1 and UmTrf2, respectively.…”

“…Thus, by regulating the Blm helicase activity, UmTay1 appears to perform similar functions in these two pathways. In contrast to UmTay1, UmTrf2 plays a pivotal role in telomere protection: deletion of trf2 is lethal, and transcriptional repression of this gene triggers a constellation of telomere aberrations that are indicative of de-protection, including telomere length heterogeneity, accumulation of ssDNA and extra-chromosomal telomere repeats (Yu et al, 2020). Therefore, despite having similar affinity and binding specificity for double-stranded telomere repeats, UmTay1 and UmTrf2 mediate largely nonoverlapping functions in telomere regulation.…”

“…Indeed, while putative telomere-binding proteins can be readily identified in many such fungi, very few studies have experimentally interrogated the functions of these proteins. Despite this substantial knowledge gap, a few recent studies have begun to provide tantalizing hints of significant structural and functional divergence at basidiomycetes telomeres (Kramara et al, 2010;Yu et al, 2018Yu et al, , 2020. In particular, it appears that even in the context of an invariant telomere repeat sequence, a new family of telomere DNA-binding protein can emerge and acquire telomere functions through the acquisition of new DNA sequence specificity and protein partners.…”