Telomeres are maintained and protected in part by proteins that bind single-stranded telomeric DNA. The telomere-binding protein Cdc13 is implicated in telomere protection and length regulation. Although prior biochemical, genetic, and molecular studies indicated that Cdc13 can exist as dimers or oligomers in solution and binds telomeric sequences with high specificity, the dynamic process by which Cdc13 loads onto telomeric DNA had not been well characterized. This study used single-molecule approaches to observe Cdc13 loading dynamics and to define a stepwise assembly mechanism.
The authors applied two complementary single-molecule techniques: single-molecule fluorescence resonance energy transfer (smFRET) and colocalization single-molecule spectroscopy (CoSMoS). These methods enabled direct observation of individual Cdc13 molecules interacting with short telomeric DNA substrates under controlled conditions. The combination of smFRET and CoSMoS allowed the investigators to detect binding events and infer assembly order on telomeric DNA at the single-molecule level.
Using smFRET and CoSMoS, the study demonstrates a sequential assembly pathway for Cdc13 on telomeric DNA. Specifically, Cdc13 initially loads onto telomeric single-stranded DNA as a monomer. After initial monomer binding, a second Cdc13 monomer is recruited to the bound complex, resulting in the formation of a stable Cdc13 dimer. This dimerization was observed on a 12-nucleotide telomeric DNA segment. The data support a two-step process rather than instantaneous cooperative binding of a pre-formed dimer or higher-order oligomer onto DNA.
The observed monomer-to-dimer transition provides a mechanistic basis that links the biochemical behavior of Cdc13 with its biological roles. Genetic studies referenced in the source indicate that monomeric binding by Cdc13 alone is insufficient to maintain telomere length. The stepwise assembly therefore suggests that initial monomer binding may serve regulatory or recruitment purposes, while the subsequent dimer state may be required for stable protection or for coordinating access of telomerase and other telomere-processing factors. In other words, monomeric Cdc13 may be important for regulation and handoff, and dimerization may be essential for sustained telomere maintenance.
The work supports a model in which Cdc13 engages telomeric single-stranded DNA in a defined sequence of events: first a single molecule recognizes and binds the telomeric sequence, then a second Cdc13 molecule is recruited to the DNA-bound monomer, stabilizing a dimeric complex on a 12-nt telomeric tract. This sequential assembly model reconciles earlier observations that Cdc13 can form dimers or oligomers in solution with the requirement for regulated binding dynamics at telomeres. The model provides a conceptual framework for how Cdc13 can perform multiple tasks — sensing or initiating assembly as a monomer, and executing protective or length-regulatory functions as a dimer.
The preprint reports the core observation of monomer loading followed by recruitment of a second monomer to form a stable dimer on a 12-nucleotide telomeric segment, but several quantitative or contextual details were not reported in the provided source text. For example, the source did not provide specific kinetic rate constants, binding affinities, concentrations used, or complete experimental conditions. The extent to which the in vitro single-molecule observations generalize to full-length telomeres in vivo or to different telomeric sequence contexts was not reported in the source. Any additional mechanistic determinants, post-translational modifications, or interacting partners that might influence Cdc13 assembly dynamics were not described in the available abstract and metadata.
The work was posted as a preprint on bioRxiv (doi: https://doi.org/10.64898/2026.09.01.748474) by authors affiliated with National Taiwan University. Correspondence was directed to the contact provided in the source. The authors declared no competing interests. The preprint is made available under a CC-BY 4.0 International license. Specific experimental parameters, extended datasets, and in-depth discussion beyond the summarized findings were not included in the provided source text.
By applying single-molecule fluorescence approaches, the study clarifies that Cdc13 loading onto telomeric DNA follows a sequential monomer-first, dimer-second pathway on at least a 12-nucleotide telomeric substrate. This sequential assembly offers a mechanistic explanation for previous genetic observations and supports a role for monomeric Cdc13 in regulatory coordination, with dimerization linked to stable telomere protection and length maintenance. The source presents these conclusions without detailed kinetic or in vivo validation data in the text provided.