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Langston, L. D.

Publications and source records attributed to Langston, L. D..

2 recordsLinked to original sources
bioRxiv2019

Supporting evidence for DNA shearing as a mechanism for origin unwinding in eukaryotes

Our earlier study demonstrated that head-to-head CMGs that encircle duplex DNA and track inward at origins, melt double-strand (ds) DNA while encircling the duplex by pulling on opposite strands and shearing DNA apart (Langston and ODonnell (2019) eLife 9, e46515). We show here that increasing the methylphosphonate neutral DNA from 10 nucleotides in the previous report, to 20 nucleotides, reveals that CMG encircling duplex DNA only interacts with the tracking strand compared to the non-tracking strand. This significantly enhances support that CMG tracks on duplex DNA by binding only one strand. Furthermore, EMSA assays using AMPPNP to load CMG onto DNA shows a stoichiometry of only 2 CMGs on an origin mimic DNA, containing a 150 bp duplex with two 3 prime single-strand (ss) tails, one on each end, enabling assay of dsDNA unwinding by a shearing force produced by only two head-to-head CMGs. The use of non-hydrolysable AMPPNP enabled a preincubation for CMG binding the two 3 prime tailed origin mimic DNA, and gave robust unwinding of dsDNA by head-to-head CMG-Mcm10s. With this precedent, it is possible to envision that the cell may utilize opposing dsDNA motors to unwind DNA for other types of DNA transactions besides origin unwinding.

biochemistry↗
bioRxiv2017

Mcm10 functions to isomerize CMG-DNA for replisome bypass of DNA blocks

Replicative helicases of all cell types are rings that unwind DNA by steric exclusion in which the helicase ring only encircles the tracking strand, excluding the other strand outside the ring. Steric exclusion mediated unwinding enables helicase rings to bypass blocks on the strand that is excluded from the central channel. Unlike other replicative helicases, eukaryotic CMG encircles duplex DNA at a forked junction and is stopped by a block on the non-tracking (lagging) strand. This report demonstrates that Mcm10, an essential replication protein unique to eukaryotes, binds CMG and enables the replisome to bypass blocks on the non-tracking strand, implying that Mcm10 isomerizes the CMG-DNA complex to position only one strand through the central channel. A similar CMG-DNA isomerization is needed at the origin for head-to-head CMGs to bypass one another during formation of bidirectional replication forks.

biochemistry↗