"We found that a part of Chp1 known as the chromodomain binds with high affinity (or strength) to methylated chromatin," explains Thomas Schalch, Ph.D., a postdoctoral researcher in the Joshua-Tor lab who led the current study. By teasing apart the origin of this unique affinity, the CSHL team stumbled across Chp1 mutants that would produce siRNAs but could not assemble heterochromatin.
"These results lead us to think that the tight interaction between RITS and the methyl marks is a requirement at least as important as the availability of siRNA," Schalch says.
A crystal structure provides the answer
The team wanted to know the exact points of contact between Chp1 and its target and how these interactions contributed to the strength of binding. For these answers, Schalch and the CSHL team made use of Joshua-Tor's expertise in X-ray crystallography the science of determining the exact position of atoms and bonds within a molecule by creating a crystal, which is then probed with powerful x-rays.
Disrupting each point of interaction between Chp1 and its target by engineering various mutations into the Chp1 protein decreased the strength of binding to different levels. Even a five-fold decrease in binding strength prevented the mutation-bearing cells from assembling new heterochromatin, even though some of the mutants were able to generate siRNAs.
This work reveals that siRNAs cannot by themselves establish heterochromatin when Chp1's binding to H3K9me the methylated chromatin is impaired. Whether this mechanism of heterochromatin assembly discovered in yeast also occurs in mammalian cells is unclear at this point, according to Schalch, as a mammalian equivalent to Chp1 has not been found. "But we've now identified a fingerprint for the high-affinity interaction between a chromatin-binding protein and its 'marked', or methylated, target," says Sch
|Contact: Hema Bashyam|
Cold Spring Harbor Laboratory