The assembly of ATP-dependent chromatin remodeling complexes is key to specificity in chromatin regulation, former Dartmouth researcher Gerald Crabtree said in a physiology seminar held at Dartmouth-Hitchcock Medical Center on Thursday.

Alongside DNA methylation and histone modification, chromatin remodeling is a mechanism by which gene expression is controlled. Chromatin remodeling complexes, or remodelers, consist of protein subunits that bind to certain sections of chromatin. Each chromatin remodeler is a gene-specific combination of subunits, Crabtree said, and substituting the wrong subunit can result in death or loss of functionality in a cell.

Crabtree and his team first observed that certain subunits in chromatin remodelers can be substituted with a number of similar, yet slightly different subunits. More specifically, Crabtree said, they noticed that the transition between stem cells and mature neural tissue requires the replacement of a subunit in the remodeler that is responsible for controlling this differentiation.

To test whether remodeler assembly produces specificity, Crabtree, who is currently at Stanford University, chose to study the embryonic stem cell chromatin remodeler esBAF. By knocking out esBAF in preliminary tests, Crabtree showed that esBAF is essential to embryonic stem cell (ES cell) proliferation and pluripotency, making it a good candidate for experimentation.

Crabtree then experimented with esBAF by replacing the essential subunit BAF155 with BAF170. The consequences were clear: ES-cell proliferation was inhibited and pluripotency was greatly limited with minimal differentiation of the germ layers. The replacement of one subunit had precluded the gene expression necessary for the ES-cells’ normal development.

The expression of pluripotency-controlling transcription factors depends on esBAF, Crabtree said. Moreover, esBAF represses differentiation genes, further but less directly promoting pluripotency. Thus, Crabtree showed that the specific combination of subunits in esBAF is tailored to interact with ES-cell regulators.

Crabtree also discussed how he had managed to sequence an important subunit of BAF called Brg (the all-important ATPase in this ATP-dependent complex). He had then found that almost every gene controlling pluripotency is bound by some complex containing the Brg subunit.