Structure of human lymphoid-specific helicase HELLS in its autoinhibitory state

Helicase, Lymphoid Specific (HELLS), also known as Lymphoid-Specific Helicase (LSH), is a member of the SNF2 chromatin-remodeling family that regulates DNA methylation and heterochromatin organization. Unlike most chromatin remodelers, HELLS is catalytically inactive in its apo form and requires the DNA-binding protein CDCA7 for activation, though the underlying mechanism has remained unclear. Here, we combine biochemical, biophysical and cryo-electron microscopy analyses to define the structural basis of HELLS autoinhibition. HELLS alone assembles into a hexameric (trimer of dimers) architecture stabilized by interactions between its N-terminal coiled-coil (CC) domain and ATPase Lobe-1, while ATPase Lobe-2 remains flexible and disengaged. The CC domain functions both as an oligomerization scaffold and as an autoinhibitory module that restricts catalytic activity. Binding of CDCA7 and DNA promotes formation of an active HELLS-CDCA7-DNA ternary complex. CDCA7 recognizes hemimethylated CpG dinucleotides in both B-form and non-B-form DNA and stimulates HELLS ATPase activity. Together, these findings reveal the mechanism of HELLS autoinhibition and its activation by CDCA7 and DNA, providing new insight into how the HELLS-CDCA7-DNA ternary complex maintains DNA methylation and heterochromatin integrity. Graphical Abstract O_FIG O_LINKSMALLFIG WIDTH=200 HEIGHT=78 SRC="FIGDIR/small/692698v1_ufig1.gif" ALT="Figure 1"> View larger version (26K): org.highwire.dtl.DTLVardef@106b73borg.highwire.dtl.DTLVardef@1e7b33aorg.highwire.dtl.DTLVardef@a7be6eorg.highwire.dtl.DTLVardef@c75a5b_HPS_FORMAT_FIGEXP M_FIG C_FIG O_LIHELLS alone assembles into an inactive hexameric trimer-of-dimers C_LIO_LIThe N-terminal coiled-coil mediates oligomerization C_LIO_LICDCA7 and DNA binding activate the HELLS-CDCA7-DNA complex C_LIO_LIThe complex recognizes hemimethylated CpG in non-B-form DNA C_LI