NAP1 modulates binding of linker histone H1 to chromatin and induces an extended chromatin fiber conformation. The Journal of biological chemistry

NAP1 (nucleosome assembly protein 1) is a histone chaperone that has been described to bind predominantly to the histone H2AH2B dimer in the cell during shuttling of histones into the nucleus, nucleosome assembly/remodeling, and transcription. Here it was examined how NAP1 interacts with chromatin fibers isolated fromHeLa cells. NAP1 induced a reversible change toward an extended fiber conformation as demonstrated by sedimentation velocity ultracentrifugation experiments. This transitionwas due to the removal of the linker histoneH1.TheH2AH2Bdimer remained stably bound to the native fiber fragments and to fibers devoid of linker histone H1. This was in contrast to mononucleosome sub-strates, which displayed a NAP1-induced removal of a single H2AH2B dimer from the core particle. The effect of NAP1 on the chromatin fiber structure was examined by scanning/atomic force microscopy. A quantitative image analysis of36,000 nucleosome