Functional characterization of human nucleosome assembly protein 1-like proteins as histone chaperones

Nucleosome Assembly Protein 1 (NAP1) is a highly conserved histone chaperone protein suspected to be involved in the dynamical regulation of the histone H2A-H2B hetero-dimer. However, the exact mechanism by which NAP1-like proteins act is currently unknown. In this work, we characterized the biochemical properties of two human NAP1-like proteins, hNAP1L1 and hNAP1L4, including a previously uncharacterized subtype, with the aim of determining their exact mechanistic role. Both hNAP1L1 and hNAP1L4 were found to be localized mainly to the cytoplasm and a minor population of them was suggested to be in the nucleus. Biochemical analyses demonstrated that both hNAP1L1 and hNAP1L4 mediated nucleosome formation. In addition, hNAP1L1 was shown to possess a significantly greater nucleosome disassembly activity than hNAP1L4, suggesting that hNAP1L1 and hNAP1L4 may play distinct roles in the regulation of histone dynamics. Building upon this initial discovery we also found that histone H2A-H2B and various histone H2A variants-H2B dimers were found to associate with both hNAP1L1 and hNAP1L4 in cell extracts. These results suggest that human NAP1-like proteins play overlapping roles in transport and deposition of histone H2A-H2B or H2A variants-H2B dimers on chromatin and nonoverlapping roles in nucleosome disassembly.