Histone chaperones manage the protein building blocks of nucleosomes and thus, play pivotal roles in DNA replication and repair. Specialized chaperones store histones, decondense sperm chromatin after fertilization and help transfer histones onto DNA (Akey & Luger, 2003). We are taking a combined structural and biochemical approach to investigate histone binding by chaperones of the Nucleoplasmin family. As a first step, we determined high resolution crystal structures of an Np-core decamer (Dutta et al., 2001), a Drosophila NLP-core pentamer (Namboodiri et al., 2003) and both pentamer and decamer forms of the NO38-core (Namboodiri et al., 2004).
We have shown that complete histone octamers can be assembled on Np. We propose that decamers are the active form, as this creates local two-fold axes which are compatible with the natural geometry of histones in the octamer. A corollary to this hypothesis is that electrostatic interactions may not be the sole determinant of histone binding. Thus, complex formation may require a stereo-specific fit between chaperones and their cognate histones. We have also shown that family members may bind to either dimers or tetramers, when the other histone partner is not present. In addition, dimer binding may initially involve a chaperone pentamer. Finally, members of the NO38/Npm and NO29/Npm3 branches prefer to bind histone tetramers, while Np may prefer dimers. The underlying biochemistry of these remarkable processes are being investigated using biophysical and structural approaches.