| Roughly 2 meters of DNA are packaged with histones to form chromatin within the nucleus. Acidic proteins known as "histone chaperones" are present in the nucleus and nucleolus. These proteins may store histones and facilitate their transfer to DNA to form nucleosomes. We have used structural and biochemical approaches to understand the function of specialized histone chaperones.; N1 is the cardinal member of the N1 family and is present in the Xenopus oocyte nucleus. Binding studies with a truncated N1 (N1 T) and a mixture of histone dimers and tetramers led to the formation of N1T-histone octamer complexes. Further reconstitution studies of N1T with core histones and Np promoted the formation of a supercomplex. Thus, we speculate that the Np decamer may provide a docking ring for 5 histone octamers, such that the 2-fold symmetry of the decamers and octamers may be used by N1 dimers to form a supercomplex. In this complex, histone octamers may be protected on opposite sides by Np and N1. Thus, both chaperones may act in concert to neutralize the basic charge of histones and could function as a repository for excess core histones in oocytes and eggs.; NO29 is a member of the Np family and is related to NO38, a major nucleolar protein. Here, we present a crystal structure of the N-terminal domain of NO29 as a pentamer. As expected, NO29-core has an NO38-like fold that is comprised of eight beta-strands, which form a barrel with a jellyroll topology. In addition, a cross-over latch is formed between adjacent subunits to create a defined ground state for the pentamer. A signature beta-hairpin, two conserved histidines and the A-1 acidic tract form this latch. This beta-hairpin may play a direct role in histone binding. We also show that NO29-core is an ambidextrous chaperone, because it is able to bind to either dimers or tetramers and form large complexes, though it may prefer tetramers. In all cases, the active form of NO29 may be a decamer comprised of two pentamers. Hence, NO29 may function as a histone chaperone in the nucleolus. |
