Snap helix with knob and hole: essential repeats in S. pombe nuclear protein nuc2+.

The S. pombe nuc2+ gene is required for mitotic chromosome disjunction. Its mutation arrests mitosis at the metaphase. The gene product is present in the nuclear scaffold-like fraction. The nuc2+ protein contains a domain, separated from ten 34 amino acid repeat segments, that is capable of binding AT-rich DNA in vitro. The ts mutation resides in one of the 34 amino acid repeats. Circular dichroism, limited proteolysis of the repeats, and model fitting indicate the presence of helical segments connected by protease-sensitive hinges. We propose that these repeats form a novel secondary structure (snap helix) having "knob and hole" helix-associating motifs. The packing of the snap helices would be stabilized by the bonding between the hydrophobic amino acids surrounding the knobs and holes. The nuc2+ protein may in one way bind to DNA and in another way mutually associate to form a part of the chromosome scaffold.