CHL1 is a nuclear protein with an essential ATP binding site that exhibitsa size-dependent effect on chromosome segregation

Saccharomyces cerevisiae chl1 mutants have a significant increase in the ra te of chromosome missegregation, CHL1 encodes a 99 kDa predicted protein wi th an ATP binding site consensus, a putative helix-turn-helix DNA binding m otif, and homology to helicases. Using site-directed mutagenesis, I show th at mutations that are predicted to abolish ATP binding in CHL1 inactivate i ts function in chromosome segregation. Furthermore, overexpression of these mutations interferes with chromosome transmission of a 125 kb chromosome f ragment in a wild-type strain. Polyclonal antibodies against CHL1 show that CHL1 is predominantly in the nuclear fraction of S.cerevisiae, CHL1 functi on is more critical for the segregation of small chromosomes. In chl1 Delta 1/chl1 Delta 1 mutants, artificial circular or linear chromosomes < 150 kb in size exhibit near random segregation (0.12 per cell division), whereas all chromosomes tested > 225 kb were lost at rates (5 x 10(-3) per cell div ision) comparable to that observed for endogenous chromosome III. These res ults reveal an important role for ATPases/DNA helicases in chromosome segre gation. Such enzymes may alter DNA topology to allow loading of proteins in volved in maintaining sister chromatid cohesion.