Multiple ISWI ATPase complexes from Xenopus laevis - Functional conservation of an ACF/CHRAC homolog

The nucleosomal ATPase ISWI is the catalytic subunit of several protein com plexes that either organize or perturb chromatin structure in vitro. This w ork reports the cloning and biochemical characterization of a Xenopus ISWI homolog. Surprisingly, whereas we find four complex forms of ISWI in egg ex tracts, we find no functional homolog of NURF. One of these complexes, xACF , consists of ISWI, Acf1, and a previously uncharacterized protein of 175 k Da. Like both ACF and CHRAC, this complex organizes randomly deposited hist ones into a regularly spaced array. The remaining three forms include two n ovel ISWI complexes distinct from known ISWI complexes plus a histone-depen dent ATPase complex. This comprehensive biochemical characterization of ISW I underscores the evolutionary conservation of the ACF/CHRAC family.