Catalytic component of ISW1-type complexes, which act by remodeling the chromatin by catalyzing an ATP-dependent alteration in the structure of nucleosomal DNA. They are involved in coordinating transcriptional repression, activation and elongation phases. The ISW1A complex represses gene expression at initiation through specific positioning of a promoter proximal dinucleosome. The ISW1B complex acts within coding regions to control the amount of RNA polymerase II released into productive elongation and to coordinate elongation with termination and pre-mRNA processing. Component of the ISW1A complex, which at least consists of ISW1 and IOC3. Component of the ISW1B complex, which at least consists of ISW1, IOC2 and IOC4. Present with 1500 molecules/cell in log phase SD medium. Belongs to the SNF2/RAD54 helicase family. ISWI subfamily.