Regulatory subunit of the ATP-dependent RSF-1 and RSF-5 ISWI chromatin-remodeling complexes, which form ordered nucleosome arrays on chromatin and facilitate access to DNA during DNA-templated processes such as DNA replication, transcription, and repair (PubMed:12972596, PubMed:28801535). Binds to core histones together with SMARCA5, and is required for the assembly of regular nucleosome arrays by the RSF-5 ISWI chromatin-remodeling complex (PubMed:12972596). Directly stimulates the ATPase activity of SMARCA1 and SMARCA5 in the RSF-1 and RSF-5 ISWI chromatin-remodeling complexes, respectively (PubMed:28801535). The RSF-1 ISWI chromatin remodeling complex has a lower ATP hydrolysis rate than the RSF-5 ISWI chromatin-remodeling complex (PubMed:28801535). The complexes do not have the ability to slide mononucleosomes to the center of a DNA template (PubMed:28801535). Facilitates transcription of hepatitis B virus (HBV) genes by the pX transcription activator. In case of infection by HBV, together with pX, it represses TNF-alpha induced NF-kappa-B transcription activation. Represses transcription when artificially recruited to chromatin by fusion to a heterogeneous DNA binding domain (PubMed:11788598, PubMed:11944984).