A super-family of ATP-dependent helicases and DNA-dependent ATPases are conserved from bacteria to human, and participate in diverse cellular processes such as chromatin remodeling, transcription, recombination, DNA repair, and replication. At least five different human diseases are results of genetic defects in genes encoding these proteins, including Bloom Syndrome and a premature aging disease, Werner Syndrome. Two classes of chromatin-remodeling complexes have been described so far. The first class consists of various complexes catalyzing histone acetylation and deacetylation. The second class includes the ATP-dependent chromatin-remodeling complexes, such as SWI/SNF. I purified the mammalian version of the SWI/SNF complex which remains the only ATP-dependent chromatin- remodeling complex purified from mammals. By microsequencing, my colleagues and I have identified and cloned all 10 subunits from the major complex of human KB cells. Six of them belong to five different multigene families. In one case, three members of the same gene family have different tissue expression patterns, suggesting the existence of tissue- specific chromatin remodeling complexes. Since I took my position at NIA last July, I have continued working on the SWI/SNF project. In addition, I initiated a new project: the purification of a new helicase/chromatin-remodeling complex. Other progress included: 1. We have cloned the 57 kD subunit of the mammalian SWI/SNF- related complex (called BAF57). We found that it contains an High-Mobility-Group (HMG) DNA binding domain and a Kinesin-like coiled-coil region. We further demonstrated that the HMG-domain of BAF57 (as well as whole complex) binds DNA as do HMG proteins. Published in PNAS (Vol 95:492, 1998). 2. Five human BAF genes have been mapped, and even members of the same family do not cluster in the same region, but rather are dispersed on different chromosomes. Published in Genomics (Vol 51:140, 1998). 3. My collaborators and I have cloned the 53 kD subunit of the complex (BAF53), and shown that it is an actin-related protein (Arp). We also identified a 48 kD subunit as actin. We further showed that BAF53 and actin directly interact with the helicase subunit, BRG1. In addition, BAF complexes are targeted to the nuclear matrix by a BRG1-mediated interaction with actin and BAF53 (Cell, in press). 4. We have purified a new human complex, named NURD, which contains both ATP-dependent nucleosome disruption activity and histone deacetylase activity (which is usually associated with transcriptional repression). The deacetylase activity is stimulated by ATP on nucleosomal templates, suggesting that in this instance nucleosome disruption helps the deacetylase to access its substrates (accepted, Molecular Cell).
