words (48). The second is dedication of the subunit to the complex. We use this criterion to indicate that the protein does not exhibit binding interactions or functions outside the complex and performs its function only within one or more complexes of the same class. Biochemically, this means that the subunit comigrates with the complex on density sedimentation (gradient centrifugation) assays and can be entirely depleted from the nuclear extract with high-affinity antibodies to other complex subunits. Genetically, dedication is often reflected in a similar phenotype, but for a variety of reasons, dedicated subunits can also have different phenotypes from the phenotypes of null mutations in other subunits, and indeed, this is often the case if they are members of a highly homologous family. For example, BAF53a deletion has a phenotype that is slightly more severe than that of the ATPase Brg (33), whereas BAF53b has an entirely different phenotype because it is a subunit of the neuron-specific nBAF complex (49). Yet both are dedicated to their complexes and resist dissociation in 5 M urea. These are important considerations to be taken into account for the functional assessment of tissue- and cancer-specific complexes.
