Many chromatin regulators are subunits within large complexes, yet there seems to be little agreement on what the term “complex” implies and the difference between a “subunit” and an “associated protein.” In this review, we use the term “subunit” to imply two critical features. The first is stability of an interaction within the complex such that specific structural features are imparted to the surface of the complex. Here, the criteria that were first used came from early characterizations of the ribosome (46), showing stability at near-denaturing conditions with urea or other denaturing reagents. Generally, the stability of the subunit within the complex to 2 M urea is taken as an arbitrary criterion (36, 47). Stability is critical because it gives rise to specific surfaces that can have new biologic meanings, much in the same way that letters are assembled into 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
