Identification of functional domains in the cohesin loader subunit Scc4 by a random insertion/dominant negative screen

Cohesin is a multi-subunit complex that plays an essential role in genome stability. Initial association of cohesin with chromosomes requires the loader-a heterodimer composed of Scc4 and Scc2. However, very little is known about the loader's mechanism of action. In this study, we performed a genetic screen to identify functional domains in the Scc4 subunit of the loader. We isolated scc4 mutant alleles that, when overexpressed, have a dominant negative effect on cell viability. We defined a small region in the N terminus of Scc4 that is dominant negative when overexpressed, and on which Scc2/Scc4 activity depends. When the mutant alleles are expressed as a single copy, they are recessive and do not support cell viability, cohesion, cohesin loading or Scc4 chromatin binding. In addition, we show that the mutants investigated reduce, but do not eliminate, the interaction of Scc4 with either Scc2 or cohesin. However, we show that Scc4 cannot bind cohesin in the absence of Scc2. Our results provide new insight into the roles of Scc4 in cohesin loading, and contribute to deciphering the loading mechanism.