Part of the plasmid partitioning system, which ensures the equal distribution of replicated plasmid molecules to daughter cells. The plasmids exist as well-organized plasmid foci within the nucleus that stay together throughout the cell-cycle and act as entity during segregation, effetively reducing copy number to one. Plasmid partitioning requires the proteins REP1, REP2, and a cis-acting locus STB (REP3). REP1-REP2 stably associate with CSE4-containing chromatin at STB during S-phase, marking the locus with a centromeric tag, and thereby probably catching mitotic spindle microtubules to the plasmid cluster and coupling plasmid segregation to chromosome segregation. REP1-REP2 are required to recruit the cohesin complex to the STB locus for pairing of the replicated plasmid cluster, a prerequisite for successful plasmid segregation. REP1-REP2 also negatively regulate expression of site-specific recombinase FLP and of RAF1. Interacts with REP2. Colocalizes with the STB locus of the 2-micron plasmid as foci in the nucleus near the spindle pole body. Is expelled from STB during a short interval between late G1 and early S phases. The plasmid 2-micron circle is a extrachromosomal element that resides in the nucleus and propagates itself stably in host cell populations. It provides no obvious advantage to the host but imposes no significant disadvantage either at its steady-state copy number of 40-60 molecules/cell.