Identification of a Subunit of a Novel Kleisin-β/SMC Complex as a Potential Substrate of Protein Phosphatase 2A

• Published in: Current biology 2003-12, Vol.13 (23), p.2058-2064
• Main Authors: Yeong, Foong May, Hombauer, Hans, Wendt, Kerstin S., Hirota, Toru, Mudrak, Ingrid, Mechtler, Karl, Loregger, Thomas, Marchler-Bauer, Aron, Tanaka, Koichi, Peters, Jan-Michael, Ogris, Egon
• Format: Article
• Language: English
• Subjects: Adenosine Triphosphatases - metabolism; Cell Cycle Proteins - metabolism; Chromosome Mapping; Chromosome Segregation - physiology; DNA-Binding Proteins - metabolism; Electrophoresis, Polyacrylamide Gel; Humans; Immunoblotting; Multiprotein Complexes; Phosphoprotein Phosphatases - metabolism; Phosphorylation; Precipitin Tests; Protein Phosphatase 2; Substrate Specificity
• ISSN: 0960-9822; 1879-0445
• DOI: 10.1016/j.cub.2003.10.032

Protein phosphatase 2A (PP2A) holoenzymes consist of a catalytic C subunit, a scaffolding A subunit, and one of several regulatory B subunits that recruit the AC dimer to substrates [1, 2]. PP2A is required for chromosome segregation [3–6], but PP2A's substrates in this process remain unknown. To identify PP2A substrates, we carried out a two-hybrid screen with the regulatory B/PR55 subunit. We isolated a human homolog of C. elegans HCP6, a protein distantly related to the condensin subunit hCAP-D2, and we named this homolog hHCP-6. Both C. elegans HCP-6 and condensin are required for chromosome organization and segregation [7–11]. HCP-6 binding partners are unknown, whereas condensin is composed of the structural maintenance of chromosomes proteins SMC2 and SMC4 and of three non-SMC subunits [12]. Here we show that hHCP-6 becomes phosphorylated during mitosis and that its dephosphorylation by PP2A in vitro depends on B/PR55, suggesting that hHCP-6 is a B/PR55-specific substrate of PP2A. Unlike condensin, hHCP-6 is localized in the nucleus in interphase, but similar to condensin, hHCP-6 associates with chromosomes during mitosis. hHCP-6 is part of a complex that contains SMC2, SMC4, kleisin-β, and the previously uncharacterized HEAT repeat protein FLJ20311. hHCP-6 is therefore part of a condensin-related complex that associates with chromosomes in mitosis and may be regulated by PP2A.