Sanz-Flores, M.; Ruiz-Torres, M.; Aguirre-Portoles, C.; El-Bakkali, A.; Salvador-Barbero, B.; Villarroya-Beltri, C.; Ortega, S.; Megias, D.; Gerlich, D. W.; Alvarez-Fernandez, M.; Malumbres, M.

Cell cycle progression is regulated by the orderly balance between kinase and phosphatase activities. PP2A phosphatase holoenzymes containing the B55 family of regulatory B subunits (PP2A-B55) function as major CDK1-counteracting phosphatases during mitotic exit in mammals. However, the identification of the specific mitotic roles of these PP2A-B55 complexes has been hindered by the existence of multiple B55 isoforms. Here, through the generation of loss-of-function genetic mouse models for the two ubiquitous B55 isoforms (B55alpha and B55delta), we report that PP2A-B55 alpha/delta complexes display overlapping roles in controlling the dynamics of proper chromosome individualization and clustering during mitosis. In the absence of PP2A-B55alpha/delta activity, mitotic cells display increased chromosome individualization in the presence of enhanced phosphorylation and perichromosomal loading of Ki-67. These data provide experimental evidence for a new regulatory mechanism by which the balance between kinase and PP2A-B55 phosphatase activity controls the Ki-67-mediated spatial organization of the mass of chromosomes during mitosis.