Subsets of human origin recognition complex (ORC) subunits are expressed in non-proliferating cells and associate with non-ORC proteins

The origin recognition complex (ORC) in yeast is a complex of six tightly a ssociated subunits essential for the initiation of DNA replication. Human O RC subunits are nuclear in proliferating cells and in proliferative tissues like the testis, consistent with a role of human ORC in DNA replication. O rc2, Orc3, and Orc5 also are detected in non-proliferating cells like cardi ac myocytes, adrenal cortical cells, and neurons, suggesting an additional role of these proteins in non-proliferating cells. Although Orc2-5 co-immun oprecipitate with each other under mild extraction conditions, a hole compl ex of the subunits is difficult to detect. When extracted under more string ent extraction conditions, several of the subunits eo-immunoprecipitate wit h stoichiometric amounts of other unidentified proteins but not with any of the known ORC subunits, The variation in abundance of individual ORC subun its (relative to each other) in several tissues, expression of some subunit s in non-proliferating tissues, and the absence of a stoichiometric complex of all the subunits in cell extracts indicate that subunits of human ORC i n somatic cells might have activities independent of their role as a six su bunit complex involved in replication initiation. Finally, all ORC subunits remain consistently nuclear, and Orc2 is consistently phosphorylated throu gh all stages of the cell cycle, whereas Orc1 is selectively phosphorylated in mitosis.