A perichromosomal region contains proteins phosphorylated during mitosis in Xenopus laevis cells.

An antibody that recognizes the phosphorylated form of nucleoplasmin has identified another nuclear protein whose antigenic form is regulated in a mitosis-specific manner, with a dramatic increase in binding occurring in all mitotic cells. The protein is localised around the periphery of condensed chromosomes during mitosis in a manner analogous to another nucleoplasmin-related polypeptide NO38. Mitosis-specific expression of the antigenic site is dependent on phosphorylation of the polypeptide; binding of the antibody is dramatically reduced by prior incubation of the polypeptide with phosphatases. Migration on SDS-PAGE suggests that the protein has an exceptionally large relative molecular mass, in excess of 400,000. The probable mitosis-specific phosphorylation and location of this antigen suggests a subcellular storage mechanism for proteins during mitosis.