The cdc2 kinase subunit (cks) proteins play an essential function in the control of mitosis through their molecular complexes with the cdc2 kinase. In this work, we characterize the conformational state(s) in solution of the cks proteins p13suc1 from Schizosaccharomyces pombe and p9cksphy from Physarum polycephalum. Monomers of p13suc1 and p9cksphy were found to be markedly nonglobular, presumably with a long, nonfolded C-terminal moiety. This was in contrast to the previously published structure of p13suc1, derived from crystallographic studies on a zinc-promoted p13suc1 dimer, in which the individual p13suc1 subunits had a globular conformation. This disparity was resolved when we found that the globular p13suc1 fold undergoes a conformational transition into nonglobular monomers upon dissociation of the dimers following chelation of the zinc ions by ethylenediaminetetraacetic acid (EDTA). We also found that p13suc1, but not p9cksphy, forms stable dimers in the absence of metal ions. The topology of these EDTA-insensitive dimers likely resembles that of the human p9ckshs2 protein, characterized by beta 4 strand exchange from each nonglobular monomer.