Figure 7

Idealized representation of Xic2 phosphoforms, developmental expression, and cell cycle regulation. A. Schematic representation of ³⁵S-methionine labeled Xic2 wildtype and mutant protein bands or Xic2 under specific conditions (untreated, in extract, with DNA, or with kinase inhibitors) (top panel) and schematic representation of Xic2 phosphorylation sites targeted by CDK2-cyclin and a caffeine-sensitive kinase (bottom panel). Unphosphorylated Xic2 is marked by the blue line, S98 phosphorylated Xic2 is marked by the purple line, and S78/S81 phosphorylated Xic2 bands are marked by the pink lines. B. Top Panel: Schematic representation of Xic1, 2, 3 RNA/protein expression during Xenopus development where the thicker lines represent higher expression, the arrow indicates the timing of gastrulation (stage 12) and the diamond indicates the tailbud stage (30). Bottom panel: Xic1, 2, and 3 are predicted to function at the G1 to S phase transition during development and in a normal somatic cell cycle as shown in the drawing (left). It is predicted that Xic1 and Xic2 are targeted by CRL4^Cdt2 and CRL1^Skp2 ubiquitin ligases. During a response to DNA damage caused by IR (right), both Xic1 and Xic2 are predicted to be upregulated to halt the cell cycle in G1 during a checkpoint to allow DNA repair to occur.