FIGURE 8.

Dissection of human Cdc7 kinase, two segments, DAM-1 and DAM-2, required for interaction with ASK. A, schematic representation of human Cdc7 polypeptide showing two essential motifs (DAM-1 and DAM-2 indicated by the pink boxes) required for interaction with ASK. Double-arrowed lines indicate that DAM-1 and DAM-2 interact with motif-M and motif-C, respectively, of ASK. The segment shown by the purple box is not required for interaction with ASK but may be required for full kinase activity in the context of full-length polypeptide. The numbers on both sides of each box indicate the amino acids (a.a.) at the ends of each segment. B, schematic drawing of a proposed mode of interaction between Cdc7 and ASK. M and C indicate motif-M and -C, respectively, of ASK. It is proposed that ATP may not have access to the ATP-binding pocket of free full-length Cdc7 polypeptide possibly due to blockage caused by the kinase insert sequences (left, suggested by the fact that minimum Cdc7 lacking KI-II and half of KI-III showed ASK-independent kinase activity, but is not explicitly depicted in the figure). ASK may bind to the larger lobe of Cdc7 kinase, through bipartite interactions (motif-M versus DAM-1 and motif-C versus DAM-2), and this binding may induce conformational change on Cdc7 and permit the binding of ATP to the ATP pocket of the kinase (right). The binding also facilitates the recognition of specific target sites on the substrate. The location of ASK on Cdc7 in this model may provide basis for further structural and biochemical studies on the mechanisms of kinase activation and substrate recognition. KI-II, kinase insert II; KI-III, kinase insert III; C-ter, C-terminal tail segment.