The presented work deals on one hand with Cyclin E and the tumorsuppressor Fbw7 and on the other hand with the APC-ligase. At first, the importance of the phosphorylated Cyclin E residues Ser372, Thr380 and Ser384 were tested for the binding to the tumorsuppressor Fbw7 by ITC binding studies. This led to the conclusion that Fbw7 WD40 domain contains two-phosphate-binding pockets to which the phosphorylated Cyclin E residues Thr380 and Ser384 but not Ser372 bind to. Moreover, this result was confirmed by the crystal structure of the Fbw7-Skp1-CycE31pS372/pT380/pS384 complex. However the two-phosphate-binding pocket model stands in contradiction to earlier results on to Fbw7 closely related scCdc4. According to this, scCdc4 contains only one-phosphate-binding pocket, to which the S-phase inhibitor Sic1 can only bind to after its phosphorylation on at least five to six residues (Orlicky et al., 2003). Yet, the superimpostion of the WD40 domains of Fbw7 and scCdc4 (Orlicky et al., 2003) showed, that its WD40 motives two to five, which contain the two-phosphate-binding pockets, are identical. Furthermore, ITC binding studies of the scCdc4-Skp1 complex with the phosphorylated Cyclin E peptides have shown that scCdc4 like Fbw7 contains two-phosphate-binding pockets. In addition three doubly phosphorylated Sic1 peptides were identified, which bound compared to its singly phosphorylated Sic1 peptides four to eighteen times tighter to scCd4. This shows that specific doubly phosphorylated Sic1 peptides bind to two scCdc4 phosphate-binding pockets. Moreover substrate-binding and substrate-degradation studies with Fbw7 and scCdc4, mutated in the 2nd-phosphate-binding pocket, showed that they were less capable to bind to and to turnover its doubly phosphorylated substrates. It is also shown that Fbw7 and scCdc4 exist as a dimer. Furthermore, the crystal structure of Fbw7-Skp1 with a N-terminal phosphorylated CycE14pT62 peptide lets speculate that the two phosphorylated regions in Cyclin E and two of the three identified phosphorylated regions of Sic1 bind each to one WD40 domain of Fbw7/scCdc4 dimer. Secondly, subunits of the APC ligase were characterized and crystallized. Crystals of the scAPC4 subunit showed a limited diffraction of 6Å to 8Å, which is not sufficient to solve the structure.