The SCFFbw7 Substrate Cycle: phosphodegron processing and nucleolar translocation

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 DESCRIPTION (provided by applicant): SCFFbw7 is a multisubunit ubiquitin ligase that targets number important cellular regulatory proteins for ubiquitin mediated proteolysis. Targeting of these substrates is mediated via the specificity factor Fbw7, which recognizes a somewhat degenerate consensus sequence known as the CPD (for Cdc4 PhosphoDegron; Cdc4 is the yeast ortholog of Fbw7). Although there is significant variation between individual CPDs, the consensus is invariant in demanding a phosphothreonine (less frequently a phosphoserine) at position 0 and a proline at position +1. However, for most vertebrate substrates of SCFFbw7, there is a second proline at position +2, a sequence motif not observed in yeast substrates of SCFCdc4. Nevertheless, SCFCdc4 is competent to ubiquity late mammalian substrates containing the proline-proline sequence. This observation suggests that the proline-proline motif has evolved to carry out an additional function. We have shown for one substrate, human cyclin E1, that although the second proline is not important for ubiquitylation and proteolysis, per se, it does have a profound effect on certain aspects of cyclin E turnover. Specifically, this second proline is required for translocation of phosphorylated cyclin E into the nucleolus where ubiquitylation is carried out by SCF constituted with a nucleolus-specific Fbw7 isoform, Fbw7. Nucleolar translocation requires both the nuclear Fbw7 isoform, Fbw7a, and the prolyl peptidyl cis-trans isomerase Pin1, which collaborate to carry out a non-cannonical isomerization of the proline-proline bond rather than the phosphothreonine-proline bond, as is typically the case for Pin1. The general goal of the proposed research is to understand the mechanism whereby a proline-proline bond in conjunction with Fbw7a and Pin1 undergoes cis-trans isomerization and promotes cyclin E nucleolar translocation, and to determine why ubiquitin-mediated proteolysis of cyclin E has evolved to incorporate nucleolar translocation. Finally, we will determine whether other SCFFbw7 substrates that contain a proline-proline sequence at an equivalent position in their phosphodegrons also exhibit Pin1- dependent isomerization and nucleolar translocation coupled to their degradation.