Conformational changes associated with post-translational modifications of pro¹⁴³ in Skp1 of dictyostelium -a dipeptide model system

Prolyl hydroxylation and subsequent glycosylation of the E3^SCF ubiquitin ligase subunit Skp1 affects its conformation and its interaction with F-box proteins and, ultimately, O₂-sensing in the organism. Taking a reductionist approach to understand the molecular basis for these effects, a series of end-capped Thr-Pro dipeptides was synthesized, tracking the sequential post-translational modifications that occur in the protein. The conformation of the pyrrolidine ring in each compound was gauged via coupling constants (³J_Hα,Hβ) and the electronegativity of the Cγ-substituents by chemical shifts (¹³C). The equilibrium between the cis-trans conformations about the central prolyl peptide bond was investigated by integration of signals corresponding to the two species in the ¹H NMR spectra over a range of temperatures. These studies revealed an increasing preference for the trans-conformation in the order Pro < Hyp < [α-(1,4)GlcNAc]Hyp. Rates for the forward and reverse reactions, determined by magnetization transfer experiments, demonstrated a reduced rate for the trans-to-cis conversion and a significant increase in the cis-to-trans conversion upon hydroxylation of the proline residue in the dipeptide. NOE experiments suggest that the Thr side chain pushes the sugar away from the pyrrolidine ring. These effects, which depended on the presence of the N-terminal Thr residue, offer a mechanism to explain altered properties of the corresponding full-length proteins.