Design, synthesis and biological evaluation of phosphopeptides as Polo-like kinase 1 Polo-box domain inhibitors.

Polo-like kinase 1 (Plk1) is an anti-cancer target due to its critical role in mitotic progression. A growing body of evidence has documented that Peptide-Plk1 inhibitors showed high Plk1 binding affinity. However, phosphopeptides-Plk1 inhibitors showed poor cell membranes permeability, which limits their clinical applications. In current study, nine candidate phosphopeptides consisting of non-natural amino acids were rationally designed and then successfully synthesized using an Fmoc-solid phase peptide synthesis (SPPS) strategy. Moreover, the binding affinities and selectivity were evaluated via fluorescence polarization (FP) assay. The results confirmed that the most promising phosphopeptide 6 bound to Plk1 PBD with the IC50 of 38.99 nM, which was approximately 600-fold selectivity over Plk3 PBD (IC50 = 25.44 µM) and nearly no binding to Plk2 PBD. Furthermore the intracellular activities and the cell membrane permeability of phosphopeptide 6 were evalutated. Phosphopeptide 6 demonstrated appropriate cell membrane permeability and arrested HeLa cells cycle in G2/M phase by regulating CyclinB1-CDK1. Further, phosphopeptide 6 showed typical apoptotic morphology and induced caspase-dependent apoptosis. In conclusion, we expect our discovery can provide new insights into the further optimization of Plk1 PBD inhibitors.

Design and synthesis of an immobilized metal affinity chromatography and metal oxide affinity chromatography hybrid material for improved phosphopeptide enrichment.

Reversible phosphorylation of proteins is one of the most important post-translational modifications, while the detection of phosphopeptides is difficult due to their low abundance and the signal suppression of nonphosphorylated peptides. Therefore, selective enrichment of phosphopeptides from highly complicated mixtures is vital for MS-based phosphoproteome analysis. Despite various strategies have been developed, there is no single method that is capable of providing full coverage of the whole phosphoproteome. Metal oxide affinity chromatography (MOAC) enrichment preferably singly phosphopeptides, whereas immobilized metal affinity chromatography (IMAC) enrichment bias towards multiply phosphopeptides. In this study, first example of IMAC and MOAC hybrid material, Fe3O4@nSiO2@mSiO2/TiO2-Ti4+ nanoparticles were successfully synthesized for the enrichment of phosphopeptides with the aim to combining their advantages for enrich both mono- and multi-phosphorylated species. The TiO2 was firstly coated on the surface of mesoporous silica and then grafted with 3-(trihydroxysilyl)propyl methylphosphonate (THPMP) to chelate Ti4+ ions. This novel type of hybird material with high surface areas (179.3m2/g) exhibited good adsorption capacity (133mg/g) towards standard tryptic digest of ß-casein and the method based on this material also showed good sensitivity (4pmol). The synthesized Fe3O4@nSiO2@mSiO2/TiO2-Ti4+ microspheres were further used to selectively enrich phosphopeptides from complex biosamples, seven mono-phosphopeptides and eight multi-phosphopeptides were successfully enriched from nonfat milk which is much better than single IMAC or MOAC strategy. Those results indicated that Fe3O4@nSiO2@mSiO2/TiO2-Ti4+ microspheres have potential applications in MS-based phosphoproteomics to enlarge phosphoproteomics coverage and this work was expected to open up a promising strategy which combined the advantages of various methods in one material for effective enrich phosphorylated peptides.