A Highly Efficient Synthesis of Polyubiquitin Chains.

A robust, microwave-assisted, highly efficient, solid-phase peptide synthesis method for preparing isopeptide-linked 62-mer and 76-mer isoubiquitins and polyubiquitin is developed. The strategy avoids the use of costly resins and pseudoprolines, and the isopeptide-linked building blocks can be assembled with high initial purity within 1 day. All seven diubiquitins are successfully synthesized on a multi-milligram scale; a four-segment, three-ligation method is used to obtain a K33-/K11-linked mixed triubiquitin in excellent yield. Circular dichroism and crystallographic analyses are used to verify the structures of the well-folded, synthetic polyubiquitin chains. The facile synthetic strategy is expected to be generally applicable for the rapid synthesis of isopeptide-linked isoUbs and to pave the way for the study of longer polyubiquitin chains.