Impaired Chaperone Activity of Human Heat Shock Protein Hsp27 Site-Specifically Modified with Argpyrimidine

Potential for targeting small heat shock protein modifications

Small heat shock proteins (sHSPs) play key roles in cellular stress and several human diseases. The direct effects of some post-translational modifications (PTMs) on certain sHSPs have been characterized, raising the possibility that small molecules could be used to modulate these modifications and indirectly up- or downregulate sHSP activity.

Expressed Protein Selenoester Ligation

Herein, we describe the development and application of a novel expressed protein selenoester ligation (EPSL) methodology for the one-pot semi-synthesis of modified proteins. EPSL harnesses the rapid kinetics of ligation reactions between modified synthetic selenopeptides and protein aryl selenoesters (generated from expressed intein fusion precursors) followed by in situ chemoselective deselenization to afford target proteins at concentrations that preclude the use of traditional ligation methods. The utility of the EPSL technology is showcased through the efficient semi-synthesis of ubiquitinated polypeptides, lipidated analogues of the membrane-associated GTPase YPT6, and site-specifically phosphorylated variants of the oligomeric chaperone protein Hsp27 at high dilution.

Expressed Protein Selenoester Ligation

Herein, we describe the development and application of a novel expressed protein selenoester ligation (EPSL) methodology for the one‐pot semi‐synthesis of modified proteins. EPSL harnesses the rapid kinetics of ligation reactions between modified synthetic selenopeptides and protein aryl selenoesters (generated from expressed intein fusion precursors) followed by in situ chemoselective deselenization to afford target proteins at concentrations that preclude the use of traditional ligation methods. The utility of the EPSL technology is showcased through the efficient semi‐synthesis of ubiquitinated polypeptides, lipidated analogues of the membrane‐associated GTPase YPT6, and site‐specifically phosphorylated variants of the oligomeric chaperone protein Hsp27 at high dilution.

Single Posttranslational Modifications in the Central Repeat Domains of Tau4 Impact its Aggregation and Tubulin Binding

A variety of methods have been employed to study the impact of posttranslational modifications on Tau protein function. Here, a semisynthesis strategy is described that enables selective modification within the central repeat domain of Tau4 (residues 291‐321), comprising a major interaction motive with tubulin as well as one of the key hexapeptides involved in Tau aggregation. This strategy has led to the preparation of four semisynthetic Tau variants with phosphoserine residues in different positions and one with a so far largely ignored carboxymethyllysine modification that results from a non‐enzymatic posttranslational modification (nPTM). The latter modification inhibits tubulin polymerization but exhibits an aggregation behavior very similar to unmodified Tau. In contrast, phosphorylated Tau variants exhibit similar binding to tubulin as unmodified Tau4 but show lower tendencies to aggregate.

Utility of the Phenacyl Protecting Group in Traceless Protein Semisynthesis through Ligation-Desulfurization Chemistry

Semisynthesis of proteins via expressed protein ligation is a widely applicable method, even more so because of the possibility of ligation at non-cysteine sites using β-mercapto amino acids that can be converted to the corresponding native amino acids by desulfurization. A drawback of this ligation- desulfurization approach is the removal of any unprotected native cysteine residues within the ligated protein segments. Here, we show that the phenacyl (PAc) moiety can be successfully used to protect cysteines within recombinantly generated protein segments. As such, this group was selectively appended onto cysteine side chains within bacterially expressed polypeptides following intein cleavage, which reveals a rather sensitive thioester at the C-terminus. The PAc group proved to be compatible with native chemical ligation, radical desulfurization, and reverse-phase HPLC conditions, and was smoothly removed at the end. The utility of the PAc protecting group was then demonstrated by the 'traceless' semisynthesis of two proteins containing one or two native cysteines: human small heat shock protein Hsp27 and murine prion protein.