Synthesis and Application of a Clickable Epoxomicin-Based Probe for Proteasome Activity Analysis

The proteasome is a multisubunit protein complex responsible for the degradation of proteins, making it essential in myriad cellular processes. Several reversible and irreversible peptide substrates inspired by known proteasome inhibitors have been developed to visualize it and monitor its activity; however, they have limited commercial availability or possess fluorophores that overlap with other known chemical probes, limiting their simultaneous use. The protocols presented here describe the synthesis of a clickable epoxomicin-based probe followed by the copper-catalyzed installment of an azide-containing fluorophore, and the application of the synthesized peptide in proteasome activity assays by SDS-PAGE and flow cytometry. © 2022 The Authors. Current Protocols published by Wiley Periodicals LLC. Basic Protocol 1: Solid-phase synthesis of clickable peptide fragment (2) Basic Protocol 2: In-solution coupling of epoxy-ketone moiety to fragment (2) Basic Protocol 3: Copper-catalyzed click reaction of (3) with fluorophore of choice Basic Protocol 4: Monitoring proteasome activity by SDS-PAGE in HEK-293T cells Alternate Protocol: Monitoring proteasome activity by flow cytometry in HEK-293T cells.

Biochemical characterization of G protein coupling to calcitonin gene-related peptide and adrenomedullin receptors using a native PAGE assay

Calcitonin gene-related peptide (CGRP), adrenomedullin (AM), and adrenomedullin 2/intermedin (AM2/IMD) have overlapping and unique functions in the nervous and circulatory systems including vasodilation, cardioprotection, and pain transmission. Their actions are mediated by the class B calcitonin-like G protein-coupled receptor (CLR), which heterodimerizes with three receptor activity-modifying proteins (RAMP1-3) that determine its peptide ligand selectivity. How the three agonists and RAMPs modulate CLR binding to transducer proteins remains poorly understood. Here, we biochemically characterized agonist-promoted G protein coupling to each CLR·RAMP complex. We adapted a native PAGE method to assess the formation and thermostabilities of detergent-solubilized fluorescent protein-tagged CLR·RAMP complexes expressed in mammalian cells. Addition of agonist and the purified Gs protein surrogate mini-Gs (mGs) yielded a mobility-shifted agonist·CLR·RAMP·mGs quaternary complex gel band that was sensitive to antagonists. Measuring the apparent affinities of the agonists for the mGs-coupled receptors and of mGs for the agonist-occupied receptors revealed that both ligand and RAMP control mGs coupling and defined how agonist engagement of the CLR extracellular and transmembrane domains affects transducer recruitment. Using mini-Gsq and -Gsi chimeras, we observed a coupling rank order of mGs > mGsq > mGsi for each receptor. Last, we demonstrated the physiological relevance of the native gel assays by showing that they can predict the cAMP-signaling potencies of AM and AM2/IMD chimeras. These results highlight the power of the native PAGE assay for membrane protein biochemistry and provide a biochemical foundation for understanding the molecular basis of shared and distinct signaling properties of CGRP, AM, and AM2/IMD.

Palmitoylated Proteins in Plasmodium falciparum-Infected Erythrocytes: Investigation with Click Chemistry and Metabolic Labeling

The examination of the complex cell biology of the human malaria parasite Plasmodium falciparum usually relies on the time-consuming generation of transgenic parasites. Here, metabolic labeling and click chemistry are employed as a fast transfection-independent method for the microscopic examination of protein S-palmitoylation, an important post-translational modification during the asexual intraerythrocytic replication of P. falciparum. Applying various microscopy approaches such as confocal, single-molecule switching, and electron microscopy, differences in the extent of labeling within the different asexual developmental stages of P. falciparum and the host erythrocytes over time are observed.