Sample purity is central to in vitro studies of protein function and regulation, and to the efficiency and success of structural studies using techniques such as x-ray crystallography and cryo-electron microscopy (cryo-EM). Here, we show that mass photometry (MP) can accurately characterize the heterogeneity of a sample using minimal material with high resolution within a matter of minutes. To benchmark our approach, we use negative stain electron microscopy (nsEM), a popular method for EM sample screening. We include typical workflows developed for structure determination that involve multi-step purification of a multi-subunit ubiquitin ligase and chemical cross-linking steps. When assessing the integrity and stability of large molecular complexes such as the proteasome, we detect and quantify assemblies invisible to nsEM. Our results illustrate the unique advantages of MP over current methods for rapid sample characterization, prioritization and workflow optimization.

Kinesin-5 motors organize mitotic spindles by sliding apart microtubules. They are homotetramers with dimeric motor and tail domains at both ends of a bipolar minifilament. Here, we describe a regulatory mechanism involving direct binding between tail and motor domains and its fundamental role in microtubule sliding. Kinesin-5 tails decrease microtubule-stimulated ATP-hydrolysis by specifically engaging motor domains in the nucleotide-free or ADP states. Cryo-EM reveals that tail binding stabilizes an open motor domain ATP-active site. Full-length motors undergo slow motility and cluster together along microtubules, while tail-deleted motors exhibit rapid motility without clustering. The tail is critical for motors to zipper together two microtubules by generating substantial sliding forces. The tail is essential for mitotic spindle localization, which becomes severely reduced in tail-deleted motors. Our studies suggest a revised microtubule-sliding model, in which kinesin-5 tails stabilize motor domains in the microtubule-bound state by slowing ATP-binding, resulting in high-force production at both homotetramer ends.

The interplay between E2 and E3 enzymes regulates the polyubiquitination of substrates in eukaryotes. Among the several RING-domain E3 ligases in humans, many utilize two distinct E2s for polyubiquitination. For example, the cell cycle regulatory E3, human Anaphase-Promoting Complex/Cyclosome (APC/C), relies on UBE2C to prime substrates with ubiquitin (Ub) and UBE2S to extend polyubiquitin chains. However, the potential coordination between these steps in ubiquitin chain formation remains undefined. While numerous studies have unveiled how RING E3s stimulate individual E2s for Ub transfer, here we change perspective to describe a case where the chain-elongating E2 UBE2S feeds back and directly stimulates the E3 APC/C to promote substrate priming and subsequent multiubiquitination by UBE2C. Our work reveals an unexpected paradigm for the mechanisms of RING E3-dependent ubiquitination and for the diverse and complex interrelationship between components of the ubiquitination cascade.

The ubiquitin (Ub)-proteasome system is vital to nearly every biological process in eukaryotes. Specifically, the conjugation of Ub to target proteins by Ub ligases, such as the Anaphase-Promoting Complex/Cyclosome (APC/C), is paramount for cell cycle transitions as it leads to the irreversible destruction of cell cycle regulators by the proteasome. Through this activity, the RING Ub ligase APC/C governs mitosis, G1, and numerous aspects of neurobiology. Pioneering cryo-EM, biochemical reconstitution, and cell-based studies have illuminated many aspects of the conformational dynamics of this large, multi-subunit complex and the sophisticated regulation of APC/C function. More recent studies have revealed new mechanisms that selectively dictate APC/C activity and explore additional pathways that are controlled by APC/C-mediated ubiquitination, including an intimate relationship with chromatin regulation. These tasks go beyond the traditional cell cycle role historically ascribed to the APC/C. Here, we review these novel findings, examine the mechanistic implications of APC/C regulation, and discuss the role of the APC/C in previously unappreciated signaling pathways.

The Anaphase Promoting Complex/Cyclosome (APC/C) is a ubiquitin ligase that initiates anaphase and mitotic exit. The APC/C is activated by Cdc20 and inhibited by the mitotic checkpoint complex (MCC), which delays mitotic exit when the spindle assembly checkpoint (SAC) is activated. We previously identified apcin as a small molecule ligand of Cdc20 that inhibits APC/C Cdc20 and prolongs mitosis. Here we find that apcin paradoxically shortens mitosis when SAC activity is high. These opposing effects of apcin arise from targeting a common binding site in Cdc20 required for both substrate ubiquitination and MCC-dependent APC/C inhibition. Furthermore, we found that apcin cooperates with p31 comet to relieve MCC-dependent inhibition of APC/C. Apcin therefore causes either net APC/C inhibition, prolonging mitosis when SAC activity is low, or net APC/C activation, shortening mitosis when SAC activity is high, demonstrating that a small molecule can produce opposing biological effects depending on regulatory context.