Protein interactions: anything new?

The Light Chain Allosterically Enhances the Protease Activity of Murine Urokinase-Type Plasminogen Activator

The active form of the murine urokinase-type plasminogen activator (muPA) is formed by a 27-residue disordered light chain connecting the amino-terminal fragment (ATF) with the serine protease domain. The two chains are tethered by a disulfide bond between C1CT in the disordered light chain and C122CT in the protease domain. Previous work showed that the presence of the disordered light chain affected the inhibition of the protease domain by antibodies. Here we show that the disordered light chain induced a 3.7-fold increase in kcat of the protease domain of muPA. In addition, hydrogen-deuterium exchange mass spectrometry (HDX-MS) and accelerated molecular dynamics (AMD) were performed to identify the interactions between the disordered light chain and the protease domain. HDX-MS revealed that the light chain is contacting the 110s, the turn between the β10- and β11-strand, and the β7-strand. A reduction in deuterium uptake was also observed in the activation loop, the 140s loop and the 220s loop, which forms the S1-specificty pocket where the substrate binds. These loops are further away from where the light chain seems to be interacting with the protease domain. Our results suggest that the light chain most likely increases the activity of muPA by allosterically favoring conformations in which the specificity pocket is formed. We propose a model by which the allostery would be transmitted through the β-strands of the β-barrels to the loops on the other side of the protease domain.

Intrinsically Disordered Flanking Regions Increase the Affinity of a Transcriptional Coactivator Interaction across Vertebrates

Interactions between two proteins are often mediated by a disordered region in one protein binding to a groove in a folded interaction domain in the other one. While the main determinants of a certain interaction are typically found within a well-defined binding interface involving the groove, recent studies show that nonspecific contacts by flanking regions may increase the affinity. One example is the coupled binding and folding underlying the interaction between the two transcriptional coactivators NCOA3 (ACTR) and CBP, where the flanking regions of an intrinsically disordered region in human NCOA3 increases the affinity for CBP. However, it is not clear whether this flanking region-mediated effect is a peculiarity of this single protein interaction or if it is of functional relevance in a broader context. To further assess the role of flanking regions in the interaction between NCOA3 and CBP, we analyzed the interaction across orthologs and paralogs (NCOA1, 2, and 3) in human, zebra fish, and ghost shark. We found that flanking regions increased the affinity 2- to 9-fold in the six interactions tested. Conservation of the amino acid sequence is a strong indicator of function. Analogously, the observed conservation of increased affinity provided by flanking regions, accompanied by moderate sequence conservation, suggests that flanking regions may be under selection to promote the affinity between NCOA transcriptional coregulators and CBP.

Protein nanocondensates: the next frontier

Over the past decade, myriads of studies have highlighted the central role of protein condensation in subcellular compartmentalization and spatiotemporal organization of biological processes. Conceptually, protein condensation stands at the highest level in protein structure hierarchy, accounting for the assembly of bodies ranging from thousands to billions of molecules and for densities ranging from dense liquids to solid materials. In size, protein condensates range from nanocondensates of hundreds of nanometers (mesoscopic clusters) to phase-separated micron-sized condensates. In this review, we focus on protein nanocondensation, a process that can occur in subsaturated solutions and can nucleate dense liquid phases, crystals, amorphous aggregates, and fibers. We discuss the nanocondensation of proteins in the light of general physical principles and examine the biophysical properties of several outstanding examples of nanocondensation. We conclude that protein nanocondensation cannot be fully explained by the conceptual framework of micron-scale biomolecular condensation. The evolution of nanocondensates through changes in density and order is currently under intense investigation, and this should lead to the development of a general theoretical framework, capable of encompassing the full range of sizes and densities found in protein condensates.

Towards sequence-based principles for protein phase separation predictions

The phenomenon of protein phase separation, which underlies the formation of biomolecular condensates, has been associated with numerous cellular functions. Recent studies indicate that the amino acid sequences of most proteins may harbour not only the code for folding into the native state but also for condensing into the liquid-like droplet state and the solid-like amyloid state. Here we review the current understanding of the principles for sequence-based methods for predicting the propensity of proteins for phase separation. A guiding concept is that entropic contributions are generally more important to stabilise the droplet state than they are for the native and amyloid states. Although estimating these entropic contributions has proven difficult, we describe some progress that has been recently made in this direction. To conclude, we discuss the challenges ahead to extend sequence-based prediction methods of protein phase separation to include quantitative in vivo characterisations of this process.

Alternatively spliced exon regulates context-dependent MEF2D higher-order assembly during myogenesis

During muscle cell differentiation, the alternatively spliced, acidic β-domain potentiates transcription of Myocyte-specific Enhancer Factor 2 (Mef2D). Sequence analysis by the FuzDrop method indicates that the β-domain can serve as an interaction element for Mef2D higher-order assembly. In accord, we observed Mef2D mobile nuclear condensates in C2C12 cells, similar to those formed through liquid-liquid phase separation. In addition, we found Mef2D solid-like aggregates in the cytosol, the presence of which correlated with higher transcriptional activity. In parallel, we observed a progress in the early phase of myotube development, and higher MyoD and desmin expression. In accord with our predictions, the formation of aggregates was promoted by rigid β-domain variants, as well as by a disordered β-domain variant, capable of switching between liquid-like and solid-like higher-order states. Along these lines, NMR and molecular dynamics simulations corroborated that the β-domain can sample both ordered and disordered interactions leading to compact and extended conformations. These results suggest that β-domain fine-tunes Mef2D higher-order assembly to the cellular context, which provides a platform for myogenic regulatory factors and the transcriptional apparatus during the developmental process.

Shapeshifting proteins: the role of structural disorder and conformational plasticity in physiology and disease

Intrinsically disordered proteins (IDPs) defy the conventional structure–function paradigm and do not autonomously fold up into unique 3D structures for carrying out functions. They exist as rapidly interconverting conformational ensembles and are thought to expand the functional repertoire of proteins. Such shapeshifting proteins are associated with a multitude of biological functions and a wide range of human diseases. The thematic issue on ‘Shapeshifting Proteins’ in Essays in Biochemistry includes some exciting and emerging aspects of this class of proteins. Articles in this issue provide current trends and contemporary views on various intriguing features of these proteins involving their unique structural and dynamical characteristics, misfolding and aggregation behavior, and their phase transitions into biomolecular condensates. I hope that this thematic issue will be of considerable interest to the practitioners in protein biochemistry and biophysics as well as to the researchers in other allied areas involving cell and molecular biology, neuroscience, virology, pathophysiology, and so forth.