New structural insights into Golgi Reassembly and Stacking Protein (GRASP) in solution

Resurrecting Golgi proteins to grasp Golgi ribbon formation and self-association under stress

The Golgi complex is an essential organelle of the eukaryotic exocytic pathway. A subfamily of Golgi matrix proteins, called GRASPs, is central in stress-induced unconventional secretion, Golgi dynamics during mitosis/apoptosis, and Golgi ribbon formation. The Golgi ribbon is vertebrate-specific and correlates with the appearance of two GRASP paralogues and two Golgins (GM130/Golgin45), which form specific GRASP-Golgin pairs. The molecular details of their appearance only in Metazoans are unknown. Moreover, despite new functionalities supported by GRASP paralogy, little is known about their structural and evolutionary differences. Here, we used ancestor sequence reconstruction and biophysical/biochemical approaches to assess the evolution of GRASPs structure/dynamics, fibrillation, and how they started anchoring their Golgin partners. Our data showed that a GRASP ancestor anchored Golgins before gorasp gene duplication in Metazoans. After gene duplication, variations within the GRASP binding pocket determined which paralogue would recruit which Golgin. These interactions are responsible for their specific Golgi location and Golgi ribbon appearance. We also suggest that GRASPs have a long-standing capacity to form supramolecular structures, affecting their participation in stress-induced processes.

In vivo observation of amyloid-like fibrils produced under stress

The participation of amyloids in neurodegenerative diseases and functional processes has triggered the quest for methods allowing their direct detection in vivo. Despite the plethora of data, those methods are still lacking. The autofluorescence from the extended β-sheets of amyloids is here used to follow fibrillation of S. cerevisiae Golgi Reassembly and Stacking Protein (Grh1). Grh1 has been implicated in starvation-triggered unconventional protein secretion (UPS), and here its participation also in heat shock response (HSR) is suggested. Fluorescence Lifetime Imaging (FLIM) is used to detect fibril autofluorescence in cells (E. coli and yeast) under stress (starvation and higher temperature). The formation of Grh1 large complexes under stress is further supported by size exclusion chromatography and ultracentrifugation. The data show for the first time in vivo detection of amyloids without the use of extrinsic probes as well as bring new perspectives on the participation of Grh1 in UPS and HSR.

The C-terminal tail extension of myosin 16 acts as a molten globule, including intrinsically disordered regions, and interacts with the N-terminal ankyrin

The lesser-known unconventional myosin 16 protein is essential in proper neuronal functioning and has been implicated in cell cycle regulation. Its longer Myo16b isoform contains a C-terminal tail extension (Myo16Tail), which has been shown to play a role in the neuronal phosphoinositide 3-kinase signaling pathway. Myo16Tail mediates the actin cytoskeleton remodeling, downregulates the actin dynamics at the postsynaptic site of dendritic spines, and is involved in the organization of the presynaptic axon terminals. However, the functional and structural features of this C-terminal tail extension are not well known. Here, we report the purification and biophysical characterization of the Myo16Tail by bioinformatics, fluorescence spectroscopy, and CD. Our results revealed that the Myo16Tail is functionally active and interacts with the N-terminal ankyrin domain of myosin 16, suggesting an intramolecular binding between the C and N termini of Myo16 as an autoregulatory mechanism involving backfolding of the motor domain. In addition, the Myo16Tail possesses high structural flexibility and a solvent-exposed hydrophobic core, indicating the largely unstructured, intrinsically disordered nature of this protein region. Some secondary structure elements were also observed, indicating that the Myo16Tail likely adopts a molten globule-like structure. These structural features imply that the Myo16Tail may function as a flexible display site particularly relevant in post-translational modifications, regulatory functions such as backfolding, and phosphoinositide 3-kinase signaling.

Conformational flexibility of GRASPs and their constituent PDZ subdomains reveals structural basis of their promiscuous interactome

The Golgi complex is a central component of the secretory pathway, responsible for several critical cellular functions in eukaryotes. The complex is organized by the Golgi matrix that includes the Golgi reassembly and stacking protein (GRASP), which was shown to be involved in cisternae stacking and lateral linkage in metazoan. GRASPs also have critical roles in other processes, with an unusual ability to interact with several different binding partners. The conserved N terminus of the GRASP family includes two PSD-95, DLG, and ZO-1 (PDZ) domains. Previous crystallographic studies of orthologues suggest that PDZ1 and PDZ2 have similar conformations and secondary structure content. However, PDZ1 alone mediates nearly all interactions between GRASPs and their partners. In this work, NMR, synchrotron radiation CD, and molecular dynamics (MD) were used to examine the structure, flexibility, and stability of the two constituent PDZ domains. GRASP PDZs are structured in an unusual β₃ α₁ β₄ β₅ α₂ β₆ β₁ β₂ secondary structural arrangement and NMR data indicate that the PDZ1 binding pocket is formed by a stable β₂ -strand and a more flexible and unstable α₂ -helix, suggesting an explanation for the higher PDZ1 promiscuity. The conformational free energy profiles of the two PDZ domains were calculated using MD simulations. The data suggest that, after binding, the protein partner significantly reduces the conformational space that GRASPs can access by stabilizing one particular conformation, in a partner-dependent fashion. The structural flexibility of PDZ1, modulated by PDZ2, and the coupled, coordinated movement between the two PDZs enable GRASPs to interact with multiple partners, allowing them to function as promiscuous, multitasking proteins.

Intrinsically disordered proteins in crowded milieu: when chaos prevails within the cellular gumbo

Effects of macromolecular crowding on structural and functional properties of ordered proteins, their folding, interactability, and aggregation are well documented. Much less is known about how macromolecular crowding might affect structural and functional behaviour of intrinsically disordered proteins (IDPs) or intrinsically disordered protein regions (IDPRs). To fill this gap, this review represents a systematic analysis of the available literature data on the behaviour of IDPs/IDPRs in crowded environment. Although it was hypothesized that, due to the excluded-volume effects present in crowded environments, IDPs/IDPRs would invariantly fold in the presence of high concentrations of crowding agents or in the crowded cellular environment, accumulated data indicate that, based on their response to the presence of crowders, IDPs/IDPRs can be grouped into three major categories, foldable, non-foldable, and unfoldable. This is because natural cellular environment is not simply characterized by the presence of high concentration of "inert" macromolecules, but represents an active milieu, components of which are engaged in direct physical interactions and soft interactions with target proteins. Some of these interactions with cellular components can cause (local) unfolding of query proteins. In other words, since crowding can cause both folding and unfolding of an IDP or its regions, the outputs of the placing of a query protein to the crowded environment would depend on the balance between these two processes. As a result, and because of the spatio-temporal heterogeneity in structural organization of IDPs, macromolecular crowding can differently affect structures of different IDPs. Recent studies indicate that some IDPs are able to undergo liquid-liquid-phase transitions leading to the formation of various proteinaceous membrane-less organelles (PMLOs). Although interiors of such PMLOs are self-crowded, being characterized by locally increased concentrations of phase-separating IDPs, these IDPs are minimally foldable or even non-foldable at all (at least within the physiologically safe time-frame of normal PMLO existence).

The yeast GRASP Grh1 displays a high polypeptide backbone mobility along with an amyloidogenic behavior

GRASPs are proteins involved in cell processes that seem paradoxical: responsible for shaping the Golgi cisternae and involved in unconventional secretion mechanisms that bypass the Golgi. Despite its physiological relevance, there is still a considerable lack of studies on full-length GRASPs. Our group has previously reported an unexpected behavior of the full-length GRASP from the fungus C. neoformans: its intrinsically-disordered characteristic. Here, we generalize this finding by showing that it is also observed in the GRASP from S. cerevisae (Grh1), which strongly suggests it might be a general property within the GRASP family. Furthermore, Grh1 is also able to form amyloid-like fibrils either upon heating or when submitted to changes in the dielectric constant of its surroundings, a condition that is experienced by the protein when in close contact with membranes of cell compartments, such as the Golgi apparatus. Intrinsic disorder and fibril formation can thus be two structural properties exploited by GRASP during its functional cycle.

Disorder-to-order transitions in the molten globule-like Golgi Reassembly and Stacking Protein

BACKGROUND

Golgi Reassembly and Stacking Proteins (GRASPs) are widely spread among eukaryotic cells (except plants) and are considered as key components in both the stacking of the Golgi cisternae and its lateral connection. Furthermore, GRASPs were also proved essential in the unconventional secretion pathway of several proteins, even though the mechanism remains obscure. It was previously observed that the GRASP homologue in Cryptococcus neoformans has a molten globule-like behavior in solution.

METHODS

We used circular dichroism, synchrotron radiation circular dichroism and steady-state as well as time-resolved fluorescence.

RESULTS

We report the disorder-to-order transition propensities for a native molten globule-like protein in the presence of different mimetics of cell conditions. Changes in the dielectric constant (such as those experienced close to the membrane surface) seem to be the major factor in inducing multiple disorder-to-order transitions in GRASP, which shows very distinct behavior when in conditions that mimic the vicinity of the membrane surface as compared to those found when free in solution. Other folding factors such as molecular crowding, counter ions, pH and phosphorylation exhibit lower or no effect on GRASP secondary structure and/or stability.

GENERAL SIGNIFICANCE

To the best of our knowledge, this is the first study focusing on understanding the disorder-to-order transitions of a molten globule structure without the need of any mild denaturing condition. A model is also introduced aiming at describing how the cell could manipulate the GRASP sensitivity to changes in the dielectric constant during different cell-cycle periods.

Golgi structure formation, function, and post-translational modifications in mammalian cells

The Golgi apparatus is a central membrane organelle for trafficking and post-translational modifications of proteins and lipids in cells. In mammalian cells, it is organized in the form of stacks of tightly aligned flattened cisternae, and dozens of stacks are often linked laterally into a ribbon-like structure located in the perinuclear region of the cell. Proper Golgi functionality requires an intact architecture, yet Golgi structure is dynamically regulated during the cell cycle and under disease conditions. In this review, we summarize our current understanding of the relationship between Golgi structure formation, function, and regulation, with focus on how post-translational modifications including phosphorylation and ubiquitination regulate Golgi structure and on how Golgi unstacking affects its functions, in particular, protein trafficking, glycosylation, and sorting in mammalian cells.

Going deep into protein secondary structure with synchrotron radiation circular dichroism spectroscopy

Circular dichroism (CD) spectroscopy is a fast, powerful, well-established, and widely used analytical technique in the biophysical and structural biology community to study protein secondary structure and to track changes in protein conformation in different environments. The use of the intense light of a synchrotron beam as the light source for collecting CD measurements has emerged as an enhanced method, known as synchrotron radiation circular dichroism (SRCD) spectroscopy, that has several advantages over the conventional CD method, including a significant spectral range extension for data collection, deeper access to the lower limit (cut-off) of conventional CD spectroscopy, an improved signal-to-noise ratio to increase accuracy in the measurements, and the possibility to collect measurements in highly absorbing solutions. In this review, we discuss different applications of the SRCD technique by researchers from Latin America. In this context, we specifically look at the use of this method for examining the secondary structure and conformational behavior of proteins belonging to the four main classes of the hierarchical protein domain classification CATH (Class, Architecture, Topology, Homology) database, focusing on the advantages and improvements associated with SRCD spectroscopy in terms of characterizing proteins composed of different structural elements.

Virulence Factors as Targets for Anticryptococcal Therapy

The global mortality due to cryptococcosis caused by Cryptococcus neoformans or C. gattii is unacceptably high. Currently available therapies are decades old and may be impacted by drug resistance. Therefore, the need for more effective antifungal drugs for cryptococcosis is evident. A number of Cryptococcus virulence factors have been studied in detail, providing crucial information about the fungal biology and putative molecular targets for antifungals. This review focuses on the use of well-described virulence factors of Cryptococcus as potential anticryptococcal agents.