Crystal structure of the S100A4-nonmuscle myosin IIA tail fragment complex reveals an asymmetric target binding mechanism

Structure, regulation, and mechanisms of nonmuscle myosin-2

Members of the myosin superfamily of molecular motors are large mechanochemical ATPases that are implicated in an ever-expanding array of cellular functions. This review focuses on mammalian nonmuscle myosin-2 (NM2) paralogs, ubiquitous members of the myosin-2 family of filament-forming motors. Through the conversion of chemical energy into mechanical work, NM2 paralogs remodel and shape cells and tissues. This process is tightly controlled in time and space by numerous synergetic regulation mechanisms to meet cellular demands. We review how recent advances in structural biology together with elegant biophysical and cell biological approaches have contributed to our understanding of the shared and unique mechanisms of NM2 paralogs as they relate to their kinetics, regulation, assembly, and cellular function.

S100A4 makes two appearances in mechanisms leading to fibrosis

Non-muscle myosin 2 (NM2) is known to play an important role in myofibroblast transdifferentiation, a hallmark of fibrotic disorders. In a recent JBC article, Southern et al. demonstrate that endogenous S100A4, a calcium- and NM2-binding protein acts as a mechanoeffector in this process. Since extracellular S100A4 is also involved in fibrogenesis by triggering the inflammatory response, this small protein appears to contribute to fibrosis via at least two distinct mechanisms.

Essential role of CD38 in platelet aggregation through the PKC-mediated internalization and activation

Introduction

CD38 is a multifunctional enzyme with a potent Ca2+ mobilizing effect, cyclic ADP-ribose (cADPR), and nicotinic acid adenine dinucleotide phosphate (NAADP). Here, we aimed to demonstrate the role of CD38 in platelets via protein kinase C (PKC)-mediated internalization and activation.

Methods

Mouse platelets were used in this study. Thrombin, an agonist of platelet function, provoked a prompt and long-lasting increase in intracellular Ca2+ concentration ([Ca2+]i), resulting from an interplay of multifold Ca2+ mobilizing messengers.The signaling pathway was delineated using different inhibitors and techniques such as platelet aggregation assay, intracellular calcium measurements, immunoprecipitation, immunoblotting, and flow cytometry.

Results

We observed a sequential formation of cADPR and NAADP through CD38 activation by PKC of non-muscle myosin heavy chain IIA (MHCIIA), resulting in phospholipase C (PLC) activation in the thrombin-stimulated platelets. These findings reveal that PKC is fundamental in activating CD38 and elicits a physiological response in the murine platelets.

Conclusion

PKC is involved in many signaling pathways. Specifically, PKC is involved in the internalization of CD38 via MHCIIA in CD38+/+ wild-type (WT) and CD38-/- knockout mice (KO). CD38 generates calcium-mobilizing agents that act on specific receptors of the calcium stores. Calcium triggered platelet aggregation while serving as a secondary messenger.

Formation of protein adducts with Hydroperoxy-PE electrophilic cleavage products during ferroptosis

Ferroptosis is an iron dependent form of cell death, that is triggered by the discoordination of iron, lipids, and thiols. Its unique signature that distinguishes it from other forms of cell death is the formation and accumulation of lipid hydroperoxides, particularly oxidized forms of polyunsaturated phosphatidylethanolamines (PEs), which drives cell death. These readily undergo iron-catalyzed secondary free radical reactions leading to truncated products which retain the signature PE headgroup and which can readily react with nucleophilic moieties in proteins via their truncated electrophilic acyl chains. Using a redox lipidomics approach, we have identified oxidatively-truncated PE species (trPEox) in enzymatic and non-enzymatic model systems. Further, using a model peptide we demonstrate adduct formation with Cys as the preferred nucleophilic residue and PE(26:2) +2 oxygens, as one of the most reactive truncated PE-electrophiles produced. In cells stimulated to undergo ferroptosis we identified PE-truncated species with sn-2 truncations ranging from 5 to 9 carbons. Taking advantage of the free PE headgroup, we have developed a new technology using the lantibiotic duramycin, to enrich and identify the PE-lipoxidated proteins. Our results indicate that several dozens of proteins for each cell type, are PE-lipoxidated in HT-22, MLE, and H9c2 cells and M2 macrophages after they were induced to undergo ferroptosis. Pretreatment of cells with the strong nucleophile, 2-mercaptoethanol, prevented the formation of PE-lipoxidated proteins and blocked ferroptotic death. Finally, our docking simulations showed that the truncated PE species bound at least as good to several of the lantibiotic-identified proteins, as compared to the non-truncated parent molecule, stearoyl-arachidonoyl PE (SAPE), indicating that these oxidatively-truncated species favor/promote the formation of PEox-protein adducts. The identification of PEox-protein adducts during ferroptosis suggests that they are participants in the ferroptotic process preventable by 2-mercaptoethanol and may contribute to a point of no return in the ferroptotic death process.

Ruscogenin alleviates LPS-triggered pulmonary endothelial barrier dysfunction through targeting NMMHC IIA to modulate TLR4 signaling

Pulmonary endothelial barrier dysfunction is a hallmark of clinical pulmonary edema and contributes to the development of acute lung injury (ALI). Here we reported that ruscogenin (RUS), an effective steroidal sapogenin of Radix Ophiopogon japonicus, attenuated lipopolysaccharides (LPS)-induced pulmonary endothelial barrier disruption through mediating non-muscle myosin heavy chain IIA (NMMHC IIA)‒Toll-like receptor 4 (TLR4) interactions. By in vivo and in vitro experiments, we observed that RUS administration significantly ameliorated LPS-triggered pulmonary endothelial barrier dysfunction and ALI. Moreover, we identified that RUS directly targeted NMMHC IIA on its N-terminal and head domain by serial affinity chromatography, molecular docking, biolayer interferometry, and microscale thermophoresis analyses. Downregulation of endothelial NMMHC IIA expression in vivo and in vitro abolished the protective effect of RUS. It was also observed that NMMHC IIA was dissociated from TLR4 and then activating TLR4 downstream Src/vascular endothelial cadherin (VE-cadherin) signaling in pulmonary vascular endothelial cells after LPS treatment, which could be restored by RUS. Collectively, these findings provide pharmacological evidence showing that RUS attenuates LPS-induced pulmonary endothelial barrier dysfunction by inhibiting TLR4/Src/VE-cadherin pathway through targeting NMMHC IIA and mediating NMMHC IIA‒TLR4 interactions.

The Annexin A2/S100A10 Complex: The Mutualistic Symbiosis of Two Distinct Proteins

Mutualistic symbiosis refers to the symbiotic relationship between individuals of different species in which both individuals benefit from the association. S100A10, a member of the S100 family of Ca²⁺-binding proteins, exists as a tight dimer and binds two annexin A2 molecules. This association forms the annexin A2/S100A10 complex known as AIIt, and modifies the distinct functions of both proteins. Annexin A2 is a Ca²⁺-binding protein that binds F-actin, phospholipid, RNA, and specific polysaccharides such as heparin. S100A10 does not bind Ca²⁺, but binds tPA, plasminogen, certain plasma membrane ion channels, neurotransmitter receptors, and the structural scaffold protein, AHNAK. S100A10 relies on annexin A2 for its intracellular survival: in the absence of annexin A2, it is rapidly destroyed by ubiquitin-dependent and independent proteasomal degradation. Annexin A2 requires S100A10 to increase its affinity for Ca²⁺, facilitating its participation in Ca²⁺-dependent processes such as membrane binding. S100A10 binds tissue plasminogen activator and plasminogen, and promotes plasminogen activation to plasmin, which is a process stimulated by annexin A2. In contrast, annexin A2 acts as a plasmin reductase and facilitates the autoproteolytic destruction of plasmin. This review examines the relationship between annexin A2 and S100A10, and how their mutualistic symbiosis affects the function of both proteins.

Studying the Structures of Relaxed and Fuzzy Interactions: The Diverse World of S100 Complexes

S100 proteins are small, dimeric, Ca²⁺-binding proteins of considerable interest due to their associations with cancer and rheumatic and neurodegenerative diseases. They control the functions of numerous proteins by forming protein–protein complexes with them. Several of these complexes were found to display “fuzzy” properties. Examining these highly flexible interactions, however, is a difficult task, especially from a structural biology point of view. Here, we summarize the available in vitro techniques that can be deployed to obtain structural information about these dynamic complexes. We also review the current state of knowledge about the structures of S100 complexes, focusing on their often-asymmetric nature.

Competing Roles of Ca2+ and Nonmuscle Myosin IIA on the Dynamics of the Metastasis-Associated Protein S100A4

The calcium-binding protein S100A4 plays an important role in a wide range of biological processes such as cell motility, invasion, angiogenesis, survival, differentiation, contractility, and tumor metastasis and interacts with a range of partners. To understand the functional roles and interplay of S100A4 binding partners such as Ca²⁺ and nonmuscle myosin IIA (NMIIA), we used molecular dynamics simulations to investigate apo S100A4 and four holo S100A4 structures: S100A4 bound to Ca²⁺, S100A4 bound to NMIIA, S100A4 bound to Ca²⁺ and NMIIA, and a mutated S100A4 bound to Ca²⁺ and NMIIA. Our results show that two competing factors, namely, Ca²⁺-induced activation and NMIIA-induced inhibition, modulate the dynamics of S100A4 in a competitive manner. Moreover, Ca²⁺ binding results in enhanced dynamics, regulating the interactions of S100A4 with NMIIA, while NMIIA induces asymmetric dynamics between the chains of S100A4. The results also show that in the absence of Ca²⁺ the S100A4-NMIIA interaction is weak compared to that of between S100A4 bound to Ca²⁺ and NMIIA, which may offer a quick response to dropping calcium levels. In addition, certain mutations are shown to play a marked role on the dynamics of S100A4. The results described here contribute to understanding the interactions of S100A4 with NMIIA and the functional roles of Ca²⁺, NMIIA, and certain mutations on the dynamics of S100A4. The results of this study could be interesting for the development of inhibitors that exploit the shift of balance between the competing roles of Ca²⁺ and NMIIA.

Tumor-Suppressor p53TAD1-60 Forms a Fuzzy Complex with Metastasis-Associated S100A4: Structural Insights and Dynamics by an NMR/MD Approach

Conformationally flexible protein complexes represent a major challenge for structural and dynamical studies. We present herein a method based on a hybrid NMR/MD approach to characterize the complex formed between the disordered p53TAD1-60 and the metastasis-associated S100A4. Disorder-to-order transitions of both TAD1 and TAD2 subdomains upon interaction is detected. Still, p53TAD1-60 remains highly flexible in the bound form, with residues L26, M40, and W53 being anchored to identical hydrophobic pockets of the S100A4 monomer chains. In the resulting "fuzzy" complex, the clamp-like binding of p53TAD1-60 relies on specific hydrophobic anchors and on the existence of extended flexible segments. Our results demonstrate that structural and dynamical NMR parameters (cumulative Δδ, SSP, temperature coefficients, relaxation time, hetNOE) combined with MD simulations can be used to build a structural model even if, due to high flexibility, the classical solution structure calculation is not possible.

S100A4 is activated by RhoA and catalyses the polymerization of non-muscle myosin, adhesion complex assembly and contraction in airway smooth muscle

KEY POINTS

S100A4 is expressed in many tissues, including smooth muscle (SM), but its physiologic function is unknown. S100A4 regulates the motility of metastatic cancer cells by binding to non-muscle (NM) myosin II. Contractile stimulation causes the polymerization of NM myosin in airway SM, which is necessary for tension development. NM myosin regulates the assembly of adhesion junction signalling complexes (adhesomes) that catalyse actin polymerization. In airway SM, ACh (acetylcholine) stimulated the binding of S100A4 to the NM myosin heavy chain, which was catalysed by RhoA GTPase via the RhoA-binding protein, rhotekin. The binding of S100A4 to NM myosin was required for NM myosin polymerization, adhesome assembly and actin polymerization. S100A4 plays a critical function in the regulation of airway SM contraction by catalysing NM myosin filament assembly. The interaction of S100A4 with NM myosin may also play an important role in the physiologic function of other tissues.

ABSTRACT

S100A4 binds to the heavy chain of non-muscle (NM) myosin II and can regulate the motility of crawling cells. S100A4 is widely expressed in many tissues including smooth muscle (SM), although its role in the regulation of their physiologic function is not known. We hypothesized that S100A4 contributes to the regulation of contraction in airway SM by regulating a pool of NM myosin II at the cell cortex. NM myosin II undergoes polymerization in airway SM and regulates contraction by catalysing the assembly of integrin-associated adhesome complexes that activate pathways that catalyse actin polymerization. ACh stimulated the interaction of S100A4 with NM myosin II in airway SM at the cell cortex and catalysed NM myosin filament assembly. RhoA GTPase regulated the activation of S100A4 via rhotekin, which facilitated the formation of a complex between RhoA, S100A4 and NM myosin II. The depletion of S100A4, RhoA or rhotekin from airway SM tissues using short hairpin RNA or small interfering RNA prevented NM myosin II polymerization as well as the recruitment of vinculin and paxillin to adhesome signalling complexes in response to ACh, and inhibited actin polymerization and tension development. S100A4 depletion did not affect ACh-stimulated SM myosin regulatory light chain phosphorylation. The results show that S100A4 plays a critical role in tension development in airway SM tissue by catalysing NM myosin filament assembly, and that the interaction of S100A4 with NM myosin in response to contractile stimulation is activated by RhoA GTPase. These results may be broadly relevant to the physiologic function of S100A4 in other cell and tissue types.

Structure Determination of the Transactivation Domain of p53 in Complex with S100A4 Using Annexin A2 as a Crystallization Chaperone

To fully understand the environmental factors that influence crystallization is an enormous task, therefore crystallographers are still forced to work "blindly" trying as many crystallizing conditions and mutations to improve crystal packing as possible. Numerous times these random attempts simply fail even when using state-of-the-art techniques. As an alternative, crystallization chaperones, having good crystal-forming properties, can be invoked. Today, the almost exclusively used such protein is the maltose-binding protein (MBP) and crystallographers need other widely applicable options. Here, we introduce annexin A2 (ANXA2), which has just as good, if not better, crystal-forming ability than the wild-type MBP. Using ANXA2 as heterologous fusion partner, we were able to solve the atomic resolution structure of a challenging crystallization target, the transactivation domain (TAD) of p53 in complex with the metastasis-associated protein S100A4. p53 TAD forms an asymmetric fuzzy complex with the symmetric S1004 and could interfere with its function.

Pentamidine inhibit S100A4 - p53 interaction and decreases cell proliferation activity

Metastasis-associated S100A4 protein is a small calcium-binding protein typically overexpressed in several tumor forms, and it is widely accepted that S100A4 plays a significant role in the metastasis of cancer. Tumor suppressor p53 is one of the S100A4's main targets. Previous reports show that through p53, S100A4 regulates collagen expression and cell proliferation. When S100A4 interacts with p53, the S100A4 destabilizes wild type p53. In the current study, based on ¹H-¹⁵N HSQC NMR experiments and HADDOCK results, S100A4 interacts with the intrinsically unstructured transactivation domain (TAD) of the protein p53 and the pentamidine molecules in the presence of calcium ions. Our results suggest that the p53 TAD and pentamidine molecules share similar binding sites on the S100A4 protein. This observation indicates that a competitive binding mechanism can interfere with the binding of S100A4-p53 and increase the level of p53. Also, we compare different aspects of p53 activity in the WST-1 test using MCF 7 cells. We found that the presence of a pentamidine molecule results in higher p53 activity, which is also reflected in less cell proliferation. Collectively, our results indicate that disrupting the S100A4-p53 interaction would prevent cancer progression, and thus S100A4-p53 inhibitors provide a new avenue for cancer therapy.

NMMHC IIA triggers neuronal autophagic cell death by promoting F-actin-dependent ATG9A trafficking in cerebral ischemia/reperfusion

Previous findings have shown that non-muscle myosin heavy-chain IIA (NMMHC IIA) is involved in autophagy induction triggered by starvation in D. melanogaster; however, its functional contribution to neuronal autophagy remains unclear. The aim of this study is to explore the function of NMMHC IIA in cerebral ischemia-induced neuronal autophagy and the underlying mechanism related to autophagy-related gene 9A (ATG9A) trafficking. Functional assays and molecular mechanism studies were used to investigate the role of NMMHC IIA in cerebral ischemia-induced neuronal autophagy in vivo and in vitro. A middle cerebral artery occlusion (MCAO) model in mice was used to evaluate the therapeutic effect of blebbistatin, a myosin II ATPase inhibitor. Herein, either depletion or knockdown of NMMHC IIA led to increased cell viability in both primary cultured cortical neurons and pheochromocytoma (PC12) cells exposed to oxygen–glucose deprivation/reoxygenation (OGD/R). In addition, NMMHC IIA and autophagic marker LC3B were upregulated by OGD/R, and inhibition of NMMHC IIA significantly reduced OGD-induced neuronal autophagy. Furthermore, NMMHC IIA-induced autophagy is through its interactions with F-actin and ATG9A in response to OGD/R. The NMMHC IIA–actin interaction contributes to ATG9A trafficking and autophagosome formation. Inhibition of the NMMHC IIA–actin interaction using blebbistatin and the F-actin polymerization inhibitor cytochalasin D significantly suppressed ATG9A trafficking and autophagy induction. Furthermore, blebbistatin significantly improved neurological deficits and infarct volume after ischemic attack in mice, accompanied by ATG9A trafficking and autophagy inhibition. These findings demonstrate neuroprotective effects of NMMHC IIA inhibition on regulating ATG9A trafficking-dependent autophagy activation in the context of cerebral ischemia/reperfusion.

Interaction of calcium binding protein S100A16 with myosin-9 promotes cytoskeleton reorganization in renal tubulointerstitial fibrosis

Renal fibrosis arises by the generation of matrix-producing fibroblasts and myofibroblasts through the epithelial-mesenchymal transition (EMT), a process in which epithelial cells undergo a transition into a fibroblast phenotype. A key feature of the EMT is the reorganization of the cytoskeletons, which may involve the Ca²⁺-binding protein S100A16, a newly reported member of the S100 protein family. However, very few studies have examined the role of S100A16 in renal tubulointerstitial fibrosis. In this study, S100A16 expression was examined by immunohistochemical staining of kidney biopsy specimens from patients with various nephropathies and kidney tissues from a unilateral ureteral obstruction (UUO) mouse model. Renal histological changes were investigated in S100A16^Tg, S100A16^+/-, and WT mouse kidneys after UUO. The expression of epithelia marker E-cadherin, mesenchymal markers N-cadherin, and vimentin, extracellular matrix protein, and S100A16, as well as the organization of F-actin, were investigated in S100A16 overexpression or knockdown HK-2 cells. Mass spectrometry was employed to screen for S100A16 binding proteins in HK-2 cells. The results indicated that S100A16 is high expressed and associated with renal tubulointerstitial fibrosis in patient kidney biopsies and in those from UUO mice. S100A16 promotes renal interstitial fibrosis in UUO mice. S100A16 expression responded to increasing Ca²⁺ and interacted with myosin-9 during kidney injury or TGF-β stimulation to promote cytoskeleton reorganization and EMT progression in renal tubulointerstitial fibrosis. Therefore, S100A16 is a critical regulator of renal tubulointerstitial fibroblast activation and is therefore a potential therapeutic target for the treatment of renal fibrosis.

Role of S100 proteins in health and disease

The S100 family of proteins contains 25 known members that share a high degree of sequence and structural similarity. However, only a limited number of family members have been characterized in depth, and the roles of other members are likely undervalued. Their importance should not be underestimated however, as S100 family members function to regulate a diverse array of cellular processes including proliferation, differentiation, inflammation, migration and/or invasion, apoptosis, Ca²⁺ homeostasis, and energy metabolism. Here we detail S100 target protein interactions that underpin the mechanistic basis to their function, and discuss potential intervention strategies targeting S100 proteins in both preclinical and clinical situations.

High-throughput competitive fluorescence polarization assay reveals functional redundancy in the S100 protein family

The calcium-binding, vertebrate-specific S100 protein family consists of 20 paralogs in humans (referred as the S100ome), with several clinically important members. To explore their protein-protein interactions (PPIs) quantitatively, we have chosen an unbiased, high-throughput, competitive fluorescence polarization (FP) assay that revealed a partial functional redundancy when the complete S100ome (n = 20) was tested against numerous model partners (n = 13). Based on their specificity, the S100ome can be grouped into two distinct classes: promiscuous and orphan. In the first group, members bound to several ligands (> 4-5) with comparable high affinity, while in the second one, the paralogs bound only one partner weakly, or no ligand was identified. Our results demonstrate that FP assays are highly suitable for quantitative interaction profiling of selected protein families. Moreover, we provide evidence that PPI-based phenotypic characterization can complement or even exceed the information obtained from the sequence-based phylogenetic analysis of the S100ome, an evolutionary young protein family.

Matrigel patterning reflects multicellular contractility

Non-muscle myosin II (NMII)-induced multicellular contractility is essential for development, maintenance and remodeling of tissue morphologies. Dysregulation of the cytoskeleton can lead to birth defects or enable cancer progression. We demonstrate that the Matrigel patterning assay, widely used to characterize endothelial cells, is a highly sensitive tool to evaluate cell contractility within a soft extracellular matrix (ECM) environment. We propose a computational model to explore how cell-exerted contractile forces can tear up the cell-Matrigel composite material and gradually remodel it into a network structure. We identify measures that are characteristic for cellular contractility and can be obtained from image analysis of the recorded patterning process. The assay was calibrated by inhibition of NMII activity in A431 epithelial carcinoma cells either directly with blebbistatin or indirectly with Y27632 Rho kinase inhibitor. Using Matrigel patterning as a bioassay, we provide the first functional demonstration that overexpression of S100A4, a calcium-binding protein that is frequently overexpressed in metastatic tumors and inhibits NMIIA activity by inducing filament disassembly, effectively reduces cell contractility.

Sensing and exerting forces is a fundamental aspect of tissue organization. We demonstrate that contractile cells form an intricate network structure when placed in a pliable culture environment, a phenomenon often associated with vascular networks and is being actively used to characterize endothelial cells in culture. We propose a computational model that operates with mechanical stresses, plastic deformation and material failure within the cell-extracellular matrix composite to explain the patterning process. In addition to re-interpret a decades-old tool of experimental cell biology, our work suggests a potentially high throughput computational assay to characterize cellular contractility within a soft ECM environment.

MYH9 Aggregation Induced by Direct Interaction With PRRSV GP5 Ectodomain Facilitates Viral Internalization by Permissive Cells

Prevention and control of infection by porcine reproductive and respiratory syndrome virus (PRRSV) remains a challenge, due to our limited understanding of the PRRSV invasion mechanism. Our previous study has shown that PRRSV glycoprotein GP5 interacts with MYH9 C-terminal domain protein (PRA). Here we defined that the first ectodomain of GP5 (GP5-ecto-1) directly interacted with PRA and this interaction triggered PRA and endogenous MYH9 to form filament assembly. More importantly, MYH9 filament assembly was also formed in GP5-ecto-1-transfected MARC-145 cells. Notably, PRRSV infection of MARC-145 cells and porcine alveolar macrophages also induced endogenous MYH9 aggregation and polymerization that were required for subsequent PRRSV internalization. Moreover, overexpression of S100A4, a MYH9-specific disassembly inducer, in MARC-145 cells significantly resulted in diminished MYH9 aggregation and marked inhibition of subsequent virion internalization and infection by both PRRSV-1 and PRRSV-2 isolates. The collective results of this work reveal a novel molecular mechanism employed by MYH9 that helps PRRSV gain entry into permissive cells.

A subset of calcium-binding S100 proteins show preferential heterodimerization

The assembly of proteins into dimers and oligomers is a necessary step for the proper function of transcription factors, muscle proteins, and proteases. In uncontrolled states, oligomerization can also contribute to illnesses such as Alzheimer's disease. The S100 protein family is a group of dimeric proteins that have important roles in enzyme regulation, cell membrane repair, and cell growth. Most S100 proteins have been examined in their homodimeric state, yet some of these important proteins are found in similar tissues implying that heterodimeric molecules can also be formed from the combination of two different S100 members. In this work, we have established co-expression methods in order to identify and quantify the distribution of homo- and heterodimers for four specific pairs of S100 proteins in their calcium-free states. The split GFP trap methodology was used in combination with other GFP variants to simultaneously quantify homo- and heterodimeric S100 proteins in vitro and in living cells. For the specific S100 proteins examined, NMR, mass spectrometry, and GFP trap experiments consistently show that S100A1:S100B, S100A1:S100P, and S100A11:S100B heterodimers are the predominant species formed compared to their corresponding homodimers. We expect the tools developed here will help establish the roles of S100 heterodimeric proteins and identify how heterodimerization might alter the specificity for S100 protein action in cells.

Isolation and Characterization of S100 Protein-Protein Complexes

S100 proteins are small, mostly dimeric, EF-hand Ca²⁺-binding proteins. Upon Ca²⁺ binding, a conformational change occurs resulting in the exposure of a shallow hydrophobic binding groove in each subunit. Interestingly, S100 proteins can interact with their partners in two ways: symmetrically, when the two partners identically bind into each groove, or asymmetrically, when only one partner binds to the S100 dimer occupying both binding pockets. Here we present a heterologous expression and purification protocol for all known human S100 proteins as well as for their partner peptides. Moreover, we provide a detailed description of three in vitro methods to determine the affinity, stoichiometry, and kinetics of S100 protein-protein interactions.

S100 proteins as therapeutic targets

The human genome codes for 21 S100 protein family members, which exhibit cell- and tissue-specific expression patterns. Despite sharing a high degree of sequence and structural similarity, the S100 proteins bind a diverse range of protein targets and contribute to a broad array of intracellular and extracellular functions. Consequently, the S100 proteins regulate multiple cellular processes such as proliferation, migration and/or invasion, and differentiation, and play important roles in a variety of cancers, autoimmune diseases, and chronic inflammatory disorders. This review focuses on the development of S100 neutralizing antibodies and small molecule inhibitors and their potential therapeutic use in controlling disease progression and severity.

Multiple S100 protein isoforms and C-terminal phosphorylation contribute to the paralog-selective regulation of nonmuscle myosin 2 filaments

Nonmuscle myosin 2 (NM2) has three paralogs in mammals, NM2A, NM2B, and NM2C, which have both unique and overlapping functions in cell migration, formation of cell-cell adhesions, and cell polarity. Their assembly into homo- and heterotypic bipolar filaments in living cells is primarily regulated by phosphorylation of the N-terminally bound regulatory light chain. Here, we present evidence that the equilibrium between these filaments and single NM2A and NM2B molecules can be controlled via S100 calcium-binding protein interactions and phosphorylation at the C-terminal end of the heavy chains. Furthermore, we show that in addition to S100A4, other members of the S100 family can also mediate disassembly of homotypic NM2A filaments. Importantly, these proteins can selectively remove NM2A molecules from heterotypic filaments. We also found that tail phosphorylation (at Ser-1956 and Ser-1975) of NM2B by casein kinase 2, as well as phosphomimetic substitutions at sites targeted by protein kinase C (PKC) and transient receptor potential cation channel subfamily M member 7 (TRPM7), down-regulates filament assembly in an additive fashion. Tail phosphorylation of NM2A had a comparatively minor effect on filament stability. S100 binding and tail phosphorylation therefore preferentially disassemble NM2A and NM2B, respectively. These two distinct mechanisms are likely to contribute to the temporal and spatial sorting of the two NM2 paralogs within heterotypic filaments. The existence of multiple NM2A-depolymerizing S100 paralogs offers the potential for diverse regulatory inputs modulating NM2A filament disassembly in cells and provides functional redundancy under both physiological and pathological conditions.

MYH9: Structure, functions and role of non-muscle myosin IIA in human disease

The MYH9 gene encodes the heavy chain of non-muscle myosin IIA, a widely expressed cytoplasmic myosin that participates in a variety of processes requiring the generation of intracellular chemomechanical force and translocation of the actin cytoskeleton. Non-muscle myosin IIA functions are regulated by phosphorylation of its 20 kDa light chain, of the heavy chain, and by interactions with other proteins. Variants of MYH9 cause an autosomal-dominant disorder, termed MYH9-related disease, and may be involved in other conditions, such as chronic kidney disease, non-syndromic deafness, and cancer. This review discusses the structure of the MYH9 gene and its protein, as well as the regulation and physiologic functions of non-muscle myosin IIA with particular reference to embryonic development. Moreover, the review focuses on current knowledge about the role of MYH9 variants in human disease.

S100A4 regulates macrophage invasion by distinct myosin-dependent and myosin-independent mechanisms

S100A4, a member of the S100 family of Ca²⁺-binding proteins, is a key regulator of cell migration and invasion. Our previous studies showed that bone marrow–derived macrophages from S100A4^−/− mice exhibit defects in directional motility and chemotaxis in vitro and reduced recruitment to sites of inflammation in vivo. We now show that the loss of S100A4 produces two mechanistically distinct phenotypes with regard to macrophage invasion: a defect in matrix degradation, due to a disruption of podosome rosettes caused by myosin-IIA overassembly, and a myosin-independent increase in microtubule acetylation, which increases podosome rosette stability and is sufficient to inhibit macrophage invasion. Our studies point to S100A4 as a critical regulator of matrix degradation, whose actions converge on the dynamics and degradative functions of podosome rosettes.

Regulation of the Equilibrium between Closed and Open Conformations of Annexin A2 by N-Terminal Phosphorylation and S100A4-Binding

Annexin A2 (ANXA2) has a versatile role in membrane-associated functions including membrane aggregation, endo- and exocytosis, and it is regulated by post-translational modifications and protein-protein interactions through the unstructured N-terminal domain (NTD). Our sequence analysis revealed a short motif responsible for clamping the NTD to the C-terminal core domain (CTD). Structural studies indicated that the flexibility of the NTD and CTD are interrelated and oppositely regulated by Tyr24 phosphorylation and Ser26Glu phosphomimicking mutation. The crystal structure of the ANXA2-S100A4 complex showed that asymmetric binding of S100A4 induces dislocation of the NTD from the CTD and, similar to the Ser26Glu mutation, unmasks the concave side of ANXA2. In contrast, pTyr24 anchors the NTD to the CTD and hampers the membrane-bridging function. This inhibition can be restored by S100A4 and S100A10 binding. Based on our results we provide a structural model for regulation of ANXA2-mediated membrane aggregation by NTD phosphorylation and S100 binding.

Ezrin interacts with S100A4 via both its N- and C-terminal domains

Ezrin belongs to the ERM (ezrin, radixin, moesin) protein family that has a role in cell morphology changes, adhesion and migration as an organizer of the cortical cytoskeleton by linking actin filaments to the apical membrane of epithelial cells. It is highly expressed in a variety of human cancers and promotes metastasis. Members of the Ca2+-binding EF-hand containing S100 proteins have similar pathological properties; they are overexpressed in cancer cells and involved in metastatic processes. In this study, using tryptophan fluorescence and stopped-flow kinetics, we show that S100A4 binds to the N-terminal ERM domain (N-ERMAD) of ezrin with a micromolar affinity. The binding involves the F2 lobe of the N-ERMAD and follows an induced fit kinetic mechanism. Interestingly, S100A4 binds also to the unstructured C-terminal actin binding domain (C-ERMAD) with similar affinity. Using NMR spectroscopy, we characterized the complex of S100A4 with the C-ERMAD and demonstrate that no ternary complex is simultaneously formed with the two ezrin domains. Furthermore, we show that S100A4 co-localizes with ezrin in HEK-293T cells. However, S100A4 very weakly binds to full-length ezrin in vitro indicating that the interaction of S100A4 with ezrin requires other regulatory events such as protein phosphorylation and/or membrane binding, shifting the conformational equilibrium of ezrin towards the open state. As both proteins play an important role in promoting metastasis, the characterization of their interaction could shed more light on the molecular events contributing to this pathological process.

Tumor Microenvironment Activated Membrane Fusogenic Liposome with Speedy Antibody and Doxorubicin Delivery for Synergistic Treatment of Metastatic Tumors

Metastasis is the principal event leading to breast cancer death. Discovery of novel therapeutic approaches that are specific in targeting tumor metastasis factors while at the same time are an effective treatment of the tumor is urgently required. S100A4 protein is a key player in promoting metastasis and sequestrating the effect of tumor-suppressor protein p53. Here, a tumor microenvironment activated membrane fusogenic liposome was prepared to deliver rapidly anti-S100A4 antibody and doxorubicin into the cytoplasm directly in a fusion-dependent manner in order to bypass the cellular endocytosis to avoid the inefficient escape and degradation in the acidic endosome. After intracellular S100A4 blockage with anti-S100A4 antibody, the cytoskeleton of breast cancer 4T1 cells was rearranged and cell motility was suppressed. In the meantime, the antitumor effect of doxorubicin was enormously enhanced by reversing the effect of S100A4 on the sequestration of tumor-suppressor protein p53. Importantly, both local growth and metastasis of 4T1 cells were inhibited in a xenograft mouse model. Together, the speedy delivery of antibody and doxorubicin into cytoplasm based on a new membrane fusogenic liposome was an innovative approach for metastatic breast cancer treatment.

Molecular inhibition mechanisms of cell migration and invasion by coix polysaccharides in A549 NSCLC cells via targeting S100A4

S100 calcium binding protein A4 (S100A4) promotes extracellular signal transduction, intercellular adhesion, motility and mobility. Different extracts from Coix lachryma-jobi have been used for the treatment of various types of cancer in Asia. In our previous study, the polysaccharide fraction extact, CP1, induced cell apoptosis of non‑small cell lung cancer cells. In the current study, CP1 inhibited migration and invasion of A549 cells in a scratch wound healing assay and matrigel invasion assay, respectively. Furthermore, reverse transcription‑polymerase chain reaction and western blotting demonstrated that CP1 downregulated the gene and protein expression levels of S100A4. In silico docking analysis demonstrated that polysaccharides may not interfere with dimerization, whereas, the affinity of polysaccharides for an S100A4‑NMIIA pocket was margnially greater than at the dimerization sites. Thus, CP1 inhibited A549 cell migration and invasion potentially via downregulation of S100A4, and may also interact with the binding site of S100A4‑NMIIA, which indicated that CP1 has potential as an alternative cancer chemotherapeutic by targeting S100A4.

Multilevel Changes in Protein Dynamics upon Complex Formation of the Calcium-Loaded S100A4 with a Nonmuscle Myosin IIA Tail Fragment

Dysregulation of Ca²⁺ -binding S100 proteins plays important role in various diseases. The asymmetric complex of Ca²⁺ -bound S100A4 with nonmuscle myosin IIA has high stability and highly increased Ca²⁺ affinity. Here we investigated the possible causes of this allosteric effect by NMR spectroscopy. Chemical shift-based secondary-structure analysis did not show substantial changes for the complex. Backbone dynamics revealed slow-timescale local motions in the H1 helices of homodimeric S100A4; these were less pronounced in the complex form and might be accompanied by an increase in dimer stability. Different mobilities in the Ca²⁺ -coordinating EF-hand sites indicate that they communicate by an allosteric mechanism operating through changes in protein dynamics; this must be responsible for the elevated Ca²⁺ affinity. These multilevel changes in protein dynamics as conformational adaptation allow S100A4 fine-tuning of its protein-protein interactions inside the cell during Ca²⁺ signaling.

MASP-3 is the exclusive pro-factor D activator in resting blood: the lectin and the alternative complement pathways are fundamentally linked

MASP-3 was discovered 15 years ago as the third mannan-binding lectin (MBL)-associated serine protease of the complement lectin pathway. Lacking any verified substrate its role remained ambiguous. MASP-3 was shown to compete with a key lectin pathway enzyme MASP-2 for MBL binding, and was therefore considered to be a negative complement regulator. Later, knock-out mice experiments suggested that MASP-1 and/or MASP-3 play important roles in complement pro-factor D (pro-FD) maturation. However, studies on a MASP-1/MASP-3-deficient human patient produced contradicting results. In normal resting blood unperturbed by ongoing coagulation or complement activation, factor D is present predominantly in its active form, suggesting that resting blood contains at least one pro-FD activating proteinase that is not a direct initiator of coagulation or complement activation. We have recently showed that all three MASPs can activate pro-FD in vitro. In resting blood, however, using our previously evolved MASP-1 and MASP-2 inhibitors we proved that neither MASP-1 nor MASP-2 activates pro-FD. Other plasma proteinases, particularly MASP-3, remained candidates for that function. For this study we evolved a specific MASP-3 inhibitor and unambiguously proved that activated MASP-3 is the exclusive pro-FD activator in resting blood, which demonstrates a fundamental link between the lectin and alternative pathways.

Structural determinants governing S100A4-induced isoform-selective disassembly of nonmuscle myosin II filaments

The Ca(2+) -binding protein S100A4 interacts with the C terminus of nonmuscle myosin IIA (NMIIA) causing filament disassembly, which is correlated with an increased metastatic potential of tumor cells. Despite high sequence similarity of the three NMII isoforms, S100A4 discriminates against binding to NMIIB. We searched for structural determinants of this selectivity. Based on paralog scanning using phage display, we identified a single position as major determinant of isoform selectivity. Reciprocal single amino acid replacements showed that at position 1907 (NMIIA numbering), the NMIIA/NMIIC-specific alanine provides about 60-fold higher affinity than the NMIIB-specific asparagine. The structural background of this can be explained in part by a communication between the two consecutive α-helical binding segments. This communication is completely abolished by the Ala-to-Asn substitution. Mutual swapping of the disordered tailpieces only slightly affects the affinity of the NMII chimeras. Interestingly, we found that the tailpiece and position 1907 act in a nonadditive fashion. Finally, we also found that the higher stability of the C-terminal coiled-coil region of NMIIB also discriminates against interaction with S100A4. Our results clearly show that the isoform-selective binding of S100A4 is determined at multiple levels in the structure of the three NMII isoforms and the corresponding functional elements of NMII act synergistically with one another resulting in a complex interaction network. The experimental and in silico results suggest two divergent evolutionary pathways: NMIIA and NMIIB evolved to possess S100A4-dependent and -independent regulations, respectively.

Various Themes of Myosin Regulation

Members of the myosin superfamily are actin-based molecular motors that are indispensable for cellular homeostasis. The vast functional and structural diversity of myosins accounts for the variety and complexity of the underlying allosteric regulatory mechanisms that determine the activation or inhibition of myosin motor activity and enable precise timing and spatial aspects of myosin function at the cellular level. This review focuses on the molecular basis of posttranslational regulation of eukaryotic myosins from different classes across species by allosteric intrinsic and extrinsic effectors. First, we highlight the impact of heavy and light chain phosphorylation. Second, we outline intramolecular regulatory mechanisms such as autoinhibition and subsequent activation. Third, we discuss diverse extramolecular allosteric mechanisms ranging from actin-linked regulatory mechanisms to myosin:cargo interactions. At last, we briefly outline the allosteric regulation of myosins with synthetic compounds.

KAHA Ligation at Serine

Oxazetidine assisted KAHA ligation was developed by the Bode group as an efficient peptide segment ligation strategy at native serine residues. This milestone achievement should enable the chemical synthesis of difficult-to-prepare proteins for biological studies.

Metastasis-associated S100A4 is a specific amine donor and an activity-independent binding partner of transglutaminase-2

S100A4 and transglutaminase-2 have a role in metastasis. S100A4 is an interaction partner and specific amine substrate of transglutaminase-2, promoting its open conformation and leading to enhanced cell adhesion. Study of their interaction could contribute to the better understanding of metastasis.

Structural Basis of Ribosomal S6 Kinase 1 (RSK1) Inhibition by S100B Protein: MODULATION OF THE EXTRACELLULAR SIGNAL-REGULATED KINASE (ERK) SIGNALING CASCADE IN A CALCIUM-DEPENDENT WAY

Mitogen-activated protein kinases (MAPK) promote MAPK-activated protein kinase activation. In the MAPK pathway responsible for cell growth, ERK2 initiates the first phosphorylation event on RSK1, which is inhibited by Ca(2+)-binding S100 proteins in malignant melanomas. Here, we present a detailed in vitro biochemical and structural characterization of the S100B-RSK1 interaction. The Ca(2+)-dependent binding of S100B to the calcium/calmodulin-dependent protein kinase (CaMK)-type domain of RSK1 is reminiscent of the better known binding of calmodulin to CaMKII. Although S100B-RSK1 and the calmodulin-CAMKII system are clearly distinct functionally, they demonstrate how unrelated intracellular Ca(2+)-binding proteins could influence the activity of the CaMK domain-containing protein kinases. Our crystallographic, small angle x-ray scattering, and NMR analysis revealed that S100B forms a "fuzzy" complex with RSK1 peptide ligands. Based on fast-kinetics experiments, we conclude that the binding involves both conformation selection and induced fit steps. Knowledge of the structural basis of this interaction could facilitate therapeutic targeting of melanomas.

Functional profile of S100A4-deficient T cells

The protein S100A4 is best known for its significant role in promoting motility and invasive capacity of cancer cells. Since S100A4 expression has been reported also in T cells, we analyzed its potential role in T cell motility and inflammation. Using S100a4(+/Gfp) mice, we show here that S100A4 is exclusively expressed by memory T cells of CD4(+) or CD8(+) subpopulations, predominantly of the effector memory T cell subtype. However, the protein was not required for in vitro memory T cell migration toward gradients of the inflammatory chemokine CXCL10. Moreover, T cell memory response was normal in S100A4-deficient mice and lack of S100a4 gene expression did not induce any defect in promoting the development of protective immunity or inflammatory reactions leading to autoimmunity. Taken together, our results demonstrate that S100A4 activity is dispensable for T cell motility/migration and inflammatory potential.

An oxazetidine amino acid for chemical protein synthesis by rapid, serine-forming ligations

Amide-forming ligation reactions allow the chemical synthesis of proteins by the union of unprotected peptide segments, and enable the preparation of protein derivatives not accessible by expression or bioengineering approaches. The native chemical ligation (NCL) of thioesters and N-terminal cysteines is unquestionably the most successful approach, but is not ideal for all synthetic targets. Here we describe the synthesis of an Fmoc-protected oxazetidine amino acid for use in the α-ketoacid-hydroxylamine (KAHA) amide ligation. When incorporated at the N-terminus of a peptide segment, this four-membered cyclic hydroxylamine can be used for rapid serine-forming ligations with peptide α-ketoacids. This ligation operates at low concentration (100 μM-5 mM) and mild temperatures (20-25 °C). The utility of the reaction was demonstrated by the synthesis of S100A4, a 12 kDa calcium-binding protein not easily accessible by NCL or other amide-forming reactions due to its primary sequence and properties.

Myosin II isoform co-assembly and differential regulation in mammalian systems

Non-muscle myosin 2 (NM2) is a major force-producing, actin-based motor in mammalian non-muscle cells, where it plays important roles in a broad range of fundamental biological processes, including cytokinesis, cell migration, and epithelial barrier function. This breadth of function at the tissue and cellular levels suggests extensive diversity and differential regulation of NM2 bipolar filaments, the major, if not sole, functional form of NM2s in vivo. Previous in vitro, cellular and animal studies indicate that some of this diversity is supported by the existence of multiple NM2 isoforms. Moreover, two recent studies have shown that these isoforms can co-assemble to form heterotypic filaments, further expanding functional diversity. In addition to isoform co-assembly, cells may differentially regulate NM2 function via isoform-specific expression, RLC phosphorylation, MHC phosphorylation or regulation via binding partners. Here, we provide a brief summary of NM2 filament assembly, summarize the recent findings regarding NM2 isoform co-assembly, consider the mechanisms cells might utilize to differentially regulate NM2 isoforms, and review the data available to support these mechanisms.

The heavy chain has its day: regulation of myosin-II assembly

Nonmuscle myosin-II is an actin-based motor that converts chemical energy into force and movement, and thus functions as a key regulator of the eukaryotic cytoskeleton. Although it is established that phosphorylation on the regulatory light chain increases the actin-activated MgATPase activity of the motor and promotes myosin-II filament assembly, studies have begun to characterize alternative mechanisms that regulate filament assembly and disassembly. These investigations have revealed that all three nonmuscle myosin-II isoforms are subject to additional regulatory controls, which impact diverse cellular processes. In this review, we discuss current knowledge on mechanisms that regulate the oligomerization state of nonmuscle myosin-II filaments by targeting the myosin heavy chain.

Life without double-headed non-muscle myosin II motor proteins

Non-muscle myosin II motor proteins (myosin IIA, myosin IIB, and myosin IIC) belong to a class of molecular motor proteins that are known to transduce cellular free-energy into biological work more efficiently than man-made combustion engines. Nature has given a single myosin II motor protein for lower eukaryotes and multiple for mammals but none for plants in order to provide impetus for their life. These specialized nanomachines drive cellular activities necessary for embryogenesis, organogenesis, and immunity. However, these multifunctional myosin II motor proteins are believed to go awry due to unknown reasons and contribute for the onset and progression of many autosomal-dominant disorders, cataract, deafness, infertility, cancer, kidney, neuronal, and inflammatory diseases. Many pathogens like HIV, Dengue, hepatitis C, and Lymphoma viruses as well as Salmonella and Mycobacteria are now known to take hostage of these dedicated myosin II motor proteins for their efficient pathogenesis. Even after four decades since their discovery, we still have a limited knowledge of how these motor proteins drive cell migration and cytokinesis. We need to enrich our current knowledge on these fundamental cellular processes and develop novel therapeutic strategies to fix mutated myosin II motor proteins in pathological conditions. This is the time to think how to relieve the hijacked myosins from pathogens in order to provide a renewed impetus for patients' life. Understanding how to steer these molecular motors in proliferating and differentiating stem cells will improve stem cell based-therapeutics development. Given the plethora of cellular activities non-muscle myosin motor proteins are involved in, their importance is apparent for human life.

Structural and thermodynamic characterization of the recognition of the S100-binding peptides TRTK12 and p53 by calmodulin

Calmodulin (CaM) is a multifunctional messenger protein that activates a wide variety of signaling pathways in eukaryotic cells in a calcium-dependent manner. CaM has been proposed to be functionally distinct from the S100 proteins, a related family of eukaryotic calcium-binding proteins. Previously, it was demonstrated that peptides derived from the actin-capping protein, TRTK12, and the tumor-suppressor protein, p53, interact with multiple members of the S100 proteins. To test the specificity of these peptides, they were screened using isothermal titration calorimetry against 16 members of the human S100 protein family, as well as CaM, which served as a negative control. Interestingly, both the TRTK12 and p53 peptides were found to interact with CaM. These interactions were further confirmed by both fluorescence and nuclear magnetic resonance spectroscopies. These peptides have distinct sequences from the known CaM target sequences. The TRTK12 peptide was found to independently interact with both CaM domains and bind with a stoichiometry of 2:1 and dissociations constants Kd,C-term  = 2 ± 1 µM and Kd,N-term  = 14 ± 1 µM. In contrast, the p53 peptide was found to interact only with the C-terminal domain of CaM, Kd,C-term = 2 ± 1 µM, 25°C. Using NMR spectroscopy, the locations of the peptide binding sites were mapped onto the structure of CaM. The binding sites for both peptides were found to overlap with the binding interface for previously identified targets on both domains of CaM. This study demonstrates the plasticity of CaM in target binding and may suggest a possible overlap in target specificity between CaM and the S100 proteins.

The C-terminal random coil region tunes the Ca²⁺-binding affinity of S100A4 through conformational activation

S100A4 interacts with many binding partners upon Ca²⁺ activation and is strongly associated with increased metastasis formation. In order to understand the role of the C-terminal random coil for the protein function we examined how small angle X-ray scattering of the wild-type S100A4 and its C-terminal deletion mutant (residues 1–88, Δ13) changes upon Ca²⁺ binding. We found that the scattering intensity of wild-type S100A4 changes substantially in the 0.15–0.25 Å⁻¹ q-range whereas a similar change is not visible in the C-terminus deleted mutant. Ensemble optimization SAXS modeling indicates that the entire C-terminus is extended when Ca²⁺ is bound. Pulsed field gradient NMR measurements provide further support as the hydrodynamic radius in the wild-type protein increases upon Ca²⁺ binding while the radius of Δ13 mutant does not change. Molecular dynamics simulations provide a rational explanation of the structural transition: the positively charged C-terminal residues associate with the negatively charged residues of the Ca²⁺-free EF-hands and these interactions loosen up considerably upon Ca²⁺-binding. As a consequence the Δ13 mutant has increased Ca²⁺ affinity and is constantly loaded at Ca²⁺ concentration ranges typically present in cells. The activation of the entire C-terminal random coil may play a role in mediating interaction with selected partner proteins of S100A4.

Structure of the S100A4/myosin-IIA complex

Background

S100A4, a member of the S100 family of Ca²⁺-binding proteins, modulates the motility of both non-transformed and cancer cells by regulating the localization and stability of cellular protrusions. Biochemical studies have demonstrated that S100A4 binds to the C-terminal end of the myosin-IIA heavy chain coiled-coil and disassembles myosin-IIA filaments; however, the mechanism by which S100A4 mediates myosin-IIA depolymerization is not well understood.

Results

We determined the X-ray crystal structure of the S100A4Δ8C/MIIA^1908-1923 peptide complex, which showed an asymmetric binding mode for the myosin-IIA peptide across the S100A4 dimer interface. This asymmetric binding mode was confirmed in NMR studies using a spin-labeled myosin-IIA peptide. In addition, our NMR data indicate that S100A4Δ8C binds the MIIA^1908-1923 peptide in an orientation very similar to that observed for wild-type S100A4. Studies of complex formation using a longer, dimeric myosin-IIA construct demonstrated that S100A4 binding dissociates the two myosin-IIA polypeptide chains to form a complex composed of one S100A4 dimer and a single myosin-IIA polypeptide chain. This interaction is mediated, in part, by the instability of the region of the myosin-IIA coiled-coil encompassing the S100A4 binding site.

Conclusion

The structure of the S100A4/MIIA^1908-1923 peptide complex has revealed the overall architecture of this assembly and the detailed atomic interactions that mediate S100A4 binding to the myosin-IIA heavy chain. These structural studies support the idea that residues 1908–1923 of the myosin-IIA heavy chain represent a core sequence for the S100A4/myosin-IIA complex. In addition, biophysical studies suggest that structural fluctuations within the myosin-IIA coiled-coil may facilitate S100A4 docking onto a single myosin-IIA polypeptide chain.

Novel interactions of the TRTK12 peptide with S100 protein family members: specificity and thermodynamic characterization

The S100 protein family consists of small, dimeric proteins that exert their biological functions in response to changing calcium concentrations. S100B is the best-studied member and has been shown to interact with more than 20 binding partners in a calcium-dependent manner. The TRTK12 peptide, derived from the consensus binding sequence for S100B, has previously been found to interact with S100A1 and has been proposed to be a general binding partner of the S100 family. To test this hypothesis and gain a better understanding of the specificity of binding for the S100 proteins, 16 members of the human S100 family were screened against this peptide and its alanine variants. Novel interactions were found with only two family members, S100P and S100A2, indicating that TRTK12 selectively interacts with a small subset of the S100 proteins. Substantial promiscuity was observed in the binding site of S100B thereby accommodating variations in the peptide sequence, while S100A1, S100A2, and S100P exhibited larger differences in the binding constants for the TRTK12 alanine variants. This suggests that single-point substitutions can be used to selectively modulate the affinity of TRTK12 peptides for individual S100 proteins. This study has important implications for the rational drug design of inhibitors for the S100 proteins, which are involved in a variety of cancers and neurodegenerative diseases.

Anti-S100A4 antibody suppresses metastasis formation by blocking stroma cell invasion

The small Ca-binding protein, S100A4, has a well-established metastasis-promoting activity. Moreover, its expression is tightly correlated with poor prognosis in patients with numerous types of cancer. Mechanistically, the extracellular S100A4 drives metastasis by affecting the tumor microenvironment, making it an attractive target for anti-cancer therapy. In this study, we produced a function-blocking anti-S100A4 monoclonal antibody with metastasis-suppressing activity. Antibody treatment significantly reduced metastatic burden in the lungs of experimental animals by blocking the recruitment of T cells to the site of the primary tumor. In vitro studies demonstrated that this antibody efficiently reduced the invasion of T cells in a fibroblast monolayer. Moreover, it was capable of suppressing the invasive growth of human and mouse fibroblasts. We presume therefore that the antibody exerts its activity by suppressing stroma cell recruitment to the site of the growing tumor. Our epitope mapping studies suggested that the antibody recognition site overlaps with the target binding interface of human S100A4. We conclude here that this antibody could serve as a solid basis for development of an efficient anti-metastatic therapy.

Structure of a C-terminal AHNAK peptide in a 1:2:2 complex with S100A10 and an acetylated N-terminal peptide of annexin A2

AHNAK, a large 629 kDa protein, has been implicated in membrane repair, and the annexin A2-S100A10 heterotetramer [(p11)(2)(AnxA2)(2))] has high affinity for several regions of its 1002-amino-acid C-terminal domain. (p11)(2)(AnxA2)(2) is often localized near the plasma membrane, and this C2-symmetric platform is proposed to be involved in the bridging of membrane vesicles and trafficking of proteins to the plasma membrane. All three proteins co-localize at the intracellular face of the plasma membrane in a Ca(2+)-dependent manner. The binding of AHNAK to (p11)(2)(AnxA2)(2) has been studied previously, and a minimal binding motif has been mapped to a 20-amino-acid peptide corresponding to residues 5654-5673 of the AHNAK C-terminal domain. Here, the 2.5 Å resolution crystal structure of this 20-amino-acid peptide of AHNAK bound to the AnxA2-S100A10 heterotetramer (1:2:2 symmetry) is presented, which confirms the asymmetric arrangement first described by Rezvanpour and coworkers and explains why the binding motif has high affinity for (p11)(2)(AnxA2)(2). Binding of AHNAK to the surface of (p11)(2)(AnxA2)(2) is governed by several hydrophobic interactions between side chains of AHNAK and pockets on S100A10. The pockets are large enough to accommodate a variety of hydrophobic side chains, allowing the consensus sequence to be more general. Additionally, the various hydrogen bonds formed between the AHNAK peptide and (p11)(2)(AnxA2)(2) most often involve backbone atoms of AHNAK; as a result, the side chains, particularly those that point away from S100A10/AnxA2 towards the solvent, are largely interchangeable. While the structure-based consensus sequence allows interactions with various stretches of the AHNAK C-terminal domain, comparison with other S100 structures reveals that the sequence has been optimized for binding to S100A10. This model adds new insight to the understanding of the specific interactions that occur in this membrane-repair scaffold.

Thermodynamic and kinetic analysis of peptides derived from CapZ, NDR, p53, HDM2, and HDM4 binding to human S100B

S100B is a member of the S100 subfamily of EF-hand proteins that has been implicated in malignant melanoma and neurodegenerative conditions such as Alzheimer's disease and Parkinson's disease. Calcium-induced conformational changes expose a hydrophobic binding cleft, facilitating interactions with a wide variety of nuclear, cytoplasmic, and extracellular target proteins. Previously, peptides derived from CapZ, p53, NDR, HDM2, and HDM4 have been shown to interact with S100B in a calcium-dependent manner. However, the thermodynamic and kinetic basis of these interactions remains largely unknown. To gain further insight, we screened these peptides against the S100B protein using isothermal titration calorimetry and nuclear magnetic resonance. All peptides were found to have binding affinities in the low micromolar to nanomolar range. Binding-induced changes in the line shapes of S100B backbone (1)H and (15)N resonances were monitored to obtain the dissociation constants and the kinetic binding parameters. The large microscopic K(on) rate constants observed in this study (≥1 × 10(7) M(-1) s(-1)) suggest that S100B utilizes a "fly casting mechanism" in the recognition of these peptide targets.