Kinetics of the association/dissociation cycle of an ATP-binding cassette nucleotide-binding domain.

Most ATP binding cassette (ABC) proteins are pumps that transport substrates across biological membranes using the energy of ATP hydrolysis. Functional ABC proteins have two nucleotide-binding domains (NBDs) that bind and hydrolyze ATP, but the molecular mechanism of nucleotide hydrolysis is unresolved. This is due in part to the limited kinetic information on NBD association and dissociation. Here, we show dimerization of a catalytically active NBD and follow in real time the association and dissociation of NBDs from the changes in fluorescence emission of a tryptophan strategically located at the center of the dimer interface. Spectroscopic and structural studies demonstrated that the tryptophan can be used as dimerization probe, and we showed that under hydrolysis conditions (millimolar MgATP), not only the dimer dissociation rate increases, but also the dimerization rate. Neither dimer formation or dissociation are clearly favored, and the end result is a dynamic equilibrium where the concentrations of monomer and dimer are very similar. We proposed that based on their variable rates of hydrolysis, the rate-limiting step of the hydrolysis cycle may differ among full-length ABC proteins.

Pathogenic Mutations in the C2A Domain of Dysferlin form Amyloid that Activates the Inflammasome.

Limb-Girdle Muscular Dystrophy Type-2B/2R is caused by mutations in the dysferlin gene ( DYSF ). This disease has two known pathogenic missense mutations that occur within dysferlin's C2A domain, namely C2A W52R and C2A V67D . Yet, the etiological rationale to explain the disease linkage for these two mutations is still unclear. In this study, we have presented evidence from biophysical, computational, and immunological experiments which suggest that these missense mutations interfere with dysferlin's ability to repair cells. The failure of C2A W52R and C2A V67D to initiate membrane repair arises from their propensity to form stable amyloid. The misfolding of the C2A domain caused by either mutation exposes ß-strands, which are predicted to nucleate classical amyloid structures. When dysferlin C2A amyloid is formed, it triggers the NLRP3 inflammasome, leading to the secretion of inflammatory cytokines, including IL-1ß. The present study suggests that the muscle dysfunction and inflammation evident in Limb-Girdle Muscular Dystrophy types-2B/2R, specifically in cases involving C2A W52R and C2A V67D , as well as other C2 domain mutations with considerable hydrophobic core involvement, may be attributed to this mechanism.

Structural mapping of post-translational modifications in human interleukin-24: role of N-linked glycosylation and disulfide bonds in secretion and activity.

Human interleukin-24 (IL-24) is unique among the IL-10 superfamily as there is considerable evidence that it possesses multiple anti-cancer properties, including direct tumor cell cytotoxicity, helper T cell (TH1) immune stimulation, and anti-angiogenic activities. The primary sequence of human IL-24 differs from homologous cytokines, because it possesses three consensus N-linked glycosylation sites and the potential for a single disulfide bond. To address the significance of these modifications in human IL-24, we analyzed the relationship between post-translational modifications and the cytokine activity of the human IL-24 protein. In contrast to related interleukins, we identified a relationship between net glycosylation, protein solubility, and cytokine activity. In addition, abrogation of the two cysteine residues by mutagenesis dramatically altered the ability of IL-24 to secrete from host cells and resulted in the concomitant loss of IL-24 activity. We conclude that, unlike other IL-10 family members, human IL-24 must be glycosylated to maintain solubility and bioavailability. Further, a single, unique disulfide bond is required for secretion and activity. These structure-function relationships show that, although IL-24 is a member of the IL-19 subfamily of IL-10-like cytokines by sequence similarity, its surface properties and its distinctive disulfide arrangement make it unique. These observations could explain the novel biological activities measured of this cytokine. Understanding the structural basis of IL-24 activity will be important in the interpretation of the function of this cytokine and in the development of scale-up strategies for biophysical and clinical applications.

The c2 domains of human synaptotagmin 1 have distinct mechanical properties.

Synaptotagmin 1 (Syt1) is the Ca(+2) receptor for fast, synchronous vesicle fusion in neurons. Because membrane fusion is an inherently mechanical, force-driven event, Syt1 must be able to adapt to the energetics of the fusion apparatus. Syt1 contains two C2 domains (C2A and C2B) that are homologous in sequence and three-dimensional in structure; yet, a number of observations have suggested that they have distinct biochemical and biological properties. In this study, we analyzed the mechanical stability of the C2A and C2B domains of human Syt1 using single-molecule atomic force microscopy. We found that stretching the C2AB domains of Syt1 resulted in two distinct unfolding force peaks. The larger force peak of approximately 100 pN was identified as C2B and the second peak of approximately 50 pN as C2A. Furthermore, a significant fraction of C2A domains unfolded through a low force intermediate that was not observed in C2B. We conclude that these domains have different mechanical properties. We hypothesize that a relatively small stretching force may be sufficient to deform the effector-binding regions of the C2A domain and modulate the affinity for soluble N-ethylmaleimide-sensitive factor (NSF) attachment protein receptors (SNAREs), phospholipids, and Ca(+2).

Structural Basis for the Distinct Membrane Binding Activity of the Homologous C2A Domains of Myoferlin and Dysferlin.

Dysferlin has been implicated in acute membrane repair processes, whereas myoferlin's activity is maximal during the myoblast fusion stage of early skeletal muscle cell development. Both proteins are similar in size and domain structure; however, despite the overall similarity, myoferlin's known physiological functions do not overlap with those of dysferlin. Here we present for the first time the X-ray crystal structure of human myoferlin C2A to 1.9 Å resolution bound to two divalent cations, and compare its three-dimensional structure and membrane binding activities to that of dysferlin C2A. We find that while dysferlin C2A binds membranes in a Ca2+-dependent manner, Ca2+ binding was the rate-limiting kinetic step for this interaction. Myoferlin C2A, on the other hand, binds two calcium ions with an affinity 3-fold lower than that of dysferlin C2A; and, surprisingly, myoferlin C2A binds only marginally to phospholipid mixtures with a high fraction of phosphatidylserine.

FerA is a Membrane-Associating Four-Helix Bundle Domain in the Ferlin Family of Membrane-Fusion Proteins.

Ferlin proteins participate in such diverse biological events as vesicle fusion in C. elegans, fusion of myoblast membranes to form myotubes, Ca2+-sensing during exocytosis in the hair cells of the inner ear, and Ca2+-dependent membrane repair in skeletal muscle cells. Ferlins are Ca2+-dependent, phospholipid-binding, multi-C2 domain-containing proteins with a single transmembrane helix that spans a vesicle membrane. The overall domain composition of the ferlins resembles the proteins involved in exocytosis; therefore, it is thought that they participate in membrane fusion at some level. But if ferlins do fuse membranes, then they are distinct from other known fusion proteins. Here we show that the central FerA domain from dysferlin, myoferlin, and otoferlin is a novel four-helix bundle fold with its own Ca2+-dependent phospholipid-binding activity. Small-angle X-ray scattering (SAXS), spectroscopic, and thermodynamic analysis of the dysferlin, myoferlin, and otoferlin FerA domains, in addition to clinically-defined dysferlin FerA mutations, suggests that the FerA domain interacts with the membrane and that this interaction is enhanced by the presence of Ca2+.

Purification, crystallization and X-ray diffraction analysis of human synaptotagmin 1 C2A-C2B.

Synaptotagmin acts as the Ca(2+) sensor for neuronal exocytosis. The cytosolic domain of human synaptotagmin 1 is composed of tandem C2 domains: C2A and C2B. These C2 domains modulate the interaction of synaptotagmin with the phospholipid bilayer of the presynaptic terminus and effector proteins such as the SNARE complex. Human synaptotagmin C2A-C2B has been expressed as a glutathione-S-transferase fusion protein in Escherichia coli. The purification, crystallization and preliminary X-ray analysis of this protein are reported here. The crystals diffract to 2.7 A and belong to the orthorhombic space group P2(1)2(1)2(1), with unit-cell parameters a = 82.37, b = 86.31, c = 140.2 A. From self-rotation function analysis, there are two molecules in the asymmetric unit. The structure determination of the protein using this data is ongoing.

MDA-7/IL-24 is a unique cytokine--tumor suppressor in the IL-10 family.

The melanoma differentiation associated gene-7 (mda-7) cDNA was isolated by virtue of being induced during melanoma differentiation. Initial gene transfer studies convincingly demonstrated potent antitumor effects of mda-7. Further studies showed that the mechanism of antitumor activity was due to induction of apoptosis. Most striking was the tumor-selective killing by mda-7 gene transfer--normal cells were unaffected by Adenoviral delivery of mda-7 (Ad-mda7). A variety of molecules implicated in apoptosis and intracellular signaling are regulated by Ad-mda7 transduction. Different apoptosis effector proteins are regulated in different tumor types, suggesting that Ad-mda7 may regulate various signaling pathways. mda-7 encodes a secreted protein, MDA-7, which has now been designated as IL-24, and is a novel member of the IL-10 cytokine family. MDA-7/IL-24 protein is actively secreted from cells after mda-7 gene transfer. In human peripheral blood mononuclear cells (PBMC), STAT3 activation by MDA-7/IL-24 is followed by elaboration of secondary Th1 cytokines, demonstrating that MDA-7/IL-24 is a pro-Th1 cytokine. Furthermore, MDA-7/IL-24 is antagonized by the prototypic Th2 cytokine IL-10. MDA-7/IL-24 protein is endogenously expressed in cultured NK and B-cells and is also expressed in dendritic cells in tissues. MDA-7/IL-24 protein is expressed in nevi and melanoma primary tumors, to varying degrees, but is rarely expressed in malignant melanoma or other human tumors evaluated. Indeed, loss of MDA-7/IL-24 protein expression correlates strongly with melanoma tumor invasion and disease progression. The "bystander" effects proposed for MDA-7/IL-24 protein include immune stimulation, antiangiogenesis and receptor-mediated cytotoxicity. Thus, mda-7 is a unique multifunctional cytokine in the IL-10 family and may have potent antitumor utility in a clinical setting.

Structure of human synaptotagmin 1 C2AB in the absence of Ca2+ reveals a novel domain association.

Release of neurotransmitter from synaptic vesicles requires the Ca2+/phospholipid-binding protein synaptotagmin 1. There is considerable evidence that cooperation between the tandem C2 domains of synaptotagmin is a requirement of regulated exocytosis; however, high-resolution structural evidence for this interaction has been lacking. The 2.7 A crystal structure of the cytosolic domains of human synaptotagmin 1 in the absence of Ca2+ reveals a novel closed conformation of the protein. The shared interface between C2A and C2B is stabilized by a network of interactions between residues on the C-terminal alpha-helix of the C2B domain and residues on loops 1-3 of the Ca2+-binding region of C2A. These interactions alter the overall shape of the Ca2+-binding pocket of C2A, but not that of C2B. Thus, synaptotagmin 1 C2A-C2B may utilize a novel regulatory mechanism whereby one C2 domain could regulate the other until an appropriate triggering event decouples them.