Glycan dependence of Galectin-3 self-association properties

Human Galectin-3 is found in the nucleus, the cytoplasm and at the cell surface. This lectin is constituted of two domains: an unfolded N-terminal domain and a C-terminal Carbohydrate Recognition Domain (CRD). There are still uncertainties about the relationship between the quaternary structure of Galectin-3 and its carbohydrate binding properties. Two types of self-association have been described for this lectin: a C-type self-association and a N-type self-association. Herein, we have analyzed Galectin-3 oligomerization by Dynamic Light Scattering using both the recombinant CRD and the full length lectin. Our results proved that LNnT induces N-type self-association of full length Galectin-3. Moreover, from Nuclear Magnetic Resonance (NMR) and Surface Plasmon Resonance experiments, we observed no significant specificity or affinity variations for carbohydrates related to the presence of the N-terminal domain of Galectin-3. NMR mapping clearly established that the N-terminal domain interacts with the CRD. We propose that LNnT induces a release of the N-terminal domain resulting in the glycan-dependent self-association of Galectin-3 through N-terminal domain interactions.

Structure of eIF3b RNA recognition motif and its interaction with eIF3j: structural insights into the recruitment of eIF3b to the 40 S ribosomal subunit

Mammalian eIF3 is a 700-kDa multiprotein complex essential for initiation of protein synthesis in eukaryotic cells. It consists of 13 subunits (eIF3a to -m), among which eIF3b serves as a major scaffolding protein. Here we report the solution structure of the N-terminal RNA recognition motif of human eIF3b (eIF3b-RRM) determined by NMR spectroscopy. The structure reveals a noncanonical RRM with a negatively charged surface in the beta-sheet area contradictory with potential RNA binding activity. Instead, eIF3j, which is required for stable 40 S ribosome binding of the eIF3 complex, specifically binds to the rear alpha-helices of the eIF3b-RRM, opposite to its beta-sheet surface. Moreover, we identify that an N-terminal 69-amino acid peptide of eIF3j is sufficient for binding to eIF3b-RRM and that this interaction is essential for eIF3b-RRM recruitment to the 40 S ribosomal subunit. Our results provide the first structure of an important subdomain of a core eIF3 subunit and detailed insights into protein-protein interactions between two eIF3 subunits required for stable eIF3 recruitment to the 40 S subunit.

Role of the Tetrahemic Subunit in Desulfovibrio vulgaris Hildenborough Formate Dehydrogenase

In the anaerobic sulfate-reducing bacterium Desulfovibrio vulgaris Hildenborough (DvH), the genome sequencing revealed the presence of three operons encoding formate dehydrogenases. fdh1 encodes an alphabetagamma trimeric enzyme containing 11 heme binding sites; fdh2 corresponds to an alphabetagamma trimeric enzyme with a tetrahemic subunit; fdh3 encodes an alphabeta dimeric enzyme. In the present work, spectroscopic measurements demonstrated that the reduction of cytochrome c(553) was obtained in the presence of the trimeric FDH2 and not with the dimeric FDH3, suggesting that the tetrahemic subunit (FDH2C) is essential for the interaction with this physiological electron transfer partner. To further study the role of the tetrahemic subunit, the fdh2C gene was cloned and expressed in Desulfovibrio desulfuricans G201. The recombinant FDH2C was purified and characterized by optical and NMR spectroscopies. The heme redox potentials measured by electrochemistry were found to be identical in the whole enzyme and in the recombinant subunit, indicating a correct folding of the recombinant protein. The mapping of the interacting site by 2D heteronuclear NMR demonstrated a similar interaction of cytochrome c(553) with the native enzyme and the recombinant subunit. The presence of hemes c in the gamma subunit of formate dehydrogenases is specific of these anaerobic sulfate-reducing bacteria and replaces heme b subunit generally found in the enzymes involved in anaerobic metabolisms.

The cytochrome c3-[Fe]-hydrogenase electron-transfer complex: structural model by NMR restrained docking

Cytochrome c(3) (M(r) 13000) is a low redox potential cytochrome specific of the anaerobic metabolism in sulfate-reducing bacteria. This tetrahemic cytochrome is an intermediate between the [Fe]-hydrogenase and the cytochrome Hmc in Desulfovibrio vulgaris Hildenborough strain. The present work describes the structural model of the cytochrome c(3)-[Fe]-hydrogenase complex obtained by nuclear magnetic resonance restrained docking. This model connects the distal cluster of the [Fe]-hydrogenase to heme 4 of the cytochrome, the same heme found in the interaction with cytochrome Hmc. This result gives evidence that cytochrome c(3) is an electron shuttle between the periplasmic hydrogenase and the Hmc membrane-bound complex.

The crystal structure of the hexadeca-heme cytochrome Hmc and a structural model of its complex with cytochrome c(3)

Sulfate-reducing bacteria contain a variety of multi-heme c-type cytochromes. The cytochrome of highest molecular weight (Hmc) contains 16 heme groups and is part of a transmembrane complex involved in the sulfate respiration pathway. We present the 2.42 A resolution crystal structure of the Desulfovibrio vulgaris Hildenborough cytochrome Hmc and a structural model of the complex with its physiological electron transfer partner, cytochrome c(3), obtained by NMR restrained soft-docking calculations. The Hmc is composed of three domains, which exist independently in different sulfate-reducing species, namely cytochrome c(3), cytochrome c(7), and Hcc. The complex involves the last heme at the C-terminal region of the V-shaped Hmc and heme 4 of cytochrome c(3), and represents an example for specific cytochrome-cytochrome interaction.