Catalytic activity of human guanylate-binding protein 1 coupled to the release of structural restraints imposed by the C-terminal domain

Human guanylate-binding protein 1 (hGBP-1) shows a dimer-induced acceleration of the GTPase activity yielding GDP as well as GMP. While the head-to-head dimerization of the large GTPase (LG) domain is well understood, the role of the rest of the protein, particularly of the GTPase effector domain (GED), in dimerization and GTP hydrolysis is still obscure. In this study, with truncations and point mutations on hGBP-1 and by means of biochemical and biophysical methods, we demonstrate that the intramolecular communication between the LG domain and the GED (LG:GED) is crucial for protein dimerization and dimer-stimulated GTP hydrolysis. In the course of GTP binding and γ-phosphate cleavage, conformational changes within hGBP-1 are controlled by a chain of amino acids ranging from the region near the nucleotide-binding pocket to the distant LG:GED interface and lead to the release of the GED from the LG domain. This opening of the structure allows the protein to form GED:GED contacts within the dimer, in addition to the established LG:LG interface. After releasing the cleaved γ-phosphate, the dimer either dissociates yielding GDP as the final product or it stays dimeric to further cleave the β-phosphate yielding GMP. The second phosphate cleavage step, that is, the formation of GMP, is even more strongly coupled to structural changes and thus more sensitive to structural restraints imposed by the GED. Altogether, we depict a comprehensive mechanism of GTP hydrolysis catalyzed by hGBP-1, which provides a detailed molecular understanding of the enzymatic activity connected to large structural rearrangements of the protein. DATABASE: Structural data are available in RCSB Protein Data Bank under the accession numbers: 1F5N, 1DG3, 2B92.

The Molecular Mechanism of Polymer Formation of Farnesylated Human Guanylate-binding Protein 1

The human guanylate-binding protein 1 (hGBP1) belongs to the dynamin superfamily proteins and represents a key player in the innate immune response. Farnesylation at the C-terminus is required for hGBP1's activity against microbial pathogens, as well as for its antiproliferative and antitumor activity. The farnesylated hGBP1 (hGBP1_fn) retains many characteristics of the extensively studied nonfarnesylated protein and gains additional abilities like binding to lipid membranes and formation of hGBP1_fn polymers. These polymers are believed to serve as a protein depot, making the enzyme immediately available to fight the invasion of intracellular pathogens. Here we study the molecular mechanism of hGBP1 polymer formation as it is a crucial state of this enzyme, allowing for a rapid response demanded by the biological function. We employ Förster resonance energy transfer in order to trace intra and intermolecular distance changes of protein domains. Light scattering techniques yield deep insights into the changes in size and shape. The GTP hydrolysis driven cycling between a closed, farnesyl moiety hidden state and an opened, farnesyl moiety exposed state represents the first phase, preparing the molecule for polymerization. Within the second phase of polymer growth, opened hGBP1 molecules can be incorporated in the growing polymer where the opened structure is stabilized, similar to a surfactant molecule in a micelle, pointing the farnesyl moieties into the hydrophobic center and positioning the head groups at the periphery of the polymer. We contribute the molecular mechanism of polymer formation, paving the ground for a detailed understanding of hGBP1 function.

The human guanylate-binding proteins hGBP-1 and hGBP-5 cycle between monomers and dimers only

Belonging to the dynamin superfamily of large GTPases, human guanylate-binding proteins (hGBPs) comprise a family of seven isoforms (hGBP-1 to hGBP-7) that are strongly upregulated in response to interferon-γ and other cytokines. Accordingly, several hGBPs are found to exhibit various cellular functions encompassing inhibitory effects on cell proliferation, tumor suppression as well as antiviral and antibacterial activity; however, their mechanism of action is only poorly understood. Often, cellular functions of dynamin-related proteins are closely linked to their ability to form nucleotide-dependent oligomers, a feature that also applies to hGBP-1 and hGBP-5. hGBPs are described as monomers, dimers, tetramers, and higher oligomeric species, the function of which is not clearly established. Therefore, this work focused on the oligomerization capability of hGBP-1 and hGBP-5, which are reported to assemble to homodimers and homotetramers. Employing independent methods such as size-exclusion chromatography, which relies on the hydrodynamic radius, and multiangle light scattering, which relies on the mass of the protein, revealed that previous interpretations regarding the size of the proteins and their complexes have to be revised. Additional studies using inter- and intramolecular Förster resonance energy transfer demonstrated that nucleotide-triggered intramolecular structural changes lead to a more extended shape of hGBP-1 being responsible for the appearance of larger oligomeric species. Thus, previously reported tetrameric and dimeric species of hGBP-1 and hGBP-5 were unmasked as dimers and monomers, respectively, with their shapes depending on both the bound nucleotide and the ionic strength of the solution.