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 comprise a family of seven isoforms (hGBP-1 to hGBP-7) which 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 capability to form nucleotide-dependent oligomers, a feature which 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 is focused on the oligomerization capability of hGBP-1 and hGBP-5 which are reported to assemble to homo dimers and homo tetramers. Employing independent methods like size exclusion chromatography relying on the hydrodynamic radius and multi angle light scattering relying 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- as well as intramolecular Forster 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. This article is protected by copyright. All rights reserved.