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 anti-proliferative and anti-tumor activity. The farnesylated hGBP1 (hGBP1fn) retains many characteristics of the extensively studied non-farnesylated protein and gains additional abilities like binding to lipid membranes and formation of hGBP1fn 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 in the changes of size and shape. The GTP hydrolysis driven cycling between a closed, farnesyl moiety hidden state and an opened, farnesyl moiety exposed state represents a 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.