Ensional models of Zarvin were generated with the MODELLER 9.1 software, using the Z domain (PDB entry 1q2n) and the calcium bound structure of S55D/E59D alpha-Parvalbumin (PDB entry 1s3p) as templates. The C-terminus of the Z domain was linked to the N-terminus of Parvalbumin using a decaglycine peptide (Figure 1A). To inhibitor investigate the stability of the system and to estimate the usage as contrast agent molecular dynamics (MD) simulations were performed with the GROMACS 4.0 software. In total, we simulated the system for 100 ns. A detailed description can be found in the supplementary materials. MD simulations showed that while the secondary and tertiary structure of the Z domain and of Autophagy modified Parvalbumin are conserved, the two domains are free to re-orient independently of each other (Figure S1). Furthermore, the number of water molecules in the first Ca2+ coordination sphere was calculated as test for the suitability 16574785 as contrast agent. The main water coordination number is one, which is comparable with the coordination number of Gd3+ in clinically used chelates (Figure S2). In about one third of the simulation time, the water coordination number increased to twoModular Contrast Agentwater molecules, which should lead to better contrast and is in accordance with the experimentally derived number of water molecules in the first coordination sphere of lanthanide ions in fish Parvalbumin [11,12]. In total, the presence of one or two water molecules in the first coordination shell was observed in .95 of the time simulated by MD. During the MD simulations the two functionally important sites of Zarvin, the antibody binding interface and the Gd3+-binding site, remained accessible. Taken together, the in silico analyses supported that the contrasting and the functional properties of Zarvin are well suited for MRI. After having obtained evidence for the structural integrity of the expressed Zarvin (Figure S3 and Table S1), we proceeded to test the in vitro capability of binding of the Zarvin Z domain to Fc fragments of antibodies. After Recombinantly expressed Zarvin was labelled with Atto-465 at its N-terminus and titrated with the monoclonal anti EGFR-IgG-antibody Cetuximab as a model compound. Changes in fluorescence anisotropy (Figure 1B) revealed a dissociation constant of 470 nM for the complex. To verify the ability of Cetuximab:Zarvin to 23727046 target cancer cells, binding of this complex to EGFR-expressing cells was demonstrated. The binding of the Cetuximab:Zarvin complex to A431 cancer cells was visualized by confocal microscopy. To this end, a D72C mutant of Zarvin was labeled with Atto-594 and incubated with Cetuximab before the Cetuximab:Zarvin-D72C complex was added to A431 cells. Staining was almost exclusively observed at the cellular membrane (Figure 1C). Fluorescence signals at theinside of the membrane visible at higher magnification could result from internalization of the complexes over time [13]. Thus, we demonstrated that coupling of Zarvin to a therapeutically used antibody resulted in efficient binding to target cells in vitro (for the controls see Figure S4). We then tested the capability of the S55D/E59D alphaParvalbumin domain to bind Gd3+ with high affinity as a prerequisite for use as a T1 contrast agent. Gadolinium(III) and Terbium(III) have a similar ion radius [14] and both are `hard’ cations with similar chemical properties according to the HSAB concept. Terbium(III), however, shows detectable luminescence upon bi.Ensional models of Zarvin were generated with the MODELLER 9.1 software, using the Z domain (PDB entry 1q2n) and the calcium bound structure of S55D/E59D alpha-Parvalbumin (PDB entry 1s3p) as templates. The C-terminus of the Z domain was linked to the N-terminus of Parvalbumin using a decaglycine peptide (Figure 1A). To investigate the stability of the system and to estimate the usage as contrast agent molecular dynamics (MD) simulations were performed with the GROMACS 4.0 software. In total, we simulated the system for 100 ns. A detailed description can be found in the supplementary materials. MD simulations showed that while the secondary and tertiary structure of the Z domain and of modified Parvalbumin are conserved, the two domains are free to re-orient independently of each other (Figure S1). Furthermore, the number of water molecules in the first Ca2+ coordination sphere was calculated as test for the suitability 16574785 as contrast agent. The main water coordination number is one, which is comparable with the coordination number of Gd3+ in clinically used chelates (Figure S2). In about one third of the simulation time, the water coordination number increased to twoModular Contrast Agentwater molecules, which should lead to better contrast and is in accordance with the experimentally derived number of water molecules in the first coordination sphere of lanthanide ions in fish Parvalbumin [11,12]. In total, the presence of one or two water molecules in the first coordination shell was observed in .95 of the time simulated by MD. During the MD simulations the two functionally important sites of Zarvin, the antibody binding interface and the Gd3+-binding site, remained accessible. Taken together, the in silico analyses supported that the contrasting and the functional properties of Zarvin are well suited for MRI. After having obtained evidence for the structural integrity of the expressed Zarvin (Figure S3 and Table S1), we proceeded to test the in vitro capability of binding of the Zarvin Z domain to Fc fragments of antibodies. After Recombinantly expressed Zarvin was labelled with Atto-465 at its N-terminus and titrated with the monoclonal anti EGFR-IgG-antibody Cetuximab as a model compound. Changes in fluorescence anisotropy (Figure 1B) revealed a dissociation constant of 470 nM for the complex. To verify the ability of Cetuximab:Zarvin to 23727046 target cancer cells, binding of this complex to EGFR-expressing cells was demonstrated. The binding of the Cetuximab:Zarvin complex to A431 cancer cells was visualized by confocal microscopy. To this end, a D72C mutant of Zarvin was labeled with Atto-594 and incubated with Cetuximab before the Cetuximab:Zarvin-D72C complex was added to A431 cells. Staining was almost exclusively observed at the cellular membrane (Figure 1C). Fluorescence signals at theinside of the membrane visible at higher magnification could result from internalization of the complexes over time [13]. Thus, we demonstrated that coupling of Zarvin to a therapeutically used antibody resulted in efficient binding to target cells in vitro (for the controls see Figure S4). We then tested the capability of the S55D/E59D alphaParvalbumin domain to bind Gd3+ with high affinity as a prerequisite for use as a T1 contrast agent. Gadolinium(III) and Terbium(III) have a similar ion radius [14] and both are `hard’ cations with similar chemical properties according to the HSAB concept. Terbium(III), however, shows detectable luminescence upon bi.