Fig. 1. Small proteins as genetically encoded contrast agents for non-invasive imaging.

(a) The tetrameric enzyme beta-galactosidase cleaves the galactopyranosyl ring on the synthetic Gd³⁺ chelator EgadMe, leading to increased water binding and T₁-weighted MRI contrast. (b) The heme-binding domain of P450-BM3 was evolved to selectively bind the neurotransmitter dopamine to alter water access to the paramagnetic Fe³⁺, yielding a molecular sensor of dopamine for T₁-weighted MRI. (c) Designed lysine repeat proteins (LRPs) rapidly exchange amide protons with water, thus yielding enhanced contrast in chemical exchange saturation transfer (CEST) MRI. (d) Reporter gene for diffusion-weighted MRI based on increased water diffusion across the cell membrane after overexpression of Aquaporin 1 (AQP1) (e) Hemodynamic contrast mechanism based on local expression and release of vasoactive peptides lead to increased blood flow detectable with fMRI or other imaging techniques sensitive to hemodynamics. (f) Bacterial phytochrome-derived infrared fluorescent proteins (iFPs) can serve as contrast agents for optoacoustic imaging. When absorbing near-infrared laser pulses, the chromophores transform photons into pressure waves detectable with ultrasound. PDB structures 3J7H (β-galactosidase), 4DU2 (BM3h-B7) and 4CQH (iFP 2.0) were visualized using ChimeraX³³. Adapted from ref.²⁶, Springer Nature Ltd. (e).