The ideal fluorescent probe for bioimaging is bright, absorbs at long wavelengths and can be implemented flexibly in living cells and in vivo. However, the design of synthetic fluorophores that combine all of these properties has proved to be extremely difficult. Here, we introduce a biocompatible near-infrared silicon-rhodamine probe that can be coupled specifically to proteins using different labelling techniques. Importantly, its high permeability and fluorogenic character permit the imaging of proteins in living cells and tissues, and its brightness and photostability make it ideally suited for live-cell super-resolution microscopy. The excellent spectroscopic properties of the probe combined with its ease of use in live-cell applications make it a powerful new tool for bioimaging.

We introduce far-red, fluorogenic probes that combine minimal cytotoxicity with excellent brightness and photostability for fluorescence imaging of actin and tubulin in living cells. Applied in stimulated emission depletion (STED) microscopy, they reveal the ninefold symmetry of the centrosome and the spatial organization of actin in the axon of cultured rat neurons with a resolution unprecedented for imaging cytoskeletal structures in living cells.

The polypyrimidine tract binding protein (PTB) is a 58-kilodalton RNA binding protein involved in multiple aspects of messenger RNA metabolism, including the repression of alternative exons. We have determined the solution structures of the four RNA binding domains (RBDs) of PTB, each bound to a CUCUCU oligonucleotide. Each RBD binds RNA with a different binding specificity. RBD3 and RBD4 interact, resulting in an antiparallel orientation of their bound RNAs. Thus, PTB will induce RNA looping when bound to two separated pyrimidine tracts within the same RNA. This leads to structural models for how PTB functions as an alternative-splicing repressor.