Allele-specific endogenous tagging and quantitative analysis of ß-catenin in colorectal cancer cells.

Wnt signaling plays important roles in development, homeostasis, and tumorigenesis. Mutations in ß-catenin that activate Wnt signaling have been found in colorectal and hepatocellular carcinomas. However, the dynamics of wild-type and mutant forms of ß-catenin are not fully understood. Here, we genome-engineered fluorescently tagged alleles of endogenous ß-catenin in a colorectal cancer cell line. Wild-type and oncogenic mutant alleles were tagged with different fluorescent proteins, enabling the analysis of both variants in the same cell. We analyzed the properties of both ß-catenin alleles using immunoprecipitation, immunofluorescence, and fluorescence correlation spectroscopy approaches, revealing distinctly different biophysical properties. In addition, activation of Wnt signaling by treatment with a GSK3ß inhibitor or a truncating APC mutation modulated the wild-type allele to mimic the properties of the mutant ß-catenin allele. The one-step tagging strategy demonstrates how genome engineering can be employed for the parallel functional analysis of different genetic variants.

Superresolution and pulse-chase imaging reveal the role of vesicle transport in polar growth of fungal cells.

Polarized growth of filamentous fungi requires continuous transport of biomolecules to the hyphal tip. To this end, construction materials are packaged in vesicles and transported by motor proteins along microtubules and actin filaments. We have studied these processes with quantitative superresolution localization microscopy of live Aspergillus nidulans cells expressing the photoconvertible protein mEosFPthermo fused to the chitin synthase ChsB. ChsB is mainly located at the Spitzenkörper near the hyphal tip and produces chitin, a key component of the cell wall. We have visualized the pulsatory dynamics of the Spitzenkörper, reflecting vesicle accumulation before exocytosis and their subsequent fusion with the apical plasma membrane. Furthermore, high-speed pulse-chase imaging after photoconversion of mEosFPthermo in a tightly focused spot revealed that ChsB is transported with two different speeds from the cell body to the hyphal tip and vice versa. Comparative analysis using motor protein deletion mutants allowed us to assign the fast movements (7 to 10 µm s-1) to transport of secretory vesicles by kinesin-1, and the slower ones (2 to 7 µm s-1) to transport by kinesin-3 on early endosomes. Our results show how motor proteins ensure the supply of vesicles to the hyphal tip, where temporally regulated exocytosis results in stepwise tip extension.

A far-red emitting fluorescent marker protein, mGarnet2, for microscopy and STED nanoscopy.

Here we present mGarnet2, a monomeric, far-red fluorescent marker protein derived from mRuby, with absorption and emission bands peaking at 598 and 671 nm, respectively. The protein shows excellent performance as a live-cell fusion marker for STED nanoscopy with 640 nm excitation and 780 nm depletion wavelengths.