Fluorophores for live cell imaging of AGT fusion proteins across the visible spectrum.

O6-alkylguanine-DNA alkyltransferase (AGT) fusion proteins can be specifically and covalently labeled with fluorescent O6-benzylguanine (O6-BG) derivatives for multicolor live cell imaging approaches. Here, we characterize several new BG fluorophores suitable for in vivo AGT labeling that display fluorescence emission maxima covering the visible spectrum from 472 to 673 nm, thereby extending the spectral limits set by fluorescent proteins. We show that the photostability of the cell-permeable dyes BG Rhodamine Green (BG505) and CP tetramethylrhodamine (CP-TMR) is in the range of enhanced green fluorescent protein (EGFP) and monomeric red fluorescent protein (mRFP), and that BG diethylaminomethyl coumarin (BGDEAC), a derivative of coumarin, is even more stable than enhanced cyan fluorescent protein (ECFP).

Labeling of fusion proteins with synthetic fluorophores in live cells.

A general approach for the sequential labeling of fusion proteins of O(6)-alkylguanine-DNA alkyltransferase (AGT) with different fluorophores in mammalian cells is presented. AGT fusion proteins with different localizations in the cell can be labeled specifically with different fluorophores, and the fluorescence labeling can be used for applications such as multicolor analysis of dynamic processes and fluorescence resonance energy transfer measurements. The facile access to a variety of different AGT substrates as well as the specificity of the labeling reaction should make the approach an important tool to study protein function in live cells.

Examining reactivity and specificity of cytochrome c peroxidase by using combinatorial mutagenesis.

Combinatorial mutagenesis was used to investigate the role of three key residues in cytochrome c peroxidase (CCP) from Saccharomyces cerevisiae, Arg48, Trp51, and Trp191, in control of the reactivity and selectivity of the heme-containing enzyme. Libraries were prepared by randomization of these residues and were subsequently screened for activity against the phenolic substrate guaiacol. Screening conditions were employed that favor either mutants with high activity or those with both high activity and stability of the reactive enzyme intermediates.