Considerations for Membrane Protein Purification

Quantitative determination of fluorescence labeling implemented in cell cultures

BACKGROUND

Labeling efficiency is a crucial parameter in fluorescence applications, especially when studying biomolecular interactions. Current approaches for estimating the yield of fluorescent labeling have critical drawbacks that usually lead them to be inaccurate or not quantitative.

RESULTS

We present a method to quantify fluorescent-labeling efficiency that addresses the critical issues marring existing approaches. The method operates in the same conditions of the target experiments by exploiting a ratiometric evaluation with two fluorophores used in sequential reactions. We show the ability of the protocol to extract reliable quantification for different fluorescent probes, reagents concentrations, and reaction timing and to optimize labeling performance. As paradigm, we consider the labeling of the membrane-receptor TrkA through 4'-phosphopantetheinyl transferase Sfp in living cells, visualizing the results by TIRF microscopy. This investigation allows us to find conditions for demanding single and multi-color single-molecule studies requiring high degrees of labeling.

CONCLUSIONS

The developed method allows the quantitative determination and the optimization of staining efficiency in any labeling strategy based on stable reactions.

Expressing Cloned Genes for Protein Production, Purification, and Analysis

Obtaining high quantities of a specific protein directly from native sources is often challenging, particularly when dealing with human proteins. To overcome this obstacle, many researchers take advantage of heterologous expression systems by cloning genes into artificial vectors designed to operate within easily cultured cells, such as Escherichia coli, Pichia pastoris (yeast), and several varieties of insect and mammalian cells. Heterologous expression systems also allow for easy modification of the protein to optimize expression, mutational analysis of specific sites within the protein and facilitate their purification with engineered affinity tags. Some degree of purification of the target protein is usually required for functional analysis. Purification to near homogeneity is essential for characterization of protein structure by X-ray crystallography or nuclear magnetic resonance (NMR) and characterization of the biochemical and biophysical properties of a protein, because contaminating proteins almost always adversely affect the results. Methods for producing and purifying proteins in several different expression platforms and using a variety of vectors are introduced here.

Preparation of Cell Extracts for Purification of Proteins Expressed in Pichia pastoris

Recovery of intracellular proteins requires disruption of the host cell before the target protein is extracted and isolated. Disruption methods vary depending on the type of cells, the total volume, and the number of samples being processed. For cells enveloped in cell walls (such as yeast), mild techniques such as hypotonic shock are not sufficient to achieve adequate lysis. More vigorous methods are often required. Although the preferred medium- or large-scale method of breaking yeast cells is mechanical shearing, lysis with the aid of glass beads in a BeadBeater is described here.

Expression of Cloned Genes in Pichia pastoris Using the Methanol-Inducible Promoter AOX1

Pichia pastoris is a methylotrophic yeast capable of metabolizing methanol as its sole carbon source. Growth in methanol-containing medium results in dramatic induction of genes in the alcohol oxidation pathway including alcohol oxidase (AOX), formaldehyde dehydrogenase (FLD), and dihydroxyacetone synthase (DHAS). These proteins may comprise up to 30% of the biomass. Investigators have exploited these methanol-dependent genes to generate tightly regulated expression vectors. Most Pichia vectors use the strong and tightly regulated AOX1 promoter to drive heterologous protein expression. Obtaining integrated Pichia transformants requires more DNA than transformations into Saccharomyces cerevisiae, where the gene is expressed from episomal plasmids; however, transformants are extremely stable and can be stored for many years.