X-ray crystallography is a very powerful technique used to determine the structure of proteins. It involves first obtaining a crystal of your protein and then shooting x-rays through the sample and observing the resulting diffraction pattern.

Equilibrium centrifugation is a separation technique. FPLC is used to purify proteins. LCMS is used to analyze large samples of peptides quickly.

A microarray is a chip with many copies of short segments of DNA that allows you to visualize the expression level of potentially unlimited genes simultaneously. It only informs you on how many copies of the gene are present in a sample, and does not give any information on gene sequence/length, etc.

Light microscopy can magnify a sample up to 1000X. this is not enough to visualize subcellular structures like organelles in cells. Electron microscopy can magnify a sample much higher than 1000X and so can be used to visualize subcellular structures. Both scanning and transmission electron microscopy magnify to this extent.

Scanning electron microscopy runs a beam of electrons over the surface of a specimen. This beam changes in length as the specimen changes in height off the slide. The microscope can detect the change in beam length and generate a 3D image of the specimen. All of the other forms of microscopy listed produce 2D images.

Light microscopy uses light and a glass lens to magnify images. The higher the magnification, the more blurry the image. Special staining techniques are used to produce images via light microscopy. Electrons are not utilized in light microscopy.

Light microscopy does not need a tungsten coil to produce visible light, only a strong but simple light source is required. Electron microscopy does employ a tungsten coil and magnets for this purpose. Each other answer choice describes a difference between the two types of microscopy. 

Fluorescent tags are incredibly powerful tools for observing protein localization. GFP (green fluorescent protein) is a widely used fluorescent tag for just this purpose. All of the other answers are tags used for various functions. His and FLAG tags are commonly used to purify recombinant proteins. TRX tags are used for solubilization and to assist in recombinant protein folding in organisms that are chaperone-deficient. 

FRET is a widely used technique to study protein-protein interactions as well as reaction kinetics. The technique works by attaching fluorophores to proteins that produce fluorescent light when stimulated by light of the appropriate wavelength. When attempting to detect distances between two objects, two fluorophores are used. The first one is stimulated by an external source and the second is stimulated by the light produced from the excited fluorophore. The presence or absence of the light produced by the second excited fluorophore can then help determine protein-protein interactions. The technique is also applicable to protein localization and detecting distances between domains. 

During FRAP experimentation, a section of membrane is labeled with fluorophores. High intensity exposure to a small region of the membrane is used to "bleach" this region, resulting in a small region of zero fluorescence. The lack of recovery of the fluorescence only gives us information about the mobility of the protein: it is immobile. If the protein were mobile, then tagged proteins would be able to move into the bleached region, returning it to fluorescence.

While it may be true that the protein is anchored to the cytoskeleton or part of a lipid raft, the result of the experiment does not directly lead to these conclusions. It does, however, point the researcher in the right direction about learning more about the functionality of his protein. FRAP does not tell us anything about the stability of the protein.

GFP is a fluorescent tag that is incredibly useful in helping visualize a protein in vivo. The tag will glow green in the tissue where the protein is present.

All of the other choices describe tags that help purify recombinant proteins. Because all of these tags isolate proteins from a sample of lysed cell, they would not be useful in following a protein in vivo.