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Ubiquitin

Ubiquitin is a protein, or rather, a polypeptide, that occurs in most eukaryotes. Its main function is to mark other proteins for proteolysis. It can also mark transmembrane proteins (for example, receptors) for removal from the membrane. The marking of the protein is done by the ubiquitin binding to a lysine residue in the target protein.

The process of marking a protein with ubiquitin consists of a series of steps:

1. Activation of ubiquitin -- the carboxyl group of the terminal glycine of ubiquitin binds to the SH group of an ubiquitin-activating enzyme. This step requires ATP as an energy source and results in a thioester bond between ubiquitin and the enzyme E1.
2. Transfer of ubiquitin from E1 to the ubiquitin-conjugating enzyme E2 via proteasome into small peptides, amino acids, and ubiquitin (which can be reused).

The gene whose omission causes Angelman syndrome has something to do with this process.

Immunohistochemistry

Antibodies to ubiquitin are used in histology to identify abnormal accumulations of protein inside cells that are markers of disease. These are called inclusion bodies. Examples of such abnormal inclusions in cells are

1. Neurofibrillary tangles in Alzheimer's disease
2. Lewy body in Parkinson's disease
3. Pick bodies in Picks disease
4. Inclusions in motor neuron disease
5. Mallory's Hyalin in alcoholic liver disease
6. Rosenthal fibres in astrocytes