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ubiquitin Small, highly conserved protein present in all eucaryotic cells that
becomes covalently attached to lysines of other proteins. Attachment of a short
chain of ubiquitins to such a lysine tags a protein for intracellular proteolytic
destruction by a proteasome.
ubiquitin ligase Any one of a large number of enzymes that attach ubiquitin to
a protein, thus marking it for destruction in a proteasome. The process
catalyzed by a ubiquitin ligase is called ubiquitylation.
unfolded protein response Cellular response triggered by an accumulation of
misfolded proteins in the endoplasmic reticulum. It involves increased
transcription of ER chaperones and degradative enzymes.
uniporter Carrier protein that transports a single solute from one side of the
membrane to the other.
unsaturated Describes a molecule that contains one or more double or triple
carbon-carbon bonds, such as isoprene or benzene.