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A protein precursor, also called a pro-protein or pro-peptide, is an inactive protein (or peptide) that can be turned into an active form by post-translational modification, such as breaking off a piece of the molecule or adding on another molecule. The name of the precursor for a protein is often prefixed by pro-. Examples include proinsulin and proopiomelanocortin, which are both .
Protein precursors are often used by an organism when the subsequent protein is potentially harmful, but needs to be available on short notice and/or in large quantities. Enzyme precursors are called or . Examples are enzymes of the digestive tract in humans.
Some protein precursors are secreted from the cell. Many of these are synthesized with an N-terminal signal peptide that targets them for secretion. Like other proteins that contain a signal peptide, their name is prefixed by pre. They are thus called pre-pro-proteins or pre-pro-peptides. The signal peptide is cleaved off in the endoplasmic reticulum. An example is preproinsulin.
Pro-sequences are areas in the protein that are essential for its correct Protein folding, usually in the transition of a protein from an inactive to an active state. Pro-sequences may also be involved in pro-protein transport and secretion.
Pro-domain (or prodomain) is the protein domain of a proprotein.
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