A protein precursor, also called a pro-protein or pro-peptide, is an inactive protein
) 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
, which are both prohormone
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 zymogen
s or proenzyme
s. 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 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 domain of a proprotein.