Signaalsequenties en sorteerreceptoren

Sorting signals are amino acid sequences that guide proteins to their proper location inside the cell. Signal sequences vary in length but are usually 15 to 20 amino acids long, flanking the N-terminal region of a polypeptide chain. 

Some proteins, such as nuclear proteins, can have distant stretches of amino acid residues that come together during protein folding to form three-dimensional arrangements called signal patches.

Sorting signals and signal patches are usually recognized based on the amino acid properties rather than the exact sequence.

Proteins targeted to a particular cell organelle have characteristic features in their signal sequences, such as stretches of hydrophobic residues, positively charged amino acids alternating with hydrophobic residues, and interspersed amino acid residues with hydroxyl-groups.

Signal receptors present on the organelle identify their corresponding signal sequences and transport proteins to the target location.

Once a protein reaches its destination, signal peptidases cleave the signal peptides at their C-terminal signal cleavage site, while the sorting receptors are recycled for catalyzing subsequent rounds of protein sorting.

Modifying the sorting signals can target proteins to a different location. For example, adding the N-terminal signal sequence of ER proteins to cytosolic proteins routes them to the endoplasmic reticulum lumen.