A
signal sequence (sometimes referred to as
signal sequence,
targeting signal,
localization signal,
localization sequence,
transit peptide,
leader sequence or
leader peptide) is a short (5-30
amino acids long)
peptide present at the
N-terminus of the majority of newly synthesized
proteins that are destined towards the
secretory pathway. These proteins include those that reside either inside certain organelles (the
endoplasmic reticulum,
golgi or
endosomes), secreted from the cell, or inserted into most cellular membranes. Although most type I
membrane-bound proteins have signal peptides, the majority of type II and multi-spanning membrane-bound proteins are targeted to the secretory pathway by their first
transmembrane domain, which biochemically resembles a signal sequence except that it is not cleaved.