A signal peptide (sometimes referred to as signal sequence,
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. In prokaryotes,
signal peptides direct the newly synthesized protein to the
SecYEG protein-conducting channel, which is present in the
plasma membrane. A homologous system exists in eukaryotes, where
the signal peptide directs the newly synthesized protein to the
Sec61 channel, which sha