What is a signal sequence?
The synthesis of all proteins begins in the cytosol (the nonparticulate part of the cytoplasm) .
However, proteins that will finish their synthesis on the RER have a sequence of amino acids called the signal sequence because this series of amino acids acts as a signal. Signal sequences are specific sequences of amino acids of a newly forming (nascent) polypeptide chain.
Proteins that will be completely synthesized in the cytosol do not have a signal sequence.
In the diagrams below secretory proteins (those that will be secreted from the cell) are synthesized on the RER. Glycolytic enzymes (those involved in glucose breakdown) are completely synthesized in the cytosol.
The nascent seqence of the secretory protein is a signal sequence but the nascent polypeptide of the glycolytic enzyme is not a signal sequence