What is the difference between a signal peptide and a transit peptide?

Question

From what I know, the two names are used interchangeably and I haven't found any resource which says otherwise either. Is there at all any difference, is there a transit peptide that is not a signal peptide or vice versa?

Answer 1

Signal peptides are typically located at the N terminus of a protein. The signal peptides are processed by the translocon machinery and are cleaved off after sorting through the membranes of organelles in the secretory system:

• endoplasmic reticulum
• Golgi apparatus
• ER-Golgi transition vesicles
• plasma membrane
• lysosomes

Transit peptides target the protein to other subcellular organelles such as (from UniProt):

• Mitochondrion
• Apicoplast
• Chromoplast
• Chloroplast
• Cyanelle
• Thylakoid
• Amyloplast
• Peroxisome
• Glyoxysome
• Hydrogenosome

N-terminal transit peptides are quite rare. C-terminal transit peptide motifs are much more common. UniProt holds transit peptides as a discrete controlled vocabulary, separate from signal peptides.