Gated transport

Transmembrane transport

Vesicular transport

Signal peptides are short discrete stretches of amino acid sequence, which are part of a protein, that determine the eventual location of that protein in a cell.

The central dogma of biology dictates that DNA encodes RNA which in turn dictates the synthesis of proteins. Each of the different cellular organelles have their specific complement of proteins, in addition to those which are common. Since the majority of proteins are only manufactured in the cells' cytoplasm (the remainder, representing a tiny fraction, are produced by mitochondrial and chloroplast ribosomes), there must be a mechanism that ensures that these molecules are delivered to the proper locations.

In vivo all proteins are synthesised by specialised ribonucleoprotein complexes known as ribosomes. In both prokaryotic (bacteria) and eukaryotic (animal and plant) cells the ribosomes are either free in the cytosol or bound to a membrane (the plasma membrane in prokaryotes and the endoplasmic reticulum (ER) in eukaryotes). The free, non-membrane bound, ribosomes are indistinguishable from those that are membrane-bound. Indeed, experimentation has shown that any one of the cells complement of ribosomes may switch between protein synthesis on the ER or in the cytosol; the decisive signal being provided by the properties of the first few amino acids of the protein being translated.


Sorting signals are not just confined to the beginning (the N-terminal region) of proteins; they have been discovered at the opposite end (C-terminal region) or somewhere in between, in others. A protein without a recognised sorting signal is directed along the default processing pathway, which destines that protein to remain in the cytosol.


Protein movement is reliant upon three well defined processes.

Gated transport is most notable at the junction between the nucleus and the cytoplasm, where selected macromolecules are actively transported while smaller molecules are allowed free passage.

Transmembrane transport involves the participation of membrane-bound protein translocators which actively allow migration of proteins from the cytoplasm to other cellular cavities. It is believed that in many cases some degree of protein unfolding is required.

Vesicular transport does not require the protein molecules to pass through membranes. Instead it is the membrane that migrates and fuses with other compartments taking the protein along with it, via a process known as pinocytosis. Movement between the endoplasmic reticulum and the Golgi apparatus occurs in this manner.

The mode of transport employed is dependent upon the proteins' final location within the cell. It is possible for a single protein to be subjected to a combination of the above trafficking modes. Each mechanisms is determined by a number of factors, key to which is the signals encoded within the protein itself. These sorting signals are known as either signal peptides or signal patches.


Signal peptides

Signal patches