How Protein Trafficking

8/20/2019 How Protein Trafficking

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Protein Trafcking or Transport

s play a central part in the compartmentalization of a eucaryotic cell.atalyze the reactions that occur in each organelle and selectively transport smules into and out of its interior, or lumen.

s also serve as organelle-specifc Surace markers that direct new deliveries ons and lipids to the appropriate organelle.

malian cell contains about 10 billion protein molecules of perhaps 10,000 kindnthesis of almost all of them begins in the cytosol

ewly synthesized protein is then delivered speci!cally to the cell compartment"uires it

cing the protein tra$c from one compartment to another, it is easier to compreb%ect better


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roteins Can Move Between Compartments in Dierent Ways

oteins begin being synthesized on ribosomes in the cytosol, e&cept for the fewynthesized on the ribosomes of mitochondria and plastids.

subse"uent fate depends on their amino acid se"uence, which can contain sor

ls that direct their delivery to locations outside the cytosol.

proteins do not have a sorting signal and conse"uently remain in the cytosol aanent residents.

others, however, have speci!c sorting signals that direct their transport from t

ol into the nucleus, the (, mitochondria, plastids )in plants*, or pero&isomes+

ng signals can also direct the transport of proteins from the ( to other destina cell


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ndably, there are T( ways by which proteins move from one compartmentr.

otein tra$c between the cytosol and nucleus occurs between topologicallyt spaces, which are in continuity through the nuclear pore comple&es. This progated transport because the nuclear pore comple&es function as selective gateactively transport speci!c macromolecules and macromolecular assemblies,they also allow free di/usion of smaller molecules.

nsmembrane transport membrane2bound protein translocators directly transporoteins across a membrane rom the cytosol into a space that is topologically dported protein molecule usually must unfold in order to snake through thee. The initial transport of selected proteins from the cytosol into the ( lumenchondria, for e&ample, occurs in this way.icular transport, transport vesicles erry proteins rom one compartment to anoles become loaded with a cargo o molecules derived from the lumen of oneent as they pinch o/ from its membrane+ they discharge their cargo into ampartment by fusing with its membrane. The transfer of soluble proteins fromthe 4olgi apparatus occurs in this way.

the transported proteins do not cross a membrane, they move only betweenents that are topologically e"uivalent


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impli!ed 7road map7 of protein tra$c. Proteins can move from one compartment to another by gateds2membrane transport ) blue), or vesicular transport ( green). The signals that direct a given proteins

system, and thereby determine its eventual location in the cell, are contained in its amino acid se"uy begins with the synthesis of a protein on a ribosome and terminates when the !nal destination is reermediate station ) bo!es) a decision is made as to whether the protein is to be retained or transport, a signal could be re"uired either for retention in or for e&it from each of the compartments shown, w fate being the deault pathway (one that re"uires no signal*. The vesicular transport of proteins from

4olgi apparatus to the cell surface appears not to re"uire any speci!c sorting signals+ speci!c sortinre re"uired to retain in the ( and the 4olgi apparatus those specialized proteins that are resident th


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al Peptides and Signal Patches Direct Proteins to the Correctddress

here are at least two types of sorting signals on proteins. 8ne type resides in a continuous stretch of amino acid se"uenpically 19 to :0 residues long. This signal peptide is often )but not always* removed from the !nished protein by a specgnal peptidase once the sorting process has been completed. The other type consists of a speci!c three2dimensional ar

f atoms on the protein;s surface that forms when the protein folds up. The amino acid residues that comprise this signale distant from one another in the linear amino acid se"uence, and they generally remain in the !nished protein ) 5igureignal peptides are used to direct proteins from the cytosol into the (, mitochondria, chloroplasts, pero&isomes, and nuey are also used to retain soluble proteins in the (. ignal patches identify certain enzymes that are to be marked witgar residues that then direct them from the 4olgi apparatus into lysosomes+ signal patches are also used in other sortiat have been less well characterized.


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=i/erent types of signal peptides are used to specify di/erent destinations in the cell. Proteins destined for initial transfer to theusually have a signal peptide at their amino terminus, which characteristically includes a se"uence composed of about 9 to 10hydrophobic amino acids. 'ost of these proteins will in turn pass from the ( to the 4olgi apparatus, but those with a speci!cse"uence of four amino acids at their carbo&yl terminus are retained as permanent ( residents. Proteins destined for mitochon

have signal peptides of yet another type, in which positively charged amino acids alternate with hydrophobic ones. Proteins destfor pero&isomes usually have a speci!c signal se"uence of three amino acids at their carbo&yl terminus. 'any proteins destinedthe nucleus carry a signal peptide formed from a cluster of positively charged amino acids, which is commonly found at internalof the polypeptide chain. ome typical signal peptides are listed in Table 123.

The importance of each of these signal peptides for protein targeting has been shown by e&periments in which the peptide istransferred from one protein to another by genetic engineering techni"ues6 placing the amino2terminal ( signal peptide at thebeginning of a cytosolic protein, for e&ample, redirects the protein to the (. ven though their amino acid se"uences can varygreatly, the signal peptides of all proteins having the same destination are functionally interchangeable6 physical properties, suchydrophobicity, often appear to be more important in the signal2recognition process than the e&act amino acid se"uence.

ignal patches are far more di$cult to analyze than signal peptides, and so less is known about their structure. #ecause they refrom a comple& three2dimensional protein2folding pattern, they cannot be easily transferred e&perimentally from one protein toanother.

The main ways of studying how proteins are directed from the cytosol to a speci!c compartment and how they are translocatedacross membranes are illustrated in Panel 121 )pp. 99>*.


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ure 12


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How Protein Trafficking