Protein Translocation across Membranes

Abstract

Cells are divided into compartments that serve to separate and coordinate metabolic processes. In eukaryotic cells, almost all enzymes, along with proteins involved in their maintenance, are encoded by genes in the nucleus. Proteins are translated in the cytoplasm, and must be targeted to the correct compartment within the cell in order to faithfully carry out their function. To achieve this, proteins are synthesized with signal sequences that guide them to receptor proteins that in turn transfer them to large transmembrane protein channels of organelles. The molecular machinery for protein translocation has evolved to capture and recognize proteins with the correct targeting signals encoded in their amino acid sequence. These diverse classes of translocases pass substrate proteins across, or into, membranes cotranslationally or posttranslationally. Protein substrates are moved by active pushing, pulling, electrophoretic and Brownian ratchet mechanisms. Most translocases operate on unfolded polypeptides, whereas some can transport partially or fully folded proteins.

Key Concepts:

  • Cells contain sophisticated molecular machines to recognize and translocate proteins across membranes and into their correct organelles.

  • Proteins are targeted to subcellular locations via targeting signals encoded in their amino acid sequence. These usually exist as linear sequences in the N‐ or C‐terminal region of the protein; however, some targeting signals occur within internal segments of the substrate polypeptide. Not all targeting signals are well defined.

  • Receptors associated with the translocase and in the cytosol specifically recognize targeting signals.

  • Translocases contain pores through which the substrate proteins can pass. Most translocase pores are only large enough to accommodate unfolded proteins, although some appear to be able to accommodate fully folded polypeptides.

  • Molecular chaperones maintain substrates in an unfolded state before translocation and assist their correct folding after translocation.

  • The force required to translocate proteins across membranes can come from several sources; the electrophoretic potential or pH gradient across the membrane, an ATP‐ or GTP‐driven pushing or pulling mechanism and an ATP‐driven Brownian ratchet mechanism.

Keywords: organelles; protein import; protein export; receptors; targeting; targeting signal

Figure 1.

Multiple preprotein translocation pathways are present in a cell. Most proteins (blue) are synthesized in the cytosol and many of these are directed into organelles.

Figure 2.

(a) Export and (b) import targeting signals of preproteins destined for different translocation pathways. Blue represents hydrophilic regions and red hydrophobic. α Helical regions are denoted by curling lines whereas β turns are by zigzags. Regions enriched in hydroxylated (OH) and positively charged (+) residues are also indicated. The thick black line represents the remainder of the protein. (c) The β‐signal found at the C‐terminus of β‐barrel membrane proteins in the outer membrane of bacteria, mitochondria and possibly chloroplasts. The exact sequence determinants of the β‐signal are yet to be defined, the sequence shown indicates most conserved residues in the bacterial β‐signal.

Figure 3.

Different processes for the translocation of proteins across eukaryotic membranes. The preprotein (blue) containing an export (magenta) or import (blue) signal is shown initially interacting with ribosomes (light yellow) or cytosolic factors (light yellow and light green), before binding to receptors (green) followed by translocation through membrane channels (light blue). Translocation motors (bright yellow) associated with anchoring proteins (red) are also indicated. Unidentified channels are left uncoloured. (a) Preprotein import into mitochondria involves movement of the positively charged presequence along a receptor‐mediated ‘acid‐chain’. Some preproteins, including those with internal targeting signals, involve binding to cytosolic factors such as the chaperone MSF and the receptor Tom70. Translocation of the preprotein across the outer (Tom40) and inner (Tim23/17) channels is facilitated by a membrane potential Δψ, along with the PAM complex with Tim44‐bound HSP70 to provide a driving force. Inner membrane proteins are inserted by the TIM22 complex and outer membrane β‐barrels are inserted by the SAM complex, from the intermembrane space side (b) translocation of endoplasmic reticulum preproteins follows a (I) posttranslocational or (II) cotranslational pathway. Posttranslational translocation utilizes both the Sec61 and Sec63 complexes along with BiP as the driving force, whereas for cotranslational translocation the extending polypeptide chain is enough to move it through the channel. (c) The translocation of preproteins into peroxisomes is the least characterized of the pathways. The interaction of preprotein‐bound cytosolic factors with membrane receptors may initiate translocation of the preprotein via a channel, endocytosis or by other means. The cytosolic factors may even shuttle across the peroxisomal membranes delivering their cargo into the matrix. (d) Chloroplast‐destined preproteins may utilize a number of receptors for their targeting before translocation through the outer envelope channel created by Toc75. The preprotein may be driven through the outer envelope by the Toc34 and Toc159GTPases and inner envelope channels via the action of molecular chaperones found at the translocation sites (HSP70, HSP100 and HSP60). The export of stromal preproteins into thylakoids follows pathways that are homologous to bacterial pathways.

Figure 4.

The translocation of Gram‐negative bacterial preproteins is mediated by a (a) Sec pathway or (b) SRP pathway which converge at the SecYE translocation channel. The (c) TAT pathway is an alternative pathway across the inner membrane for some proteins. Colours indicate functions of subunits as described in Figure .

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Further Reading

Agne B and Kessler F (2009) Protein transport in organelles: the Toc complex way of preprotein import. FEBS Journal 276: 1156–1165.

Bukau B (ed.) (1999) Molecular Chaperones and Folding Catalysts: Regulation, Cellular Function and Mechanism. Amsterdam: Harwood.

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Rapoport TA (2007) Protein translocation across the eukaryotic endoplasmic reticulum and bacterial plasma membranes. Nature 450: 663–669.

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Perry, Andrew J(Apr 2010) Protein Translocation across Membranes. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1002/9780470015902.a0000632.pub2]