History of the Signal Hypothesis

Abstract

The signal hypothesis, which describes how secretory and membrane proteins are targeted to the endoplasmic reticulum, was proposed in 1971 by Günter Blobel and David Sabatini and demonstrated by Blobel and Bernhard Dobberstein in 1975. Subsequent research identified the key components of the membrane insertion and translocation machinery, including the signal recognition particle (SRP), the SRP receptor and the protein‐conducting channel. Ultimately, the signal hypothesis was shown to be true for not only eukaryotes but also prokaryotes. Most importantly, the concept of signal‐mediated targeting was expanded into a general hypothesis of cellular topogenesis that helps to explain how proteins are distributed to their correct locations within the cell following their synthesis in the cytoplasm.

Key Concepts:

  • Modern cell biology was developed in the post‐World War II period.

  • Cell biological research in the 30 years after World War II was characterised by a multidisciplinary approach that combined electron microscopy, cell fractionation, biochemical analysis and radioactive pulse‐labelling of proteins.

  • Formulation of the signal hypothesis was an outgrowth of protein synthesis and secretion research carried out at the Rockefeller University beginning in the late 1940s.

  • The signal hypothesis was originally proposed by Günter Blobel and David Sabatini in 1971, and demonstrated by Blobel and colleagues in 1975.

  • The signal hypothesis showed that cytoplasmically synthesised proteins targeted to the ER use a signal sequence to direct them to the ER membrane.

  • A signal sequence is a short peptide that is part of the original translation product of proteins targeted to the ER, often at the N‐terminus of the protein, and is frequently removed proteolytically after translocation of the nascent polypeptide through the ER membrane.

  • The mechanism of protein translocation was established through the isolation and characterisation of SRP, the SRP‐receptor and the protein‐conducting channel.

  • The signal hypothesis was found to be applicable to protein translocation in all eukaryotes and prokaryotes.

  • Although the original signal hypothesis proposed translocation of proteins during the elongation of the polypeptide chain (cotranslational), translocation was found to also occur in some instances after protein synthesis was complete (post‐translational), primarily through studies of bacteria and yeast.

  • The concept of signal‐mediated targeting of proteins was expanded into a generalised hypothesis of protein topogenesis by Günter Blobel in 1980.

Keywords: signal sequence; translocation; targeting; sorting; topogenesis; ribosomes; protein synthesis; cotranslational; post‐translational; membrane insertion; precursor; signal recognition particle (SRP); sec61; Günter Blobel; George Emil Palade

Figure 1.

Models of the signal hypothesis. (a) The original model of the signal hypothesis proposed by Günter Blobel and David Sabatini in 1971 with an×indicating the N‐terminal extension (later called the signal sequence). Reproduced with the permission of Günter Blobel. © Günter Blobel. (b) The revised model published by Blobel and Bernhard Dobberstein in 1975. The model now shows proteolytic removal of the signal sequence and also includes a hypothetical protein‐conducting channel. Reproduced with permission from Blobel and Dobberstein . © Rockefeller University Press. (c) A modern version of the signal hypothesis including SRP, the SRP receptor and the protein‐conducting channel (Sec61/SecY complex). For clarity, the mRNA is not shown. Reprinted from Rapoport by permission of MacMillan Publishing Ltd. © Nature Publishing Group.

Figure 2.

Precursor processing and protection by the in‐vitro assay system used to demonstrate the signal hypothesis. Figure 6 from the second of Blobel and Dobberstein's 1975 papers. Polypeptides were synthesised from added mRNA and radioactive amino acids in the presence or absence of microsomal membranes. To determine if the newly made polypeptides had been translocated into the microsomal vesicles, some samples were treated with proteolytic enzymes to degrade polypeptides outside of the vesicles. Polypeptides were analysed by SDS gel electrophoresis and autoradiography. (a) Synthesis without microsomal membranes: in the left‐hand lane, a polypeptide marked with a downward pointing arrow is a precursor of immunoglobulin light chain. In the sample with protease (right‐hand lane), the polypeptide is completely degraded. (b) When microsomal membranes are present, the precursor is processed to a lower molecular weight form (left‐hand lane, upward pointing arrow). In the sample with protease (right‐hand lane), only the processed form is still present, indicating that it has been translocated into the interior of the microsomal vesicle and is protected from destruction by the protease. (c) In these samples mRNA for the cytoplasmic protein globin has been translated (asterisk, left‐hand lane) in the presence of microsomal membranes. Addition of protease (right‐hand lane) indicates that it is not protected and therefore has not been translocated into the microsomal vesicles. Reproduced with permission from Blobel and Dobberstein . © Rockefeller University Press. Labelling in (a), (b) and (c) by the author. Upward‐ and downward‐pointing arrows are in the original publication.

Figure 3.

The hypothetical protein‐conducting channel. The original speculative model of the protein‐conducting channel created by Blobel and Dobberstein in 1975. In (a) subunits of the channel are proposed to ‘float’ in the lipid bilayer until they are assembled by the ribosome/nascent polypeptide complex (b). Though the exact details of this model were not correct, the prescience of Blobel's proposal was remarkable. Reproduced with permission from Blobel and Dobberstein © Rockefeller University Press.

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Further Reading

Blobel G (2000) Protein targeting (Nobel lecture). ChemBioChem 1: 86–102.

Matlin KS (2002) The strange case of the signal recognition particle. Nature Reviews Molecular Cell Biology 3: 538–542.

Matlin KS (2011) Spatial expression of the genome: the signal hypothesis at forty. Nature Reviews Molecular Cell Biology 12: 333–340.

Palade G (1975) Intracellular aspects of the process of protein synthesis. Science 189: 347–348.

Rasmussen N (1997) Picture Control: The Electron Microscope and the Transformation of Biology in America 1940–1960. Stanford: Stanford University Press.

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Matlin, Karl S(Sep 2013) History of the Signal Hypothesis. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1002/9780470015902.a0025089]