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Molecular biologist studying transport of proteins across membranes. With Günter Blobel, Dobberstein reconstructed steps of the secretory pathway, connecting information in a nascent polypeptide with membrane-associated targeting.^[ref]

By David D. Sabatini, Günter Blobel. Journal of Cell Biology 45(1), 146–157 (1970); published 1 April 1970. Semantic Scholar hosts this journal scan. The byline gives D. D. Sabatini and G. Blobel; the full names are independently verified.

Inspected text: pdfs.semanticscholar.org/1c83/ae62dcb210423f9e8190797ec25f0af67b1f.pdf

DOI: doi.org/10.1083/jcb.45.1.146

Cited passages: Full original PDF pp. 146–157; Methods p. 147: rat-liver preparation, sucrose centrifugation and Philips EM 300 electron microscope; Activity control p. 148: free polysomes with/without microsomes; Figure 5C–D and text p. 152: chromatography and digestion with membrane-solubilizing detergents; Discussion pp. 155–156: membrane-protected versus ribosome-protected fragments.

Cell biologist who, with Günter Blobel, used controlled digestion of nascent polypeptides to investigate their location in cellular fractions. Protection from digestion connected membrane enclosure with the path of protein synthesis.^[ref]

By Günter Blobel. Proceedings of the National Academy of Sciences of the United States of America 77(3), 1496–1500 (1980). PubMed Central hosts the record. Only the abstract was inspected.

Inspected record: pmc.ncbi.nlm.nih.gov/articles/PMC348522/

DOI: doi.org/10.1073/pnas.77.3.1496

Cited passage: abstract.

In 1975, Günter Blobel and Bernhard Dobberstein combined cell-free translation with dog-pancreas microsomes. Density-gradient centrifugation isolated the membranes; removal of resident ribosomes allowed reconstitution with rabbit-reticulocyte ribosome subunits. Added messenger RNA encoded an immunoglobulin light chain; globin provided a comparison. Products made with or without membranes were challenged with proteases and analyzed by SDS-PAGE and autoradiography. Membranes present during synthesis yielded processed, protected light chain; globin remained susceptible. Adding membranes after precursor synthesis failed to reproduce import. These comparisons support selective passage during synthesis in this preparation.^[ref]

The companion paper specifies radioactive amino acid incorporation, ice-cold trypsin plus chymotrypsin treatment, a slab-gel electrophoresis apparatus and dried-gel exposure to X-ray film. Product-size analysis distinguishes a preserved full-size product from residual labeled fragments.^[ref]

Protection was incomplete. The authors considered leaky vesicles, and warned that heat treatment could damage processing as well as ribosome attachment. Their experiments did not identify SRP or the Sec61 complex. Nor does failure of late membrane addition establish that all organelle protein import must occur during synthesis.^[ref]

A protease protection assay asks whether a barrier prevents an added protease from reaching a protein. A membrane-disruption comparison tests whether protection depends on that barrier.

The membrane-disruption test was performed by David D. Sabatini and Günter Blobel in 1970. Rat-liver microsomes purified by sucrose centrifugation incorporated radioactive leucine during cell-free translation. Samples received cold trypsin and chymotrypsin, with or without membrane-solubilizing detergents. Electron microscopy checked vesicle preservation; size-exclusion chromatography separated protected fragment sizes. Large membrane-associated fragments lost protection after disruption, whereas small ribosome-protected fragments persisted. An exposed ribosome preparation remained digestible when mixed with microsomes, checking that membranes did not simply inhibit the proteases.^[ref]

To read the qualitative diagram, first compare untreated aliquots: they establish that detectable cargo exists before digestion. Next compare external protease with intact versus disrupted membranes. Persistence only while the barrier remains intact supports enclosure, provided the exposed-cargo control confirms active protease in both chemical conditions. The detergent-only aliquot checks that disruption itself does not destroy recovery or detection. Adding membranes after synthesis tests a different alternative: capture of a completed product rather than entry during biological translation.

This is an argument from controlled accessibility. Binding predicts membrane association but generally leaves an exposed cargo digestible. Enclosure predicts protection that depends on membrane integrity. However, protein aggregation, intrinsic resistance to proteolysis, inactive protease or undetected small fragments can obscure this distinction. For a transmembrane protein, expect protection of the enclosed portion rather than necessarily the full chain. Combine fragment size, compartment controls and membrane topology; a protected signal alone does not uniquely specify the whole protein's location.

Protein cell biology studies how proteins function within biological cells: where they are delivered, which partners they encounter, and how these interactions affect cellular processes. A protein's protein primary structure and abundance do not by themselves specify its location or available partners. RNA-binding proteins, G proteins and protein targeting illustrate different aspects of that problem.

The microsomal protein import experiment makes location experimentally testable. In 1975, Günter Blobel and Bernhard Dobberstein combined cell-free translation components with dog-pancreas microsomes. An immunoglobulin light chain product made while suitable membranes were present became processed and resistant to external proteolysis, whereas globin did not enter the same protected compartment. Adding membranes only after synthesis did not reproduce the effect in this preparation.^[ref]

The important observation is controlled accessibility, not just recovery near a membrane. A protein can bind a vesicle's outside without crossing it; enclosure can protect it from an external protease. Protease protection assays therefore complement measurements of total product and membrane topology.

For an original two-compartment example, suppose a fixed total of100 competent cargo molecules is divided between20 enclosed and80 exposed molecules. Redistributing the same total to70 enclosed and30 exposed changes accessibility even though total abundance stays100. If every enclosed molecule is protected and every exposed molecule is digested in the stipulated model, the protected fraction changes from0.20 to0.70. Real measurements also need checks for membrane integrity, protease activity and product recovery; the fractions alone do not identify every transport mechanism. This is why cellular organization belongs alongside the isolated molecule's chemistry.

A signal peptide is a short targeting segment of a newly made polypeptide chain. A familiar ER signal sequence lies near the N-terminus and is removed during entry; Günter Blobel and Bernhard Dobberstein's 1975 experiments linked a larger immunoglobulin light chain precursor to a processed product.^[ref]

Cleavage is a separate question from recognition. Not every targeting signal is a cleaved signal peptide: an internal signal anchor can remain part of a transmembrane protein. Conversely, loss of a segment does not by itself prove membrane crossing. Compare the independent size and accessibility readouts in the microsomal protein import experiment.

By Günter Blobel, Bernhard Dobberstein. Journal of Cell Biology 67(3), 835–851 (1975). The University of California, Berkeley hosts this journal scan.

Inspected text: mcb.berkeley.edu/labs/krantz/pdf/blobel%26dobberstein-transfer_of_proteins_across_membranes_I-JCB-1975.pdf

DOI: doi.org/10.1083/jcb.67.3.835

Cited passages: Introduction p. 836; Methods pp. 837–838: cell-free translation, proteolysis, slab SDS-PAGE and dried-gel autoradiography; Figures 5,7,14; Discussion p. 847: size-resolved protection versus sedimentation.

By Günter Blobel, Bernhard Dobberstein. Journal of Cell Biology 67(3), 852–862 (1975). SciSpace hosts this journal scan.

Inspected text: scispace.com/pdf/transfer-of-proteins-across-membranes-ii-reconstitution-of-5pau33unje.pdf

DOI: doi.org/10.1083/jcb.67.3.852

Cited passages: Methods p. 853: dog-pancreas microsomes, centrifugation, stripped preparations; Figure 6 and text pp. 856–859: light chain, globin, plus/minus protease; Figure 7 and text pp. 860–861: reconstitution and heat-inactivation caveat; Figure 8 and text pp. 861–862: membranes after synthesis, failed detergent-solubilized processing.