Part XIV: George E. Palade, 1974 Prize in Physiology or Medicine.
By Joseph Luna
Nestled in the 3rd sub-basement of Smith Hall, around 1953, an electron microscope (EM) is briefly idle. The machine, an RCA model EMU-2A, resembles a spare part from some future space station: a long vertical steel tube adorned with studs and knobs, with a viewfinder at the base. To the casual viewer, there’s little to indicate the purpose of this strange contraption. But to its operator, just having imaged the last specimen of tissues ranging from the pancreas to blood cells to the intestine, the purpose of this machine is strikingly clear, and is measured in Angstroms. The man sitting at the controls is George Palade, and he has just discovered “a small particulate component of the cytoplasm,” as he tentatively named it. In a few years, this particle would be renamed the “ribosome” and would soon be recognized as the essential protein-making machine in all of life.
Of course, such a romantic view of discovery relies squarely on hindsight, for it is almost impossible to pinpoint where one is during a scientific revolution in real time. This was certainly true at the beginning of modern cell biology, as the specimen preparation methods used for EM carried with them the specter of artifact. In essence, how did George Palade know that these particles weren’t a farce? The preceding seven years had done much to prepare Palade to address this question. Alongside Albert Claude, Keith Porter and others, Palade placed the nascent field of cell biology on sound methodological footing that enabled the discovery of the ribosome, and so much more.
In 1946, barely a year from the first EM picture of a cell, Palade joined the Rockefeller Institute as a postdoc, at Claude’s invitation. When Palade got his start, Claude’s group was concerned with trying to connect enzymatic activities that biochemists could measure, with a physical location in the cell that could be accounted for by fractionation or using new EM methods to see what the ultrastructure looked like. Claude and his co-workers were able to break cells apart into roughly four fractions that could be subjected to biochemical tests: nuclei, a large fraction that appeared to contain mitochondria, microsomes, and free cytoplasm. The large fraction caught their attention precisely because there was a problem. In intact cells, mitochondria could be stained with a dye called Janus Green, but the dye never worked in the large fraction, despite EM results that showed intact, though clumped, mitochondria. Moreover, biochemists had found that the large fraction contained many of the enzymes known to be involved in energy production, but this fraction wasn’t pure enough to make firm conclusions. Palade helped to clarify this issue by devising a better way to isolate pure mitochondria using dissolved sucrose (table sugar) as an isotonic buffer instead of the saline solutions used by Claude. As a result, the large fraction retained Janus Green staining, and energy making enzymes were much more enriched. It was an instructive experience because it showed that cells could be taken apart rationally, a bit like taking apart a radio with a screwdriver instead of with a sledgehammer. Intact, functional units like mitochondria could be separated and studied apart from other cell components. For these early cell biologists, it was a compelling justification to keep going.
This much was evident to Institute president Herbert Gasser. With Claude’s move back to Belgium in 1949, the retirement of lab head James Murphy in 1950, and other departures, the first Rockefeller cell biology group shrunk to just Porter and Palade. Gasser made the rare move of making them joint lab heads of their own cytology laboratory, and outfitted Smith hall with an RCA microscope.
Porter and Palade next made a concerted effort to describe, in intact cells and tissues, the ultrastructure of the mitochondria and a subcellular structure found in the microsomal fraction that Porter named the endoplasmic reticulum (ER). While Porter working with Joseph Blum, devised a new microtome to make thin slices of tissue for EM, Palade refined fixation and staining conditions (colloquially called “Palade’s pickle procedure”) to take EM to new heights. Using these tools, Palade went on to describe the inner structure of the mitochondria, observing inner folds and chambers he called cristae. The Palade model of the mitochondrion was illuminating for biochemists, because it provided structural constraints for possible mechanisms that explained how mitochondria made energy. In other words, what a mitochondrion looked like was essential for its function.
This line of thinking was critical to deciphering what role, if any, of those particles Palade observed in 1953. He noticed that they were typically observed stuck to the ER, were enriched in the microsomal fraction, and had high levels of RNA. He also noticed that secretory cells, such as digestive enzyme producing exocrine cells of the pancreas were packed with ER and ribosomes. In short order a hypothesis emerged, from Palade and others, that ER and ribosomes were involved in the synthesis and ordered transport of proteins in the cell. Working with Philip Siekevitz, Palade used radioactive amino acids to biochemically trace protein synthesis and transport in these cells, following the radioactivity in cell fractions, and using EM to visualize structure in each fraction; all in a seven part series of papers between 1958 and 1962. This triple threat of cell fractionation, biochemistry, and EM became the model for the entire field. EMs the world over have since rarely been idle for long.
