Location, location, location. It has been, and remains, one of the best initial clues to understand the function of a protein or process. The same was true for those trying to localize protein synthesis in the 1950s and 1960s, but their experiments came with a twist. This early in the history of cell biology, the localization effort was not far behind, or even inspired, the definition and naming of the locations.
The cells of choice were pancreatic or liver cells that were furiously churning out secreted proteins. Littlefield et al. (1955) made a promising start by showing that radioactive amino acids accumulated first in the detergent-resistant particles of the microsomes (i.e., the ribosomes) before moving into the membranous portion. The authors felt confident to state that “the cytoplasmic ribonucleoprotein particles are the site of initial incorporation of free amino acids into protein.
”Siekevitz and Palade (1958) came to a similar conclusion, and extended the work to show that the label continued on from the microsome fraction to reach the pancreatic zymogen granules. The same sequence held true for individual purified secretory proteins (Siekevitz and Palade, 1960).
Localization moved from biochemistry to cytology with Caro and Palade (1964), who used electron micrographs to trace radioactive leucine as it moved through the endoplasmic reticulum, then the Golgi, and finally to zymogen granules. The Golgi had been destroyed by the earlier biochemical fractionations, so this was its first appearance in the pathway.
All these in vivo studies were hampered by overly long pulse and chase times. Thus it remained formally possible that label was not moving from one compartment to another, but accumulating in the different compartments independently and at different rates. Jamieson and Palade (1967a,b) used in vitro tissue slices so that pulse times could be more precisely controlled; thus they demonstrated that there was indeed a flow of protein from one compartment to the next. As the autoradiography became more exact, Jamieson and Palade (1971) could also conclude that the proteins were inside rather than outside the Golgi cisternae, and thus the proteins were not traveling through the cytoplasm as some had suggested.
“The evidence was overwhelming” for this basic secretory pathway, says Siekevitz, and it was generally accepted. These early results from studies with secretory cells had turned up a model that was good at explaining secretion.
But the story of protein synthesis itself remained incomplete—not least because of the inexact nature of biochemical fractionation. “Even with the corrections envisaged,” said Siekevitz and Palade (1958), “…it seems unlikely that the decrease in microsomal radioactivity [over time] could balance the increase in counts in all the other cell fractions. It follows that incorporation of amino acids into proteins, and presumably protein synthesis, is not necessarily restricted to microsomes.”
The other half of the equation was not filled in until the report of Ganoza and Williams (1969). They made the correlation between, on the one hand, membrane-bound ribosomes that made secreted proteins and, on the other, nonmembrane-bound ribosomes that made nonsecreted proteins.