Last week one of the fathers of Cell Biology died. I found out Friday during happy hour - but I just haven't had the time to write anything until now.
George Palade and Keith Porter, were the first scientists to peer into the depths of the cell using electron microscopy (EM). This all started in 1945 when Keith Porter, a researcher at Rockefeller University, snapped the first known EM micrograph (right). A year later Palade joined Albert Claude's group to work on the electron microscopy of large structures isolated from tissue culture cells. Palade and Porter started a revolution - they were the first to see ribosomes, the endoplasmic reticulum and countless other subcellular structures. Much about what we know about the cell can be traced back to Palade's famous EM studies from the 50s and 60s.
One of Palade's greatest series of papers dealt with how secretory proteins were produced. In an early paper Palade's group investigated small vesicles derived from mashed up pancreatic cells. By using electron microscopy, they observed that these vesicles, also known as microsomes (small vessicles, get it?), were studded with tiny bodies called ribosomes. An example of these microsomes can be seen in figure A from the powerpoint slide below (it's straight from one of my talks - the image is from Redman et al., JBC 1966). If you dissolved the microsome's membranes with detergent, you could release polysomes, or ribosomes connected together by mRNA. As you all know, mRNA contains the information needed to make proteins, ribosomes are the machines that actually make the proteins.
Palade discovered that the these mRNAs encoded secreted proteins, or proteins that end up on the outside of the cell. An example of such a protein is amylase, a digestive enzyme.
So from these results we can say that secreted proteins are synthesized on the surface of some membrane. When cells are broken apart, this mystery membrane can be isolated as the microsome fraction.
But what does the microsome fraction correspond to?
Well there is a membrane in cells that is also studded with ribosomes, the rough endoplasmic reticulum (RER). In a tour-de-force Palade's group demonstrated that radioactive amino acids first get incorporated into the RER. How did they do this? Well, they pulsed in tritiated leucine (one of the twenty amino acids used to make proteins) into live cells. They then fixed the samples and imaged the cells by electron microscopy. But in addition they captured the radiation emanating from the radioactive amino acids within the newly synthesized proteins. The radiation shows up as these squiggles visible in the images below (from Jamieson et al. JCB 67). It turns out that the newly made proteins appear first in the RER (first panel). If you wash away the amino acids and let the cells sit for a further 7 min the radio-labelled proteins move from the RER to the Golgi (second panel), then if you wait even longer - say 37min - the newly made proteins end up in secretory vesicles (third panel).
There you have it, our first glimpse of the secretory pathway, one of the cornerstones of Cell Biology.
George Palade's obituary in last week's NY Times.
George Palade's Autobiography at the Nobel website
From ASCB, George Palade's profile and a collection of his images and micrographs.
And for all you kidz, The Incredible Megacell Game (an interactive cartoon-like game all about organelles and electron microscopy, at the Nobel website.)
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