By David N. Leff
A TV couch potato knows the letters "ER" as a soap opera centered on the emergency room of a large virtual hospital.
To research immunologists, that same ER acronym refers to the endoplasmic reticulum organelle in all mammalian cells. In a pivotal way, it's a cellular emergency room.
Endoplasmic reticulum is the convoluted name for a convoluted network of tubules in the cell, just beyond its nucleus.
When a pathogenic virus or bacterium — or a malignant tumor, for that matter — invades the body, the immune system mounts a counterattack to wipe out those alien antigens by getting rid of the cells they're infecting. Much of that action takes place in the ER.
First of all, the cells marked for termination must advertise the fact that they carry antigens in or on the proteins of the foreign invaders. So in the cytosol (liquid bulk) of those death-row cells, enzymes called proteosomes set about slicing, dicing and mincing those hostile antigens into thousands of small peptides. This swarm of highly antigenic fragments then heads for the ER, and their rendezvous with the antigen-presenting class I major histocompatibility complexes (MHC).
To bring them to the attention of the immune system's killer T cells, genes on human chromosome 6 (mouse chromosome 17) churn out a set of proteins called HLA (Human Leukocyte Antigens) or murine MHC (Major Histocompatibility Antigens) class I.
Circumstantial Evidence Strong
The conformational assembly of the MHC molecules occurs elsewhere in the ER network. It makes use of the protein that pumped the peptides from the cytosol into the ER, and then participates in assembling the MHC.
That molecule's heavy chain is a transmembrane protein that's synthesized on ribosomes and immediately translocated into the ER. Those ribosomes, which express the antigen-recognition molecules, line the network of ER microtubules. Once synthesized and assembled, they sally forth to wrap around the swarm of antigen peptide fragments located in another ER compartment, and then transport them to the waiting executioners — the killer T cells deployed on the doomed cells' surface.
But on their way to find, bind and enfold the peptide fragments, those nascent, intermediate MHC class I complexes are like empty bags, with floppy, unstable structures. Like small children left alone on the street, they risk being bullied, and worse, bullied by their own immune defenses — which might see them as alien protein targets. Those MHC's that fail to fold fully will be degraded while still inside the ER.
Luckily, nature protects the intermediates with a platoon of nanny proteins called chaperones, which escort them through the protein-folding process.
However neat it all may look on paper, this entire rap sheet is riddled with unknown molecular mechanisms and unanswered questions. One major uncertainty is cleared up in this month's Nature Structural Biology, published May 1, 1998. The paper is titled: "Structural characterization of a soluble and partially folded class I major histocompatibility heavy chain/[beta-2]m heterodimer."
The article's senior and lead authors are, respectively, molecular and cell biologist Don Wiley of Harvard University, in Boston, and structural biologist Marlene Bouvier, now at the University of Connecticut, in Storrs.
In an accompanying editorial, immunologist Ted Hansen of Washington University, in St. Louis, noted speculation by immunologists that "MHC class I molecules lacking peptide ligand have a structure significantly different from that of peptide-bound MHC molecules."
Hansen welcomed the Bouvier/Wiley report as providing "intriguing insights into the structure of peptide-free class I, using a series of comprehensive chemical approaches, including native isoelectric focusing gel electrophoresis, circular dichroism [spectroscopy] and susceptibility of proteolytic cleavage."
Anticipating his own work in progress, as well as that of others, Hansen told BioWorld Today, "It will be particularly exciting to define the chaperone-detected conformational change in class I that occurs when peptides bind. There's a lot of circumstantial evidence that there's a very substantial change, and Bouvier's paper is the very first evidence from a structural point of view.
"After that change — the class I molecule folding around the peptide — it is in a functional state so that it can be recognized by the immune system's killer T cells," Hansen explained.
Speculation On A Clinical Spin-Off
Turning from basic to clinical immunology, and mouse to human, Hansen cited "recent studies suggesting that peptide-empty HLA-B27 class I molecules may play a critical role in the induction of autoimmune disease.
"As for such a putative role in autoimmunity," he went on, "there'd be two ways that an MHC molecule lacking a peptide could arrive at the cell surface: It could fail to bind a peptide in the ER, or it would make it to the surface with a peptide, and that fragment would then fall out.
"At this stage," Hansen added, "that is speculative. There are various theories and speculations as to how it might happen. Some mouse models find a correlation between the expression of an empty form on the cell surface, and autoimmunity." *