This story is a supplement to the feature "Could Our Own Proteins Be Used to Help Us Fight Cancer?" which was printed in the July 2008 issue of Scientific American.
Primary Role: Keeping Order
Heat shock proteins (HSPs) chaperone other cellular proteins, guarding them from going astray, folding improperly or misassembling while forming larger aggregates, as in the examples below.
HSP40 delivers a newly formed amino acid chain (or one that has become unfolded) to HSP70, which grabs the molecule, helps it to fold into its proper functional form and then releases it.
HSP60 attracts a new amino acid chain or a protein that has lost its structure and internalizes it. Chemical forces inside the cage help the protein to assume its correct folded shape.
HSP90 receives folded proteins from other chaperones and helps to join them into a larger protein structure, such as a cellular receptor.
Second Job: Activating Immune Responses
When a cell is cancerous or infected by a pathogen, it generates proteins not found in normal cells. Fragments of such proteins can then potentially act as antigens, substances that provoke an immune response. But immune cells must first be made aware of the problem. Heat shock proteins,
primarily members of the HSP90 and HSP70 families, participate in sounding the alarm and identifying the culprits.
1. HSP delivers antigens from diseased cells to the immune system’s antigen-presenting cells (APCs), via a surface receptor known as CD91.
2. After internalizing the antigen, the APC releases inflammatory signals to recruit other immune cells and presents the antigen on its surface to a T cell.
3. Thus primed to recognize the target antigen, the T cell proliferates and with its brethren seeks out diseased cells to destroy.