heat Shock Proteins

The following sample is an excerpt I wrote as part of a much larger project.  The project allowed a very informal tone, which was fun.  It also required we start from the basics, so that i did, trying to cover the essentials and just the essentials, and to make it fun.

Meet Your Heat Shock Proteins

Like many greats, heat shock proteins (HSPs) were discovered by accident. In 1962, the Italian scientist Ferruccio Ritossa was studying fruit fly genetics, when one morning he found an incubator to be set too high. Ritossa denied responsibility even in his write up of his new finding.  He says "I do not remember whether it was John Pulitzer or Inge or Clara Ghindi or Giordana who shifted the temperature of my incubator, but one day I noticed a different puffing pattern!"[4]  A puffing pattern signals genes being activated. Those genes were heat shock proteins. Butit would be years before the significance of that puffing pattern would be known to science.

Proteins 101

Before we dive in, let’s make sure we are on the same page. What exactly are proteins? Like, a piece of chicken? Smaller. We are diving into the cells of that chicken—the microscopic cells that make up every tissue and organ in your body.

Proteins 101

Our cells are crowded places.  They are not the clean, spacious line drawing from your high school biology textbook, with a nucleus and a single mitochondrion off to the side doing its job in peace, substrate molecules moseying over in a leisurely backstroke.

Our cells can contain billions of proteins. We have identified around genes for over 10,000 different human proteins [5], meaning different structures and jobs- like a hammer and a stapler, or some can be completely different in shape and function, like an exercise ball and tweezers.  But they are all made from amino acids, and they all have a job to do, and are shaped for their job. While there are thousands of different proteins in the body, there are a smaller number of regulars that take up most of the space in a typical cell. Sometimes referred to as “housekeeping proteins” because they maintain the essential functions of day to day life (metabolism, regulating ion concentrations, communicating with neighbors), they typically make up about 70% of the proteins in a cell. 

The proteins are the tools and workers of the cells; they basically do everything. They make chemical reactions happen, they move the cell, they open and close channels– they ARE the channels– they replicate DNA, they communicate with other cells.  Each of the thousands of  different proteins has a unique role and a unique shape that is integral to its job. Whether it's a channel that water and molecules pass through, an enzyme that binds a particular substrate, or a structural protein that has to hold up the cell, shape defines function. Mis-shaped proteins cannot perform their job and are essentially useless until they're fixed or recycled.

Heat shock proteins are one of the most abundant and important housekeeping proteins.  Up to 10% of proteins in a cell at baseline are heat shock proteins [6]. There are millions of HSPs in each cell.   One out of every 10 proteins you'd bump into swimming through the cytosol is a heat shock protein.   And they are in the cells of every living cell, from bacteria to humans, in nearly identical forms across all species, which is rare.  They are in fact one of the most conserved proteins that we know of. 

Chaperoning and Folding

I have already dropped hints at how important protein shape is. But we aren’t done.  Lets talk about protein shape.

How does a protein get its shape? The sequence of amino acids that make up a protein is encoded in our DNA. When a cell decides to make more proteins, it uses the DNA code to string together the amino acids in the right order.  This “string” of amino acids is the protein's primary structure.  It then starts to fold.  First in two dimensions, then three, to fold into its secondary and tertiary structure.  These depend on the amino acid sequence, as certain places along the chain will interact. Some proteins come together with other subunits to form a larger multi-unit protein, which would be its quaternary structure.

Ok, I promised not to bore you with details, and I may have already broken that, but I am reeling it in here. The point is that the process of folding each protein into its correct shape is a complicated and meticulously orchestrated process, and heat shock proteins are part of that process.  

A chaperone protein, more accurately, is the family of proteins that guides protein folding.  Chaperone proteins and heat shock proteins were being discovered independently in the 1970s, and it was not recognized until the 1980s that there was a large overlap in the two families[7].  The names stuck though, which often causes confusion even 50 years later.  Technically, only about one fifth of chaperone proteins are activated by heat (making them true HSPs)[8] but the terms are often used interchangeably in the literature.

When Good Proteins Go Bad: Unfolding and Misfolding

While crucial that the shape of a protein be precise and accurate, it also cannot be too rigid or unyielding. Many proteins must be able to change their shape to function. Enzymes bind their substrates and catalyze a reaction and then release the product, channels open and close, myosin and actin contract and relax muscles. The protein must be flexible enough to move and function, even to change shape a little, but not so flexible that it falls apart under stress.

Yes, proteins get stressed too. You will hear the term “cell stressor” used, or stress referring to cells or proteins. A stressor is anything that threatens the homeostasis of the cell, and/or causes damage to the structure of the cell, including proteins. This could be changes in temperature or pH, low glucose or oxygen, toxic chemicals or heavy metals, radiation, and oxidative stress which is now understood to perhaps be the stress rather than cause it, as it is a result of everything else on that list.  More on that later.  

When a protein unfolds, amino acids normally in the middle of the protein, which are often hydrophobic and “sticky,” become exposed. This can lead to clumping, especially with other unfolded proteins. If nothing clears the clumps, they can become large aggregates of damaged proteins that are toxic to the cell.

Toxic Aggregates

Remember how crowded a cell can be? Imagine the cell is a busy airport. Everyone is focused on their task at hand. People swerve around and squeeze past each other, not paying much attention if they can get to where they need to. Carts go whizzing by with the shrill beep of the horn telling pedestrians to make way. Occasionally they get slowed up a little, but normally the system functions. Now imagine a cart loses a wheel. The axle drops, and the cart stops short. Now another cart comes along, and bumps the broken cart, and gets a flat tire, and a pole gets bent and sticks out to the side. The pole now hooks the next cart coming through, and so on and so on until there is a huge barricade of broken carts and nothing can get through. Now people start missing their flights, and the pilots can’t get past, and eventually, if not cleared, the whole airport comes to a halt.

Heat shock proteins are the cart mechanics. They watch for breakdowns, get to the cart and either fix it or tow it before flights start getting delayed.

Heat shock proteins are your primary defense against protein misfolding, and this is a hugely important job.  There are five families of heat shock proteins that work together to keep the cell clear and functioning. There are small HSPs that are constantly surveilling the cell for signs of damage.  If they see anything concerning.. They can rush over, dodging through the crowd, and perform emergency maintenance until the rest of the team gets there. One thing they do is bind to the sticky parts to prevent further clumping. So, they would be removing that bent pole, trying to block other carts from the mess.  

Other families can do more repair, and the goal is to try to unclump the pile and fix all the damage and send the proteins back to work.  If the damage is too much, they at least can get the broken proteins out of the way, and to the proteasomes to be recycled.  It’s really quite an amazing tiny system….