3,000 members. Complicated family reunions to organize. And unfortunately, not as famous as the Kardashians.
Meet the SERPin family.
In 1980, aligning the protein sequences on his computer, Lois made a curious discovery. Working on the sequence of ovalbumin, and comparing it to the entire available database, Lois T. Hunt found that chicken ovalbumin was clearly related to two other vertebrate proteins, and so despite their phylogenic origin, he proposed that all three should be grouped in the same protein superfamily (1).
The two new family members were antithrombin and antitrypsin, and to be more accurate, anti-thrombin-III, and alpha1-proteinase inhibitor.
Since it was easy to call something new after its function, SERPin became the acronym for SERine Protease INhibitor since we thought that they were all inhibitors (2). But SERPin functions can be very different, and do not necessarily involve protease inhibition.
SERPin : Back to the roots?
SERPins are an ancient group of 500 amino acid proteins distributed in the different kingdoms of life, including bacteria (Actinobacteria, Firmicutes, Bacteroidetes, Cyanobacteria, Proteobacteria), viruses and eukaryotes (3,4). We do not know if bacteria gene were transferred to human or vice-versa, but there seems to exist six ancestral genes in vertebrates (5). In humans, SERPins are divided in 16 clades, designated by capital letters (6,7) and localized on 10 different chromosomes.
If the A clade is clearly engaged into the regulation of proteases that mediate the inflammatory responses, the B clade functions are less characterized. Nevertheless, knock-out experiments in rodents have given more proof that some SERPin are essential to our survival, like B5 or maspin (8), C1 or antithrombin (9), and H1 or Hsp47 (10).
But despite their huge diversity, SERPin share a common core.
SERPin : The suicide mousetrap
The serpin fold is usually comprised of three β-sheets and eight or nine α-helices. However, studies conducted by Simonovic et al. (11) and Renatus et al. (12) demonstrated that in crmA (poxvirus SERPin), SERPin helix D is absent and helix A is shortened. Helix G is also missing from some Prokaryotes SERPins (13).
But the essential is preserved.
All SERPins have in common an exposed reactive center loop (RCL) of 350 amino acids, often mentioned as a « bait » for proteases (the prey), that protrudes from the top end of the molecule. 51 amino-acids of this RCL are highly conserved across the family and are mostly localized within the hydrophobic core, creating the enzyme cleavage site (P1-P1’).
SERPin : An apparent equilibrium
« Metastability is the property of a state that appears to be stable but that a disturbance can cause it to move quickly to an even more stable state.
In the absence of a significant perturbation, the speed of the transformation leading to the stable state can be very low or even almost null. In response to a disturbance,
the transformation can be very fast or even almost instantaneous. »
Curiously enough, SERPin family is distinguishable by the fact that the native fold is not the most stable state of the protein. This apparent state of equilibrium is a metastable conformation, converted to a more stable state during protease inhibition (14). When the prey recognizes the bait, the inhibition process starts and the SERPin is forming a reversible Michaelis-Menten complex with its protease (15).
The protease then cleaves the bond between P1 and P1’ parts of the SERPin, that lead the C-terminal loop of the RCL to be inserted into the SERPin body, thus closing the trap.
If this last reaction is fast enough, the protease becomes covalently bound to the P1 residue and the complex will be withdrawn from circulation. SERPin wins the fight.
But in case the process is too slow, the covalent bind is disrupted and the SERPin remains cleaved. SERPin turns into a substrate. The hunter becomes the prey.
The price to pay for this incredible structural flexibility is the susceptibility to mutation. Inactive SERPins lead to unbalanced protease regulation, and so to a specific class of conformational diseases known as serpinopathies. Consequently, the mutant SERPin polymerizes, and in some cases, may form cytotoxic inclusion bodies.
To tackle this issue, strategies can be declined through the introduction of recombinant SERPins (enzyme replacement therapy), the engineering of more stable or selective SERPin variants or the blockade of mutant SERPins.
And you, are you working on SERPin?