Molecular gymnastics: serpin structure, folding and misfolding

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The native state of serpins represents a long-lived intermediate or metastable structure on the serpin folding pathway. Upon interaction with a protease, the serpin trap is sprung and the molecule continues to fold into a more stable conformation. However, thermodynamic stability can also be achieved through alternative, unproductive folding pathways that result in the formation of inactive conformations. Our increasing understanding of the mechanism of protease inhibition and the dynamics of native serpin structures has begun to reveal how evolution has harnessed the actual process of protein folding (rather than the final folded outcome) to elegantly achieve function. The cost of using metastability for function, however, is an increased propensity for misfolding.

Section snippets

Introduction: an overview of the serpin inhibitory mechanism

Serpins are the largest and most widely distributed family of protease inhibitors, bona fide inhibitory serpins having been identified in all major branches of life [1, 2••, 3]. Around 90% of characterised serpins inhibit chymotrypsin-like serine proteases [4], although increasing numbers of cross-class serpin inhibitors of cysteine proteases have been identified ([5••]; see also Update). In contrast to the small (10–15 kDa) and relatively rigid Kazal-type and Kunitz-like serine protease

Getting metastable and staying there

The most stable conformation of the native serpin chain is the latent RCL-inserted form [13] (Figure 2, Figure 3). The native conformation of plasminogen activator inhibitor-1 (PAI-1) spontaneously undergoes the transition to the latent form; this represents an elegant inbuilt mechanism for controlling inhibitory activity. However, because of a large energetic barrier, most inhibitory serpins do not readily attain this state and instead the folding pathway is prematurely interrupted at the

Dynamics of the native serpin conformation: implications for the inhibitory mechanism and the transition to latency

Both cleaved and latent serpins adopt RCL-inserted states that are structurally similar and display high thermal stabilities. Therefore, the transition from N ⇒ L can be considered similar to the transition from N ⇒ I*, where I* represents the cleaved serpin in the final serpin–enzyme complex. Thus, interaction with a target protease and RCL cleavage essentially lowers the kinetic barrier or offers an alternative energetic route to this more stable conformation. However, it is unclear precisely how

Non-productive serpin folding: polymerisation and latency

In 1992, the identification of the polymerogenic properties of the Z (Glu342 → Lys) allele of antitrypsin opened the door to the study of serpin dysfunction through misfolding [10]. Since this discovery, more than forty variants of five different human serpins (antitrypsin, antichymotrypsin, antithrombin, C1 inhibitor and neuroserpin) have been identified that result in serpin-related misfolding diseases (serpinopathies) [3, 34, 35, 36]. All these variants share several common features, including

Conclusions

Inhibitory serpins undergo a dramatic conformational rearrangement that is required for protease inhibition. However, serpins are markedly susceptible to mutations that result in the formation of inactive states. The analysis of over eighty different X-ray crystal structures of serpins in five different conformational states, together with biophysical studies of serpin folding, has started to reveal a detailed picture of the molecular dynamics of conformational change. Crucially, the native

Function of a plant serpin

The precise role of serpins in plants has long been the subject of much debate and controversy. Vercammen et al. [51••] have shown that serpin1 of Arabidopsis thaliana is an effective inhibitor of AtMC9, a type II (Arg/Lys-specific) metacaspase (a cysteine protease distantly related to animal caspases). Although the function of most metacaspases is unknown, a recent study revealed that one of these proteases (mcII-Pa) may play a role in programmed cell death in plants [52]. The work of

References and recommended reading

Papers of particular interest, published within the annual period of review, have been highlighted as:

  • • of special interest

  • •• of outstanding interest

Acknowledgements

JCW is a National Health and Medical Research Council (NHMRC) senior research fellow and Monash University senior Logan fellow. SPB is an NHMRC R Douglas Wright fellow and a Monash University senior Logan fellow. We thank Mary Pearce, Ashley Buckle and Michelle Dunstone for critical reading of the manuscript, and the Australian Research Council and NHMRC for support.

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