Misfolding of membrane proteins in health and disease: the lady or the tiger?

doi:10.1016/S0959-440X(00)00112-3

Current Opinion in Structural Biology

Volume 10, Issue 4, 1 August 2000, Pages 438-442

https://doi.org/10.1016/S0959-440X(00)00112-3 Get rights and content

Abstract

Protein misfolding is increasingly recognized as a factor in many diseases, including cystic fibrosis, Parkinson’s, Alzheimer’s and atherosclerosis. Many proteins involved in misfolding-based pathologies are membrane-associated, such that the bilayer may play roles in normal and aberrant folding. It can be argued that the in vivo partitioning of eukaryotic membrane proteins between folding and misfolding pathways is under kinetic control. Moreover, the balance between these pathways can be surprisingly delicate.

Introduction

There are hundreds of examples in which point mutations or other variations in protein sequences are etiologic to various diseases [1]. In work beginning with the classic studies of the relationship between hemoglobin and sickle cell anemia and accelerating dramatically in the past few years, it has come to be appreciated that many, possibly a majority of, such mutations lead to disease-related problems in cell physiology because of protein misfolding, misassembly and/or aberrant oligomerization 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16. For the sake of convenience, we use the term ‘misfolding’ very broadly in this review to refer to all situations in which a protein is expressed at normal levels, but fails to efficiently reach its critical cellular or extracellular destination in a functional conformation. Misfolding can contribute to pathology through a variety of inter-related mechanisms. These include loss of function of the misfolded protein and damage triggered by the accumulation of the misfolded form inside or outside of cells, which can interfere with normal cell function by a variety of possible mechanisms (apoptosis, etc.).

Many of the diseases associated with misfolding involve proteins that are membrane-associated for part or all of their lifetime. The primary site of folding for most of these proteins is the endoplasmic reticulum (ER), although other organelles are sometimes involved 17, 18, 19.

In the ER, protein folding involves the participation of a host of ER-resident proteins and protein complexes 7, 8, 9, 20, 21, 22, 23, 24. These include the translocon complex (through which nascent proteins enter the membrane or lumen of the ER), chaperones, escorts, enzymes involved in post-translational modifications and quality control, and proteins involved in trafficking between the ER and the Golgi. In addition, chaperones and proteases in the cytoplasm sometimes play a role in the folding and degradation of ER proteins that have cytoplasmically exposed domains or that are extruded from the ER following misfolding 21, 23. Protein misfolding in disease states is only rarely associated with mutations in the many components of the protein assembly apparatus (presenilin mutants connected with Alzheimer’s disease may represent an important exception 25, 26). Although we shall limit our discussion to misfolding that occurs as a consequence of mutations in an expressed protein, misfolding need not involve mutations, but can be triggered by a variety of factors, such as protein overexpression, temperature, oxidative stress and activation of various signaling pathways linked to protein folding and quality control machinery [27]. For example, changes in temperature can perturb the rates of flow through competing folding and misfolding pathways and can induce increased cellular production of heat-shock proteins, which are often chaperones [27].

Section snippets

In vivo membrane protein folding and misfolding as competitive pathways

Misfolding in vivo often may be unrelated to the thermodynamic stability of the functional conformation of the protein in question. Instead, the partitioning of a nascent membrane protein in the cell between reaching either the folded state or a misfolded state can be very loosely thought of as being under kinetic control [28]. Consider the case of the cystic fibrosis transmembrane conductance regulator (CFTR). Misfolding of CFTR in the ER results in the loss of chloride channel activity on the

More lessons from CFTR: acquiring a preference for tigers can be easy

As noted, less than 50% of wild-type CFTR is correctly assembled and trafficked to the cell surface under normal physiological conditions 29, 31. If we persist in the oversimplified notion that the partitioning of proteins between folded and misfolded states is under kinetic control, then this indicates that the differences between the free energy barriers of the rate-limiting steps for each pathway are extremely low: roughly RTlnK_partition, where R is the gas constant, T is the temperature and

The membrane bilayer as an active player in folding and misfolding in vivo?

Some misfolded proteins involved in disease are integral membrane proteins throughout their entire lifetime. The relationship of other misfolding disease-related proteins to the membrane is more complex. For example, while the Aβ-amyloid peptide of Alzheimer’s disease is water soluble under some conditions, it has a high affinity for some membrane surfaces, upon which the rate of fibril formation is significantly enhanced 49, 50, 51. Moreover, the two forms of this peptide are derived from an

Conclusions

The development of a comprehensive understanding of the molecular and energetic principles governing the folding and misfolding of membrane proteins in living cells will require close collaboration between structural and cell biologists. Given the high incidence of protein misfolding phenomena in human disease, the stakes associated with such collaborations are very high.

Update

Schubert et al. [69^••] have now shown, in multiple cell lines under near-physiological conditions, that 30% or more of all newly synthesized proteins are rapidly degraded, primarily through the polyubiquitination/proteasome pathway. Perusal of their data suggests that, except for a tendency for high-molecular weight proteins to be more efficiently degraded than small, this is a phenomenon that is generally non-specific in terms of the proteins involved. This calls to mind the hero of Frank

Acknowledgements

The support of the authors by the US National Institutes of Health is very much appreciated (RO1 GM47485 and T32 HL07653). We thank F Ismail-Beigi, W Surewicz, M Zagorski, P Davis, JR Riordan and J Bowie for helpful comments related to this paper. We express our regret to the many authors of related work (especially in the areas of folding kinetics and prokaryotic membrane protein assembly) that was not cited because of space limitations.

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

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ReviewMisfolding of membrane proteins in health and disease: the lady or the tiger?

Abstract

Introduction

Section snippets

In vivo membrane protein folding and misfolding as competitive pathways

More lessons from CFTR: acquiring a preference for tigers can be easy

The membrane bilayer as an active player in folding and misfolding in vivo?

Conclusions

Update

Acknowledgements

References and recommended reading

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Review
Misfolding of membrane proteins in health and disease: the lady or the tiger?