Resolving the paradox for protein aggregation diseases: NMR structure and dynamics of the membrane-embedded P56S-MSP causing ALS imply a common mechanism for aggregation-prone proteins to attack membranes

Mechanistic Insights into Protein Stability and Self-aggregation in GLUT1 Genetic Variants Causing GLUT1-Deficiency Syndrome

Human sodium-independent glucose cotransporter 1 (hGLUT1) has been studied for its tetramerization and multimerization at the cell surface. Homozygous or compound heterozygous mutations in hGLUT1 elicit GLUT1-deficiency syndrome (GLUT1-DS), a metabolic disorder, which results in impaired glucose transport into the brain. The reduced cell surface expression or loss of function have been shown for some GLUT1 mutants. However, the mechanism by which deleterious mutations affect protein structure, conformational stability and GLUT1 oligomerization is not known and require investigation. In this review, we combined previous knowledge of GLUT1 mutations with hGLUT1 crystal structure to analyze native interactions and several natural single-point mutations. The modeling of native hGLUT1 structure confirmed the roles of native residues in forming a range of side-chain interactions. Interestingly, the modeled mutants pointed to the formation of a variety of non-native novel interactions, altering interaction networks and potentially eliciting protein misfolding. Self-aggregation of the last part of hGLUT1 was predicted using protein aggregation prediction tool. Furthermore, an increase in aggregation potential in the aggregation-prone regions was estimated for several mutants suggesting increased aggregation of misfolded protein. Protein stability change analysis predicted that GLUT1 mutant proteins are unstable. Combining GLUT1 oligomerization behavior with our modeling, aggregation prediction, and protein stability analyses, this work provides state-of-the-art view of GLUT1 genetic mutations that could destabilize native interactions, generate novel interactions, trigger protein misfolding, and enhance protein aggregation in a disease state.

Environment-transformable sequence-structure relationship: a general mechanism for proteotoxicity

In his Nobel Lecture, Anfinsen stated "the native conformation is determined by the totality of interatomic interactions and hence by the amino acid sequence, in a given environment." As aqueous solutions and membrane systems co-exist in cells, proteins are classified into membrane and non-membrane proteins, but whether one can transform one into the other remains unknown. Intriguingly, many well-folded non-membrane proteins are converted into "insoluble" and toxic forms by aging- or disease-associated factors, but the underlying mechanisms remain elusive. In 2005, we discovered a previously unknown regime of proteins seemingly inconsistent with the classic "Salting-in" dogma: "insoluble" proteins including the integral membrane fragments could be solubilized in the ion-minimized water. We have thus successfully studied "insoluble" forms of ALS-causing P56S-MSP, L126Z-SOD1, nascent SOD1 and C71G-Profilin1, as well as E. coli S1 fragments. The results revealed that these "insoluble" forms are either unfolded or co-exist with their unfolded states. Most unexpectedly, these unfolded states acquire a novel capacity of interacting with membranes energetically driven by the formation of helices/loops over amphiphilic/hydrophobic regions which universally exit in proteins but are normally locked away in their folded native states. Our studies suggest that most, if not all, proteins contain segments which have the dual ability to fold into distinctive structures in aqueous and membrane environments. The abnormal membrane interaction might initiate disease and/or aging processes; and its further coupling with protein aggregation could result in radical proteotoxicity by forming inclusions composed of damaged membranous organelles and protein aggregates. Therefore, environment-transformable sequence-structure relationship may represent a general mechanism for proteotoxicity.

SALS-linked WT-SOD1 adopts a highly similar helical conformation as FALS-causing L126Z-SOD1 in a membrane environment

So far >180 mutations have been identified within the 153-residue human SOD1 to cause familial amyotrophic lateral sclerosis (FALS), while wild-type (WT) SOD1 was intriguingly implicated in sporadic ALS (SALS). SOD1 mutations lead to ALS by a dominant gain of cytotoxicity but its mechanism still remains elusive. Previously functional studies have revealed that SOD1 mutants became unexpectedly associated with organelle membranes. Indeed we decoded that the ALS-causing truncation mutant L126Z-SOD1 with an elevated toxicity completely loses the ability to fold into the native β-barrel structure but acquire a novel capacity to interact with membranes by forming helices over hydrophobic/amphiphilic segments. Very recently, the abnormal insertion of SOD1 mutants into ER membrane has been functionally characterized to trigger ER stress, an initial event of a cascade of cell-specific damages in ALS pathogenesis. Here we attempted to understand the mechanism for gain of cytotoxicity of the WT SOD1. We obtained atomic-resolution evidence that the nascent WT SOD1 without metalation and disulfide bridge is also highly disordered as L126Z. Most importantly, it owns the same capacity in interacting with membranes by forming very similar helices over the first 125 residues identical to L126Z-SOD1, plus an additional hydrophobic helix over Leu144-Ala152. Our study thus implies that the WT and mutant SOD1 indeed converge on a common mechanism for gain of cytotoxicity by abnormally interacting with membranes. Moreover, any genetic/environmental factors which can delay or impair its maturation might act to transform SOD1 into cytotoxic forms with the acquired capacity to abnormally interact with membranes.

Mechanism for transforming cytosolic SOD1 into integral membrane proteins of organelles by ALS-causing mutations

Mutations in superoxide dismutase 1 (SOD1) cause familial amyotrophic lateral sclerosis (FALS), while wild-type SOD1 has been implicated in sporadic ALS (SALS). SOD1 mutants are now recognized to acquire one or more toxicities that include their association with mitochondrial and endoplasmic reticulum membranes but the underlying structural mechanism remains unknown. Here we determine NMR conformations of both wild-type and a truncation mutant (L126Z) of SOD1 in aqueous solution and a membrane environment. The truncation mutant (which causes FALS at very low levels, indicating its elevated toxicity) is highly unstructured in solution, failing to adopt the β-barrel SOD1 native structure. Wild-type SOD1 is also highly unstructured upon reduction of disulfides and depletion of zinc. Most remarkably, both mutant and wild type adopt similar, highly-helical conformations in a membrane environment. Thus, either truncation or depletion of zinc is sufficient to eliminate the native β-barrel structure, and transform cytosolic SOD1 into membrane proteins energetically driven by forming amphiphilic helices in membranes. That zinc-deficiency is sufficient to produce a similar transformation in wild-type SOD1 implies that the wild-type and FALS-linked SOD1 mutants may trigger ALS by a common mechanism.