The cytosolic chaperone α-Crystallin B rescues appropriate folding and compartmentalization of misfolded multispan transmembrane proteins

Abstract

SummaryThe a-crystallin B chain (CRYAB or HspB5) is a cytosolic chaperone belonging to the small heat shock protein family, which is known to help in the folding of cytosolic proteins. Here we show that CRYAB binds the mutant form of at least two multispan transmembrane proteins (TMPs), exerting an anti-aggregation activity. It rescues the folding of mutant Frizzled4, which is responsible for a rare autosomal dominant form of familial exudative vitreoretinopathy (Fz4-FEVR), and the mutant ATP7B Cu transporter (ATP7B-H1069Q) associated with a common form of Wilson's disease. In the case of Fz4-FEVR, CRYAB prevents the formation of inter-chain disulfide bridges between the lumenal ectodomains of the aggregated mutant chains, which enables correct folding and promotes appropriate compartmentalization on the plasma membrane. ATP7B-H1069Q, with help from CRYAB, folds into the proper conformation, moves to the Golgi complex, and responds to copper overload in the same manner as wild-type ATP7B. These findings strongly suggest that CRYAB plays a pivotal role, previously undetected, in the folding of multispan TMPs and, from the cytosol, is able to orchestrate folding events that take place in the lumen of the ER. Our results contribute to the explanation of the complex scenario behind multispan TMP folding; additionally, they serve to expose interesting avenues for novel therapeutic approaches.

Pharmacological folding chaperones act as allosteric ligands of Frizzled4

“…As previously shown 26,27 , HA-Fz4-WT was localized in the Golgi complex and at the PM, whereas HA-Fz4-FEVR appeared to be trapped intracellularly, mainly in the ER ( Fig. 1b Tables 4 and 5).…”

“…Among the Fz4 mutations described 25 , the one resulting in Fz4-FEVR shows the most clearly identifiable phenotype, trapping the mutant receptor in the endoplasmic reticulum (ER) and hampering its transport to the PM of the cells 25,26 . The mutated C-terminal tail of Fz4-FEVR improperly interacts with the ER lipid bilayer, disturbing receptor stability and inducing its aggregation and retention in the ER (Fig.…”

“…1a) 27 . Notwithstanding, Fz4-FEVR localization at the PM can be rescued by overexpressing the protein chaperone α-B crystallin 26 . Fz4-FEVR can be thus included among the proteins that are responsible for conformational diseases whose phenotype can be rescued by strategies aiming to improve folding.…”

“…In humans, despite being rare, they lead to the development of familial exudative vitreoretinopathy (FEVR), a pathology resulting in aberrant vascularization of the retina during embryo development 25 . Among the mutations, the frameshift L501fsX533 of Fz4 shows autosomal dominant inheritance and causes a conformational defect by generating a different and shorter C-terminal cytosolic tail that hampers signaling of the mutant receptor (henceforth referred as Fz4-FEVR) and its correct folding and transport to the cell plasma membrane (PM) [25][26][27] .…”

Small heat shock proteins: Role in cellular functions and pathology

“…sHsps may chaperone membrane proteins αB-crystallin binds and prevents the aggregation of the mutant form of two multispan transmembrane proteins and helps them attain the proper functional folded state [76]. It prevents the formation of inter-chain disulfide bridges between the lumenal ectodomains of the aggregated mutant chains of Frizzled4 (responsible for a rare autosomal dominant form of familial exudative vitreoretinopathy (Fz4-FEVR)), enabling correct folding and appropriate compartmentalization on the plasma membrane.…”