Structural insight into the SAM-mediated assembly of the mitochondrial TOM core complex.

ß barrel outer membrane proteins (ß-OMPs) play vital roles in mitochondria, chloroplasts, and Gram-negative bacteria. Evolutionarily conserved complexes such as the mitochondrial sorting and assembly machinery (SAM) mediate the assembly of ß-OMPs. We investigated the SAM-mediated assembly of the translocase of the outer membrane (TOM) core complex. Cryo­electron microscopy structures of SAM­fully folded Tom40 and the SAM-Tom40/Tom5/Tom6 complexes at ~3-angstrom resolution reveal that Sam37 stabilizes the mature Tom40 mainly through electrostatic interactions, thus facilitating subsequent TOM assembly. These results support the ß barrel switching model and provide structural insights into the assembly and release of ß barrel complexes.

Cataract-causing mutation S228P promotes ßB1-crystallin aggregation and degradation by separating two interacting loops in C-terminal domain.

ß/γ-Crystallins are predominant structural proteins in the cytoplasm of lens fiber cells and share a similar fold composing of four Greek-key motifs divided into two domains. Numerous cataract-causing mutations have been identified in various ß/γ-crystallins, but the mechanisms underlying cataract caused by most mutations remains uncharacterized. The S228P mutation in ßB1-crystallin has been linked to autosomal dominant congenital nuclear cataract. Here we found that the S228P mutant was prone to aggregate and degrade in both of the human and E. coli cells. The intracellular S228P aggregates could be redissolved by lanosterol. The S228P mutation modified the refolding pathway of ßB1-crystallin by affecting the formation of the dimeric intermediate but not the monomeric intermediate. Compared with native ßB1-crystallin, the refolded S228P protein had less packed structures, unquenched Trp fluorophores and increased hydrophobic exposure. The refolded S228P protein was prone to aggregate at the physiological temperature and decreased the protective effect of ßB1-crystallin on ßA3-crystallin. Molecular dynamic simulation studies indicated that the mutation decreased the subunit binding energy and modified the distribution of surface electrostatic potentials. More importantly, the mutation separated two interacting loops in the C-terminal domain, which shielded the hydrophobic core from solvent in native ßB1-crystallin. These two interacting loops are highly conserved in both of the N- and C-terminal domains of all ß/γ-crystallins. We propose that these two interacting loops play an important role in the folding and structural stability of ß/γ-crystallin domains by protecting the hydrophobic core from solvent access.

Congenital microcornea-cataract syndrome-causing mutation X253R increases ßB1-crystallin hydrophobicity to promote aggregate formation.

The high solubility and lifelong stability of crystallins are crucial to the maintenance of lens transparency and optical properties. Numerous crystallin mutations have been linked to congenital cataract, which is one of the leading causes of newborn blindness. Besides cataract, several crystallin mutations have also been linked to syndromes such as congenital microcornea-cataract syndrome (CMCC). However, the molecular mechanism of CMCC caused by crystallin mutations remains elusive. In the present study, we investigated the mechanism of CMCC caused by the X253R mutation in ßB1-crystallin. The exogenously expressed X253R proteins were prone to form p62-negative aggregates in HeLa cells, strongly inhibited cell proliferation and induced cell apoptosis. The intracellular X253R aggregates could be successfully redissolved by lanosterol but not cholesterol. The extra 26 residues at the C-terminus of ßB1-crystallin introduced by the X253R mutation had little impact on ßB1-crystallin structure and stability, but increased ßB1-crystallin hydrophobicity and decreased its solubility. Interestingly, the X253R mutant fully abolished the aggregatory propensity of ßB1- and ßA3/ßB1-crystallins at high temperatures, suggesting that X253R was an aggregation-inhibition mutation of ß-crystallin homomers and heteromers in dilute solutions. Our results suggest that an increase in hydrophobicity and a decrease in solubility might be responsible for cataractogenesis induced by the X253R mutation, while the cytotoxic effect of X253R aggregates might contribute to the defects in ocular development. Our results also highlight that, at least in some cases, the aggregatory propensity in dilute solutions could not fully mimic the behaviours of mutated proteins in the crowded cytoplasm of the cells.

The N-terminal extension of ßB1-crystallin chaperones ß-crystallin folding and cooperates with αA-crystallin.

ß/γ-Crystallins are the major structural proteins in mammalian lens. The N-terminal truncation of ßB1-crystallin has been associated with the regulation of ß-crystallin size distributions in human lens. Herein we studied the roles of ßB1 N-terminal extension in protein structure and folding by constructing five N-terminal truncated forms. The truncations did not affect the secondary and tertiary structures of the main body as well as stability against denaturation. Truncations with more than 28 residues off the N-terminus promoted the dissociation of the dimeric ßB1 into monomers in diluted solutions. Interestingly, the N-terminal extension facilitated ßB1 to adopt the correct folding pathway, while truncated proteins were prone to undergo the misfolding/aggregation pathway during kinetic refolding. The N-terminal extension of ßB1 acted as an intramolecular chaperone (IMC) to regulate the kinetic partitioning between folding and misfolding. The IMC function of the N-terminal extension was also critical to the correct refolding of ß-crystallin heteromer and the action of the lens-specific molecular chaperone αA-crystallin. The cooperation between IMC and molecular chaperones produced a much stronger chaperoning effect than if they acted separately. To our knowledge, this is the first report showing the cooperation between IMC and molecular chaperones.