Functional diversity between HSP70 paralogs caused by variable interactions with specific co-chaperones.

Heat shock protein 70 (HSP70) chaperones play a central role in protein quality control and are crucial for many cellular processes, including protein folding, degradation, and disaggregation. Human HSP70s compose a family of 13 members that carry out their functions with the aid of even larger families of co-chaperones. A delicate interplay between HSP70s and co-chaperone recruitment is thought to determine substrate fate, yet it has been generally assumed that all Hsp70 paralogs have similar activities and are largely functionally redundant. However, here we found that when expressed in human cells, two highly homologous HSP70s, HSPA1A and HSPA1L, have opposing effects on cellular handling of various substrates. For example, HSPA1A reduced aggregation of the amyotrophic lateral sclerosis-associated protein variant superoxide dismutase 1 (SOD1)-A4V, whereas HSPA1L enhanced its aggregation. Intriguingly, variations in the substrate-binding domain of these HSP70s did not play a role in this difference. Instead, we observed that substrate fate is determined by differential interactions of the HSP70s with co-chaperones. Whereas most co-chaperones bound equally well to these two HSP70s, Hsp70/Hsp90-organizing protein (HOP) preferentially bound to HSPA1L, and the Hsp110 nucleotide-exchange factor HSPH2 preferred HSPA1A. The role of HSPH2 was especially crucial for the HSPA1A-mediated reduction in SOD1-A4V aggregation. These findings reveal a remarkable functional diversity at the level of the cellular HSP70s and indicate that this diversity is defined by their affinities for specific co-chaperones such as HSPH2.

DNAJB6, a Key Factor in Neuronal Sensitivity to Amyloidogenesis.

CAG-repeat expansions in at least eight different genes cause neurodegeneration. The length of the extended polyglutamine stretches in the corresponding proteins is proportionally related to their aggregation propensity. Although these proteins are ubiquitously expressed, they predominantly cause toxicity to neurons. To understand this neuronal hypersensitivity, we generated induced pluripotent stem cell (iPSC) lines of spinocerebellar ataxia type 3 and Huntington's disease patients. iPSC generation and neuronal differentiation are unaffected by polyglutamine proteins and show no spontaneous aggregate formation. However, upon glutamate treatment, aggregates form in neurons but not in patient-derived neural progenitors. During differentiation, the chaperone network is drastically rewired, including loss of expression of the anti-amyloidogenic chaperone DNAJB6. Upregulation of DNAJB6 in neurons antagonizes glutamate-induced aggregation, while knockdown of DNAJB6 in progenitors results in spontaneous polyglutamine aggregation. Loss of DNAJB6 expression upon differentiation is confirmed in vivo, explaining why stem cells are intrinsically protected against amyloidogenesis and protein aggregates are dominantly present in neurons.

Glycosylation of BRI2 on asparagine 170 is involved in its trafficking to the cell surface but not in its processing by furin or ADAM10.

Two different mutated forms of BRI2 protein are linked with familial British and Danish dementias, which present neuropathological similarities with Alzheimer's disease. BRI2 is a type II transmembrane protein that is trafficked through the secretory pathway to the cell surface and is processed by furin and ADAM10 (a disintegrin and metalloproteinase domain 10) to release secreted fragments of unknown function. Its apparent molecular mass (42-44 kDa) is significantly higher than that predicted by the number and composition of amino acids (30 kDa) suggesting that BRI2 is glycosylated. In support, bioinformatics analysis indicated that BRI2 bears the consensus sequence Asn-Thr-Ser (residues 170-173) and could be N-glycosylated at Asn170. Given that N-glycosylation is considered essential for protein folding, processing and trafficking, we examined whether BRI2 is N-glycosylated. Treatment of HEK293 (human embryonic kidney) cells expressing BRI2 with the N-glycosylation inhibitor tunicamycin or mutation of Asn170 to alanine reduced its molecular mass by ~2 kDa. These data indicate that BRI2 is N-glycosylated at Asn170. To examine the effect of N-glycosylation on BRI2 trafficking at the cell surface, we performed biotinylation and (35)S methionine pulse-chase experiments. These experiments showed that mutation of Asn170 to alanine reduced BRI2 trafficking at the cell surface and its steady state levels at the plasma membrane. Furthermore, we obtained data indicating that this mutation did not affect cleavage of BRI2 by furin or ADAM10. Our results confirm the theoretical predictions that BRI2 is N-glycosylated at Asn170 and show that this post-translational modification is essential for its expression at the cell surface but not for its proteolytic processing.