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HLA-DR beta chains enter into an aggregated complex containing GRP-78/BiP prior to their degradation by the pre-Golgi
Abstract:
HLA class II molecules are membrane proteins which are assembled in the endoplasmic reticulum shortly after synthesis of the alpha and beta and invariant chain (Ii) monomers. DR beta chains, in the absence of DR alpha, are rapidly and completely degraded by the pre-Golgi degradative pathway. Here we have examined those factors which target DR beta chains for degradation in a DR alpha deficient cell line, 9.22.3. The DR beta monomers in 9.22.3 were initially incorporated into a proteinaceous complex containing BiP. With time, the DR beta complexes were further aggregated. In wild type cells, which can assemble DR alpha-beta dimers, the secondary phase of aggregation of DR beta was not seen. Additional evidence that aggregation of DR beta in 9.22.3 cells was progressive was that a more mature form of DR beta was found exclusively in the largest DR beta complexes. Furthermore, the most highly aggregated DR beta chains were degraded more rapidly than bulk DR beta chains. These data suggest that DR beta aggregates are intermediates in the pre-Golgi pathway of DR beta degradation. They further suggest that formation of large DR beta aggregates is a proximal event to DR beta degradation. We conclude that DR beta chains are targeted for degradation as a consequence of a change of state, coincident with their aggregation into slow forming, high molecular weight complexes.
Insights
Human Leukocyte Antigen (HLA) class II DR beta chains aggregate and form large complexes, marking them for degradation in the endoplasmic reticulum. This aggregation is a key step in the pre-Golgi pathway for DR beta chain breakdown.
Area of Science:
- Immunology
- Cell Biology
- Protein Degradation
Background:
- Human Leukocyte Antigen (HLA) class II molecules are crucial for immune responses.
- Proper assembly of HLA class II alpha and beta chains with the invariant chain (Ii) occurs in the endoplasmic reticulum.
- DR beta chains, lacking DR alpha, are rapidly degraded via a pre-Golgi pathway.
Purpose of the Study:
- To investigate the factors targeting DR beta chains for degradation in a DR alpha-deficient cell line.
- To elucidate the role of protein aggregation in the pre-Golgi degradation of DR beta chains.
Main Methods:
- Utilized a DR alpha-deficient cell line (9.22.3) to study DR beta chain behavior.
- Analyzed protein complex formation and aggregation states of DR beta monomers.
- Compared DR beta aggregation in wild-type versus DR alpha-deficient cells.
Main Results:
- DR beta monomers initially formed complexes with BiP in the absence of DR alpha.
- Progressive aggregation of DR beta complexes was observed over time in 9.22.3 cells.
- Highly aggregated DR beta chains were degraded more rapidly, indicating aggregation as an intermediate step.
Conclusions:
- DR beta aggregation into high molecular weight complexes precedes and facilitates pre-Golgi degradation.
- A change in the state of DR beta, specifically aggregation, targets it for degradation.
- The findings reveal a novel mechanism for regulating HLA class II beta chain stability.