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Transient aggregation of major histocompatibility complex class II chains during assembly in normal spleen cells
M S Marks1, R N Germain, J S Bonifacino
1Cell Biology and Metabolism Branch, NICHD, National Institutes of Health, Bethesda, Maryland 20892, USA.
Insights
Major histocompatibility complex class II (MHC II) subunits transiently aggregate in the endoplasmic reticulum before forming mature complexes. The invariant chain (Ii) promotes the dissociation of these aggregates.
Area of Science:
- Cell biology
- Immunology
- Protein biochemistry
Background:
- Multisubunit cell surface proteins assemble in the endoplasmic reticulum (ER).
- The precise mechanisms of protein complex assembly in the ER remain largely unelucidated.
- Major histocompatibility complex class II (MHC II) antigens are critical cell surface proteins involved in immune responses.
Purpose of the Study:
- To investigate the mechanistic details of MHC II complex assembly in the ER.
- To determine the role of the invariant chain (Ii) in MHC II assembly.
- To explore the involvement of ER chaperones in MHC II subunit aggregation.
Main Methods:
- Analysis of MHC class II alpha and beta subunits in mouse spleen cells.
- Observation of protein aggregation and dissociation dynamics in the ER.
- Investigation of the interaction between MHC II subunits, invariant chain (Ii), and ER chaperones (BiP, calnexin).
Main Results:
- MHC II alpha and beta subunits transiently aggregate in the ER post-synthesis.
- These aggregates resolve upon assembly with the invariant chain (Ii).
- In the absence of Ii, aggregates persist, indicating Ii's role in disaggregation.
- ER chaperones BiP and calnexin bind MHC II chains but do not cause significant aggregation.
Conclusions:
- MHC II subunit assembly involves a transient, high molecular weight aggregate phase in the ER.
- The invariant chain (Ii) plays a crucial role in promoting the dissociation of these preformed aggregates.
- Ii's function is to facilitate stable complex formation by resolving subunit aggregates, not solely to prevent their formation.
Abstract:
Many cell surface proteins exist as complexes of multiple subunits. It is well established that most such complexes are assembled within the endoplasmic reticulum (ER). However, the mechanistic details of the assembly process are largely unknown. We show here that alpha and beta subunits of major histocompatibility complex class II antigens in spleen cells of normal mice pass through a transiently aggregated phase in the ER prior to assembly with the invariant chain (Ii). Aggregates form immediately after synthesis and disappear concomitantly with assembly of mature alpha beta Ii complexes. In spleen cells lacking Ii, aggregates fail to be efficiently dissociated over time, implicating subunit assembly as a requirement for disaggregation. Two ER chaperones, BiP and calnexin, bind to newly synthesized class II MHC chains but do not contribute appreciably to the large size of the aggregates. Our observations suggest that some subunits of multisubunit complexes pass through a transient, dynamic high molecular weight aggregate phase during the physiological process of assembly. The results further suggest a novel role for Ii in promoting stable dissociation of preformed aggregates containing alpha and beta subunits rather than in preventing their formation.