18.7 Membrane Protein Folding and Assembly

Membrane Protein Folding and Assembly describes the process by which a membrane protein adopts its functional three-dimensional structure and, where applicable, combines with additional subunits and cofactors into a complete functional complex, covering both the timing of folding relative to insertion and the various ways assembly can go wrong. It treats successful insertion and correct orientation as necessary but not sufficient, since a protein must also fold correctly and, if it functions as part of a larger complex, assemble correctly to become genuinely functional.

Timing of Folding

Membrane Protein Cotranslational Folding

Membrane protein cotranslational folding is the adoption of structural conformation that begins while a protein is still being synthesized, occurring in tandem with cell-free cotranslational membrane insertion as the protein chain emerges and inserts into the boundary.

Membrane Protein Post-Insertion Folding

Membrane protein post-insertion folding is the adoption of structural conformation that occurs after a protein has already been fully inserted into the boundary, representing a folding process that completes separately from and after the insertion event itself.

Structural Folding Processes

Transmembrane Segment Packing

Transmembrane segment packing is the arrangement of a protein's individual membrane-spanning segments against one another within the boundary, forming the specific three-dimensional bundle or barrel structure characteristic of the protein's folded state.

Membrane Protein Domain Folding

Membrane protein domain folding is the adoption of correct structural conformation by a protein's extramembrane domains, occurring independently of, though often coordinated with, transmembrane segment packing.

Incorporating Additional Molecular Components

Membrane Protein Cofactor Binding

Membrane protein cofactor binding is the association of a required cofactor molecule with a folded protein, a step necessary for proteins whose function depends on cofactor presence beyond the polypeptide chain alone.

Membrane Protein Prosthetic Group Incorporation

Membrane protein prosthetic group incorporation is the permanent, tightly bound association of a non-protein chemical group with a folded protein, distinguished from more loosely associated cofactors by the durability of its attachment.

Multi-Subunit Assembly

Membrane Protein Subunit Assembly

Membrane protein subunit assembly is the combination of multiple individually folded protein chains into a single functional complex, applicable to proteins whose function depends on more than one polypeptide chain working together.

Membrane Protein Homooligomerization

Membrane protein homooligomerization is subunit assembly in which the combining chains are all identical copies of the same protein, forming a complex composed of repeated identical units.

Membrane Protein Heterooligomerization

Membrane protein heterooligomerization is subunit assembly in which the combining chains are distinct, non-identical proteins, forming a complex composed of different component types working together.

Membrane Protein Complex Stoichiometry

Membrane protein complex stoichiometry is the specific number of each subunit type present within a fully assembled complex, a defining structural characteristic of any multi-subunit membrane protein assembly.

Final Maturation and Failure Modes

Membrane Protein Conformational Maturation

Membrane protein conformational maturation is the final structural adjustment a protein or complex undergoes after initial folding and assembly, settling into its fully stable, functionally competent conformation.

Membrane Protein Misfolding

Membrane protein misfolding describes a protein that fails to adopt its correct three-dimensional structure, resulting in a non-functional or improperly structured outcome despite having been successfully inserted into the boundary.

Membrane Protein Incomplete Assembly

Membrane protein incomplete assembly describes a multi-subunit complex that fails to combine all of its required components, resulting in a partial structure lacking the full stoichiometry needed for function.

Membrane Protein Aggregation

Membrane protein aggregation describes multiple protein copies clumping together in an unintended, disorganized manner rather than folding or assembling correctly, representing a distinct failure mode from misfolding or incomplete assembly.

Successful Outcome

Functional Membrane Protein Complex Formation

Functional membrane protein complex formation is the successful outcome of the folding and assembly process, in which a protein or multi-subunit complex has correctly folded, incorporated any required cofactors, achieved its intended stoichiometry, and matured into a stable, functionally competent state ready to carry out its intended role within the synthetic cell boundary.