Protein Complexes with Interchangeable Parts

Protein complexes are composed of multiple non-covalently linked proteins, where each component provides a different function to the complex. 

One or more of these individual proteins can be replaced by closely related variants to create an alternate complex that is functionally distinct.

As an organism evolves, the gene for a beneficial protein may duplicate within the genome. This duplicated copy is free to undergo mutations without affecting the function of the original protein. 

These mutations generate families of related proteins. This is one of the ways a cell can generate a protein complex containing interchangeable parts.  

SCF ubiquitin ligase is a multimeric protein made of five subunits.  Its function is to attach ubiquitin molecules to target proteins, marking them for degradation by proteolytic enzymes. 

One of the protein’s subunits, the F-box, which is responsible for binding to the target protein has several variants that are interchangeable. A change in this subunit enables the complex to mark different proteins for degradation. 

Saccharomyces cerevisiae, commonly known as baker’s yeast, has 11 variants of F-box subunits, enabling the ligase to mark proteins involved in a variety of cell processes.

For example, the cdc4 F-box protein targets cell cycle regulators, such as Sic1 and Far1, that inhibit enzymes that promote the cell cycle.  Their degradation allows the cell cycle to proceed. 

Variation in the F-box across organisms generates hundreds of distinct complexes with similar functions but distinct targets.