Exposing the subunit diversity and modularity of protein complexes by structural mass spectrometry approaches.

Although the number of protein-encoding genes in the human genome is only about 20 000 not far from the amount found in the nematode worm genome, the number of proteins that are translated from these sequences is larger by several orders of magnitude. A number of mechanisms have evolved to enable this diversity. For example, genes can be alternatively spliced to create multiple transcripts; they may also be translated from different alternative initiation sites. After translation, hundreds of chemical modifications can be introduced in proteins, altering their chemical properties, folding, stability, and activity. The complexity is then further enhanced by the various combinations that are generated from the assembly of different subunit variants into protein complexes. This, in turn, confers structural and functional flexibility, and endows the cell with the ability to adapt to various environmental conditions. Therefore, exposing the variability of protein complexes is an important step toward understanding their biological functions. Revealing this enormous diversity, however, is not a simple task. In this review, we will focus on the array of MS-based strategies that are capable of performing this mission. We will also discuss the challenges that lie ahead, and the future directions toward which the field might be heading.