While outlining the logic of front-loading in The Design Matrix, I noted how the existence of multifunctional (moonlighting) proteins would serve the needs of front-loading. In essence, a protein with multiple functions can be viewed as a protein that is packed with preadaptations ready to be more fully exploited when the proper conditions arise.
This developing paradigm has allowed me to come up with a prediction. If evolution was front-loaded, and a significant aspect of this front-loading existed as multifunctional proteins, whereby secondary or tertiary functions could be unleashed as evolution proceeded into the future, an excellent candidate for storage of some of these secondary functions would be the ribosome, the protein-synthesizing factory of the cell. This is because a designer could count on the ribosome being retained, largely unchanged, throughout billions of years of evolution because it plays such an absolutely essential role in life. If it remains largely unchanged, secondary functions can be carried into the future. Thus, I would predict that ribosomal proteins, which normally function as chaperones to fold the ribosomal RNA and hold it together to form the functioning ribosome, would also exhibit secondary functions (moonlight).
And a survey of the literature does indeed support this prediction.
To illustrate the extent of moonlighting among ribosomal proteins, let’s begin with an assembly map of the ribosomal small subunit.
Fig. 1. The revised assembly map of the 30S subunit. The 16S ribosomal RNA is shown at the top, oriented from 5´ to 3´ direction. Each of the arrows indicates an observed dependency of binding for each ribosomal protein. The primary binding proteins depend solely on interactions with 16S rRNA (top row); the secondary and tertiary binding proteins depend on prior binding of other proteins. (Source)
Next, let’s put some blue boxes around the ribosomal proteins that are found in all three domains (archaea, bacteria, and eukarya), indicating these proteins were present in LUCA:
Next, let’s put red circles around the ribosomal proteins for which there is experimental evidence to support a moonlighting role (the moonlighting roles are listed below the figure)
S2 – plays a role in oogenesis in Drosophila
S3 – DNA repair
S4 – contains ETS DNA-binding motif
S6 – possible tumor suppressor
S7 – the fold of the core is similar to that of a DNA architectural factor; binds own mRNA
S9 – a B23/NPM-binding protein required for normal cell proliferation
S10 – Transcription elongation
S12 – enhance splicing in phage
S13 – controls splicing of own pre-mRNA
S14 – inhibits transcription of own mRNA
S19 – attracts monocytes/blood cell production
S20 – participates in antitermination of transcription
So these ribosomal proteins are involving in DNA repair, transcription control, RNA processing, the cell cycle, and development.
Now let’s look at both figures side-by-side
This assembly maps involves 15 universal small subunit ribosomal proteins, and of these 15, ten have a moonlighting role (we shall explore several of these functions at a future date). What’s striking is that all proteins involved in binding to the 3’ domain of the rRNA are not only universal proteins, but also have moonlighting functions.
Those universal ribosomal proteins that don’t seem to have a moonlighting role are s17, s5, s8, s11, and s15.
Thus, the front-loading hypothesis further predicts these too will eventually be discovered to have moonlighting functions.