The logic of front-loading allowed me to hypothesize that ribosomal proteins would moonlight. Admittedly, it is more like an expectation than a precise prediction, but what matters is that the hypothesis directed my attention with subsequent payoff. After determining that about 2/3 of the universal SSU ribosomal proteins appear to moonlight, the criterion of Rationality kicks in. That is, if it is possible to front-load alternative functions into ribosomal proteins, a good designer would attempt to maximally exploit this particular vehicle for front-loading. So if the ribosome was front-loaded where two out of three ribosomal proteins were preadapted, why not all of them? Using this logic, I then noted we should expect to find moonlighting roles for the other third of the universal SSU ribosomal proteins – s17, s5, s8, s11, and s15 – for which I could not find a potential moonlighting function.
So after writing up my original blog entry on moonlighting ribosomal proteins, I went back to the data bases to search more closely.
Matragkou CN, Papachristou ET, Tezias SS, Tsiftsoglou AS, Choli-Papadopoulou T, Vizirianakis IS. 2008. The potential role of ribosomal protein S5 on cell cycle arrest and initiation of murine erythroleukemia cell differentiation. J Cell Biochem. 104:1477-90.
Evidence now exists to indicate that some ribosomal proteins besides being structural components of the ribosomal subunits are involved in the regulation of cell differentiation and apoptosis. As we have shown earlier, initiation of erythroid differentiation of murine erythroleukemia (MEL) cells is associated with transcriptional inactivation of genes encoding ribosomal RNAs and ribosomal proteins S5 (RPS5) and L35a. In this study, we extended these observations and investigated whether transfection of MEL cells with RPS5 cDNA affects the onset of initiation of erythroid maturation and their entrance in cell cycle arrest. Stably transfected MEL cloned cells (MEL-C14 and MEL-C56) were established and assessed for their capacity to produce RPS5 RNA transcript and its translated product. The impact of RPS5 cDNA transfection on the RPS5 gene expression patterns and the accumulation of RPS5 protein in inducible transfected MEL cells were correlated with their ability to: (a) initiate differentiation, (b) enter cell cycle arrest at G(1)/G(0) phase, and (c) modulate the level of cyclin-dependent kinases CDK2, CDK4, and CDK6. The data presented indicate that deregulation of RPS5 gene expression (constitutive expression) affects RPS5 protein level and delays both the onset of initiation of erythroid maturation and entrance in cell cycle arrest in inducer-treated MEL cells.
So there is some evidence that ribosomal protein s5 is plugged into the red blood cell differentiation circuit in mammals. What is most intriguing is that expression of s5 is correlated with altered expression of the cyclin-dependent kinases. In the future, we will have to look more closely at these moonlighting functions.
So we can now update our list to include:
S5 – Potential involvement in cell cycle arrest and initiation of red blood differentiation
In summary, the logic outlined in chapter 7 of The Design Matrix allowed me to hypothesize that ribosomal proteins would moonlight. The hypothesis was successful. And the logic outlined in chapter 9, combined with the discovery of many moonlighting ribosomal proteins, allowed me to further hypothesize that ribosomal proteins not identified as moonlighters would be found to moonlight. More success.