Table 1.
Exaptation of proteins and RNAs
| Ancestral functiona | Exapted function(s) | |
|---|---|---|
| Protein | ||
| Cytosolic ribosomal proteins (Wang et al. 2015; Lu et al. 2015) | Structure and assembly of the ribosome | In transcription, cell growth and proliferation, apoptosis, mRNA splicing, DNA repair, cellular development, and cellular differentiation |
| Ribonuclease III (Petrov et al. 2019) | Cleavage of double-stranded RNA | As a mitochondrial ribosomal protein |
| MutT (Petrov et al. 2019) | Repair of DNA containing 8-oxoguanine | As a mitochondrial ribosomal protein |
| Aminoacyl-tRNA synthetases (Guo and Schimmel 2013) | Covalently link amino acids to their cognate tRNAs | In metabolism, development, angiogenesis, tumorigenesis, immune response, neuronal function, and inflammation |
| Glyceraldehyde-3-phosphate dehydrogenase (GAPDH) (Singh and Bhalla 2020) | Catalyzes the sixth step of glycolysis, the conversion of d-glyceraldehyde 3-phosphate to 3-phospho-d-glyceroyl phosphate | In apoptosis, iron transport, membrane fusion, transcriptional regulation, vesicle transport, and cellular response to oxidative stress and hypoxia |
| Argininosuccinate lyase (Piatigorsky 2003; Gavelis et al. 2017) | Catalyzes the fourth step of the urea cycle and is involved in the biosynthesis of arginine | Light focusing δ-crystallins in birds and reptiles |
| Lactate dehydrogenase (Hendriks et al. 1988) | Catalyzes the reversible conversion of pyruvate to lactate and of NADH to NAD+ | Light focusing ε-crystallins in birds and reptiles |
| Pancreatic trypsinogen (Chen et al. 1997) | A zymogen of trypsin, a digestive protease | An antifreeze protein in cold water fishes |
| Transposase (Fugmann 2010) | Cut-and-paste transposition | A V(D)J recombinase, which rearranges immunity-related genes |
| Protein of an endogenous retrovirus (Cornelis et al. 2015) | Retrovirus envelope formation | A mediator of placentation in mammals |
| RNA | ||
| tRNAs (Luchetti and Mantovani 2013; Robeck et al. 2016) | Translational adaptors | BC1 and many additional short interspersed nuclear elements (SINEs) in animals with many functions |
| tRNAval and tRNAPhe (Greber et al. 2014; Rorbach et al. 2016; Brown et al. 2017) | Translational adaptors | Replacements for 5S rRNA in mammalian mitochondrial ribosomes |
| tRNA fragments (Schorn and Martienssen 2018; Magee and Rigoutsos 2020) | Translational adaptors | Guides to silencing of mRNAs by binding to argonaute and PIWI proteins, inhibitors of retrotransposition by binding to the primer binding site of LTR-retrotransposons, inhibitors of replication of retroviruses and LTR-retroelements, inhibitors of translation, and mediators of epigenetics |
| Dormant transposon (Ellis et al. 2018; Brosius 2019) | Transposon | Post-transcriptional regulation |
aIn several cases the characterization of ancestral versus exapted/adapted functions (i.e., the polarity) is tentative. Ancestry is relative to more recent exapted functions and should not be interpreted to imply ultimate ancestry