@article {Longo2020.07.15.204123, author = {Liam M. Longo and Jagoda Jab{\l}o{\'n}ska and Pratik Vyas and Manil Kanade and Rachel Kolodny and Nir Ben-Tal and Dan S. Tawfik}, title = {On the Emergence of P-Loop NTPase and Rossmann Enzymes from a Beta-Alpha-Beta Ancestral Fragment}, elocation-id = {2020.07.15.204123}, year = {2020}, doi = {10.1101/2020.07.15.204123}, publisher = {Cold Spring Harbor Laboratory}, abstract = {Dating back to the last universal common ancestor (LUCA), the P-loop NTPases and Rossmanns now comprise the most ubiquitous and diverse enzyme lineages. Intriguing similarities in their overall architecture and phosphate binding motifs suggest common ancestry; however, due to a lack of sequence identity and some fundamental structural differences, these families are considered independent emergences. To address this longstanding dichotomy, we systematically searched for {\textquoteleft}bridge proteins{\textquoteright} with structure and sequence elements shared by both lineages. We detected homologous segments that span the first βαβ segment of both lineages and include two key functional motifs: (i) a phosphate binding loop {\textendash} the {\textquoteleft}Walker A{\textquoteright} motif of P-loop NTPases or the Rossmann equivalent, both residing at the N-terminus of α1; and (ii) an Asp at the tip of β2. The latter comprises the {\textquoteleft}Walker B{\textquoteright} aspartate that chelates the catalytic metal in P-loop NTPases, or the canonical Rossmann β2-Asp that binds the cofactor{\textquoteright}s ribose moiety. Tubulin, a Rossmann GTPase, demonstrates the potential of the β2-Asp to take either one of these two roles. We conclude that common P-loops/Rossmann ancestry is plausible, although convergence cannot be completely ruled out. Regardless, both lineages most likely emerged from a polypeptide comprising a βαβ segment carrying the above two functional motifs, a segment that comprises the core of both enzyme families to this very day.Competing Interest StatementThe authors have declared no competing interest.}, URL = {https://www.biorxiv.org/content/early/2020/10/20/2020.07.15.204123}, eprint = {https://www.biorxiv.org/content/early/2020/10/20/2020.07.15.204123.full.pdf}, journal = {bioRxiv} }