The Core of Eukaryotic Ribosomal Protein uS19 Functions as a Pivot Point Enhancing Eukaryotic Ribosome Flexibility

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While most ribosomal elements are highly conserved in the three domains of life, over the course of evolution, significant differences have emerged as ribosomes have been subjected to different types of selective pressure. In prokaryotes and archaea, a single small subunit protein, uS13, partners with H38 (the A-site finger) and uL5 to form the B1a and B1b/c bridges, respectively. In eukaryotes, it appears that the small subunit component was split into two separate proteins during the course of evolution. One of these, also known as uS13 (previously known as S18), only participates in bridge B1b/c with uL5 in eukaryotes (previously known as L11). The other, called uS19 (previously known as S15) is the small subunit partner in the B1a bridge with H38. Here, poly-alanine mutants of the uS19/Us13 interface of uS19 were used to elucidate the evolutionary advantage of this split. A previously described chemical protection profile of the B7a bridge was utilized in order to determine the ribosomal rotational status of the selected mutants. rRNA structure probing analyses reveal that uS19/uS13 interface mutations shift the ribosomal rotational equilibrium toward the unrotated state. This perturbation of ribosomal rotational equilibrium also affected the ribosomes affinity for two intrinsic ligands: unrotated ribosomes exhibit increased affinity for ternary complex and disfavor binding of the translocase, eEF2. We posit that this mutation causes the normally flexible head region of the small subunit to "stiffen", thereby decreasing the ribosomes range of motion. A model is presented in which residues L112GH114 at the uS19/uS13 interface act as the ball in a "ball-and-socket joint", providing the increased flexibility required in the head region of the eukaryotic SSU as a consequence of the evolutionary process.