The ATPase Fap7 Tests the Ability to Carry Out Translocation-like Conformational Changes and Releases Dim1 during 40S Ribosome Maturation.

Late in their maturation, nascent small (40S) ribosomal subunits bind 60S subunits to produce 80S-like ribosomes. Because of the analogy of this translation-like cycle to actual translation, and because 80S-like ribosomes do not produce any protein, it has been suggested that this represents a quality control mechanism for subunit functionality. Here we use genetic and biochemical experiments to show that the essential ATPase Fap7 promotes formation of the rotated state, a key intermediate in translocation, thereby releasing the essential assembly factor Dim1 from pre-40S subunits. Bypassing this quality control step produces defects in reading frame maintenance. These results show how progress in the maturation cascade is linked to a test for a key functionality of 40S ribosomes: their ability to translocate the mRNA⋅tRNA pair. Furthermore, our data demonstrate for the first time that the translation-like cycle is a quality control mechanism that ensures the fidelity of the cellular ribosome pool.