Although the bulk of the ribosome is comprised of rRNA, and indeed, many of its critical functions are mediated through RNA-RNA interactions, it is clear that protrusions of ribosomal proteins, which can be thought of as loops, hooks and fingers, function to help 'switch' the ribosome between different conformational/functional states. For example, the C-terminal extension of E. coli S13 is thought to help coordinate movement of the peptidyl-tRNA with structural rearrangements at the ribosome interface that are critical for translocation by sampling the tRNA occupancy status at the decoding center (52, 53) . The N-terminal 'hook' of yeast L10 (E. coli L16) is believed to play an active role to coordinate switching of the ribosome between the pre-and post-translocational states (16) . Ribosomal protein L2, which is intimately intertwined with multiple domains of the LSU, is thought to coordinate long range interactions between tRNAs and the ribosome (14) . Ribosomal protein L3 is of particular interest because of its function as a 'gatekeeper' to the ribosomal A-site (13) . A follow-up study suggested that two critical structures of L3, the W-finger and the N-terminal extension, function together as a 'rocker switch' to coordinate LSU associated functions (17) . The L3 basic thumb is of interest because it appears to provide the structural link in this rocker switch mechanism. Protruding roughly perpendicular from the L3 W-finger toward the intersubunit face of the LSU, it is surrounded by a cagelike structure formed by a large bulge framed between Helices H61-64, Helix 73, Helix 90 and Helix 94. It is in the center of a nexus connecting the W-finger with the L3 globular domain, the peptidyltransferase center, the aa-tRNA accommodation corridor, and the SRL. Furthermore, it is proximal to the B5 intersubunit bridge, which involves multiple contacts involving bases in Helices 62 and 64 (54) .