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. 2018 Sep;10(9):a032664. doi: 10.1101/cshperspect.a032664

Figure 5.

Figure 5.

Kinetic model of translocation. The rotation states of the small subunit (SSU) relative to the large subunit (LSU) (gray) are indicated by color intensity of the SSU body (light blue for nonrotated, N; dark blue for rotated, R). The swiveling motions of the SSU head are depicted by a color gradient from light green (classical nonswiveled SSU head position) to forest green (maximum degree of SSU head domain swiveling relative to the SSU body domain) (Belardinelli et al. 2016a). Peptidyl- and deacylated tRNA in the pretranslocation state (PRE) complex are shown in magenta and blue, respectively. EF-G (purple) is shown in two conformations, a compact (Lin et al. 2015) and an extended one after engagement with the ribosome (Ramrath et al. 2013; Zhou et al. 2014). The rates of transitions between PRE(N) and PRE(R) and PRE(N)–EF-G and PRE(R)–EF-G (kCW and kCCW at 37°C) are modified from data in Sharma et al. (2016b). The rate constants for the kinetically defined steps 1, 2, 3, 4, and 5 are from ensemble kinetics studies at 37°C (Belardinelli et al. 2016a). Step 1, EF-G binding. Step 2, GTP hydrolysis (Rodnina et al. 1997; Savelsbergh et al. 2003) and opening of the SSU because of the opposite movements of the SSU head and body domains (see inset) (Belardinelli et al. 2016a). Step 3, unlocking of the tRNA–mRNA complex on the SSU, which is rate-limiting for tRNA movement and Pi release from EF-G (Savelsbergh et al. 2003). The existence of rapid transitions between steps 3 and 4 was shown using stalled translocation intermediates (Savelsbergh et al. 2003; Holtkamp et al. 2014). Translocation intermediates (CHI1 to CHI4) are adopted from smFRET data (Adio et al. 2015). The posttranslocation (POST) state may entail further conformational substates (Wasserman et al. 2016). The light red background indicates complexes undergoing unlocking; the light green background shows complexes that move toward relocking. (Inset) Distinct timing of counterclockwise (CCW) and clockwise (CW) movements of the SSU body relative to LSU (blue symbols) and of the SSU head (green symbols) (Belardinelli et al. 2016a). (From Belardinelli et al. 2016b; with permission from Taylor & Francis and Creative Commons Public Domain licensing.)