The structure of the eukaryotic ribosome derived from cryo-EM. (A) The atomic model for the yeast 80S ribosome was obtained by building on the X-ray structure of the E. coli ribosome using rRNA modelling of expansion segments, homology modelling of proteins for which there are bacterial counterparts, and placement of some extra 80S proteins whose structure is known . Experimental cryo-EM densities corresponding to 40S and 60S subunits and eEF2 for Thermomyces lanuginosus, used as constraints for modelling, are shown in transparent yellow, blue and red, respectively. (B) Same as (A) but with extra proteins highlighted (orange, 40S; magenta, 60S), for which the structures are known and which could be located through cross-linking or by exhaustive computational search. (Note that helices marked rpL19e, rpL21e, rpL7, rpL16, and rpL16 have bacterial/archaeal homologues represented in the available X-ray structures for the largest portion of the proteins.) Marked are only pieces (a-helices) supported by EM density proximal to the corresponding protein and by secondary structure predictions.) Likewise, rRNA expansion segments and 5.8S rRNA are highlighted (blue, 40S; green, 60S). In (B), eEF2 has been omitted for clarity. The illustration in (A) was reproduced from Frank (2009).