Title: Cryo-electron microscopy and single particle reconstructions of the Leishmania major ribosome and of the encephalomyocarditis virus internal ribosome entry site bound to the 40S subunit
Description: The ribosome is a macromolecular machine, present in high copy number in the cell, that synthesizes proteins from information encoded in messenger RNA. It is a universal translator, found in all life forms and in all eras recent enough to bear life. The ribosome is structurally complex and its structure is highly evolutionarily conserved; that conservation reinforces the concept that its function in executing translation is essential. As a subject of study, the ribosome lends itself well to direct imaging, as it is large, asymmetric, dynamic, and it interacts with other heterogeneous agents throughout the translation process; if we are to infer function from structure, then the most certain way to observe the ribosome’s structure is to image it as directly as possible. Cryo-electron microscopy and single particle reconstruction are appropriate tools for this endeavor, as they can produce high-resolution three-dimensional structures of ribosomes or other macromolecular samples, and they can even reveal multiple biologically relevant states of a single sample.

Although the ribosome is highly conserved in terms of its presence and core structure and functions, there is considerable variation among taxa, and the function of some of this variation is not yet understood. For example, the ribosome of the unicellular trypanosomatid parasite Leishmania major exhibits unusually large expansion segments of ribosomal RNA, as well as unusual cleavage sites in ribosomal RNA that is otherwise conserved. Here, we present a three-dimensional cryo-electron microscopy reconstruction of the 80S ribosome of Leishmania major and compare it to the available ribosome structures of closely related parasites. There is also structural variation related to the mechanism of translation: certain viruses with RNA genomes employ highly structured segments of RNA called internal ribosome entry sites to initiate translation of viral proteins on host cell ribosomes via noncanonical mechanisms. We explore one instance of this with a reconstruction of the encephalomyocarditis virus internal ribosome entry site bound with necessary protein factors to a eukaryotic 40S ribosomal subunit.