Assembly of the mitoribosome is largely enigmatic and involves numerous assembly
factors. However, little is known about their function and the architectural transitions
of the pre-ribosomal intermediates. Here, we solved cryo-EM structures of the human
39S large subunit pre-ribosomes, representing four distinct late state intermediates.
Besides the MALSU1 complex, we identified several assembly factors, including the
DDX28 helicase, MRM3, GTPBP10 and the NSUN4-mTERF4 complex, all of which
keep the 16S rRNA in immature conformations. The late transitions mainly involved
rRNA domains IV and V, which form the central protuberance, the intersubunit side
and the peptidyltransferase center of the 39S subunit.