hide
Free keywords:
cryo-electron microscopy; mitochondrial evolution; nonadaptive evolution; ribosome
Abstract:
The five macromolecular complexes that jointly mediate oxidative phosphorylation (OXPHOS) in mitochondria consist of many more
subunits than those of bacteria, yet, it remains unclear by which evolutionary mechanism(s) these novel subunits were recruited. Even
less well understood is the structural evolution of mitochondria
l ribosomes (mitoribosomes): wh
ile it was long thought that their
exceptionally high protein content would physically compensate for their uniquely low amount of ribosomal RNA (rRNA), this
hypothesis has been refuted by structural studies. Here, we pres
ent a cryo-electron microscopy structure of the 73S mitoribosome
from
Neurosporacrassa
, together with genomic and proteomic analyses of mitoribosome composition across the eukaryotic domain.
Surprisingly, our findings reveal that both structurally and compositi
onally, mitoribosomes have evolved very similarly to mitochondrial
OXPHOS complexes via two distinct phases: A constructive phase
that mainly acted early in eukaryote evolution, resulting in the
recruitment of altogether approximately 75 novel subunits, and a re
ductive phase that acted during metazoan evolution, resulting in
gradual length-reduction of mitochondrially encoded rRNAs and OXPHOS proteins. Both phases can be well explained by the
accumulation of (slightly) deleterious mutations and deletions, respectively, in mitochondrially encoded rRNAs and OXPHOS proteins.
We argue that the main role of the newly recruited (nuclear enco
ded) ribosomal- and OXPHOS proteins is to provide structural
compensation to the mutationally destabilized mitochondrially en
coded components. While the newly recruited proteins probably
provide a selective advantage owing to their compensatory nature,
and while their presence may have opened evolutionary pathways
toward novel mitochondrion-specific functions, we emphasize that
the initial events that resulted in their recruitment was non-
adaptive in nature. Our framework is support
ed by population genetic studies, and it can explain the complete structural evolution of
mitochondrial ribosomes and OXPHOS complexes, as well as many observed functions of individual proteins
