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2017 ; 34
(5
): 1252-1260
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Frozen in Time: The History of Proteins
#MMPMID28201543
Kovacs NA
; Petrov AS
; Lanier KA
; Williams LD
Mol Biol Evol
2017[May]; 34
(5
): 1252-1260
PMID28201543
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The ribosome is imprinted with a detailed molecular chronology of the origins and
early evolution of proteins. Here we show that when arranged by evolutionary
phase of ribosomal evolution, ribosomal protein (rProtein) segments reveal an
atomic level history of protein folding. The data support a model in which
aboriginal oligomers evolved into globular proteins in a hierarchical step-wise
process. Complexity of assembly and folding of polypeptide increased
incrementally in concert with expansion of rRNA. (i) Short random coil
proto-peptides bound to rRNA, and (ii) lengthened over time and coalesced into
?-? secondary elements. These secondary elements (iii) accreted and collapsed,
primarily into ?-domains. Domains (iv) accumulated and gained complex
super-secondary structures composed of mixtures of ?-helices and ?-strands. Early
protein evolution was guided and accelerated by interactions with rRNA. rRNA and
proto-peptide provided mutual protection from chemical degradation and
disassembly. rRNA stabilized polypeptide assemblies, which evolved in a stepwise
process into globular domains, bypassing the immense space of random unproductive
sequences. Coded proteins originated as oligomers and polymers created by the
ribosome, on the ribosome and for the ribosome. Synthesis of increasingly longer
products was iteratively coupled with lengthening and maturation of the ribosomal
exit tunnel. Protein catalysis appears to be a late byproduct of selection for
sophisticated and finely controlled assembly.
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