= S1 RNA-binding domain
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= S1 domain
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The \b[S1 RNA-binding domain] is a <protein domain> found in many RNA-associated proteins, named after its original identification in <ribosomal protein S1>. It is a reusable RNA-interaction module; its presence does not identify the entire host protein as a ribosomal component or a nuclease.<The solution structure of the S1 RNA binding domain: a member of an ancient nucleic acid-binding fold by M. Bycroft and others>[Abstract: origin of the domain name and occurrence in RNA-associated proteins]{ref}
A 1997 structural experiment examined the S1 domain of <Escherichia coli> <polynucleotide phosphorylase> using <nuclear magnetic resonance>. The resulting structure was a five-stranded antiparallel <beta barrel>. Conserved residues on one face and neighboring loops suggested an RNA-binding surface; that proposal distinguished an observed fold from an inferred contact function.<The solution structure of the S1 RNA binding domain: a member of an ancient nucleic acid-binding fold by M. Bycroft and others>[Abstract: NMR structure and putative binding face]{ref}
The 2016 <RNase R> study supplies a functional example. Altering residues in its S1 domain changed interactions with duplex <RNA>, while separate measurements examined <nucleotide> binding, conformational changes and degradation. A binding domain can therefore influence catalysis by positioning a substrate or stabilizing a productive state, without itself supplying all catalytic chemistry.<How RNase R Degrades Structured RNA: Role of the Helicase Activity and the S1 Domain by Sk Tofajjen Hossain and others>[Abstract: S1-domain mutations and designed-substrate/fluorescence comparisons]{ref}
For an architectural counting example, a hypothetical protein with two S1 domains has two copies of that module, not automatically two RNA-cleaving active sites. Domain count, binding behavior and <enzyme> activity are different observations that require different evidence.
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