= U1A protein
{c}
\b[U1A protein] is an RNA-binding <protein> component of <U1 snRNP>. Its N-terminal <RNA recognition motif> binds stem-loopII of <U1 small nuclear RNA>, helping distinguish a specific RNA-binding interaction from the assembly of the entire <small nuclear ribonucleoprotein>.<Crystal structure of human spliceosomal U1 snRNP at 5.5 Å resolution by Daniel A. Pomeranz Krummel and others>[Introduction, U1A binding to stem-loopII]{ref}
A1994 <X-ray crystallography> study determined a complex of the U1A RNA-binding domain and an <RNA hairpin> at $1.92\,\mathrm{\mathring A}$ resolution. This was a domain–RNA complex, not a structure of a complete <spliceosome>.<Crystal structure at 1.92 A resolution of the RNA-binding domain of the U1A spliceosomal protein complexed with an RNA hairpin by C. Oubridge and others>[Primary title, molecular complex and structural resolution]{ref}
For an original one-site binding example, suppose a domain binds one RNA molecule with <dissociation constant> $K_d$ and free RNA concentration $[R]$. Under the stipulated equilibrium model, the bound fraction of domains is
$$
f=\frac{[R]}{K_d+[R]}.
$$
If $K_d=10\,\mathrm{nM}$, then $[R]=10\,\mathrm{nM}$ gives $f=1/2$, whereas $[R]=90\,\mathrm{nM}$ gives $f=0.9$. These invented constants illustrate <binding affinity>; they are not measured U1A values and assume free, rather than total, RNA concentration.
Binding of an isolated domain does not by itself demonstrate functional <RNA splicing>. A later U1-core crystallographic study omitted U1A from its engineered particle and added a previously known U1A–RNA structure when modeling the complete assembly. Distinguishing observed components from modeled additions is essential to interpreting that experiment.<Crystal structure of human spliceosomal U1 snRNP at 5.5 Å resolution by Daniel A. Pomeranz Krummel and others>[Structure determination and Figure5, complete-particle model]{ref}
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