The small nuclear ribonucleoprotein SS-B/La binds RNA with a conserved protease-resistant domain of 28 kilodaltons.

SS-B/La is a nuclear protein of 48 kilodaltons with two structural domains of Mr 28,000 and Mr 23,000 generated by proteolytic cleavage. UV irradiation was used to cross-link preexisting intracellular La-RNA complexes. Subsequent protease digestion and diagonal gel electrophoresis showed that the RNA-binding site resided in the nonphosphorylated, methionine-rich 28-kilodalton domain.