La/SSB is a phosphoprotein that associates with various small RNA molecules. It has been found that the primary phosphorylation site of the molecule during various physiological processes is in Ser366.
To determine whether the phosphorylation state of Ser366 could affect the antigenicity and the recognition of the protein by antibodies from patients with primary Sjögren's syndrome (pSS).
Peptides 349–368aa and phos349–368aa (with the Ser366 residue phosphorylated) were synthesized. Sera with anti-La specificity from 30 patients with pSS and sera from 19 normal individuals were examined against the two synthetic peptides in ELISA. The antibody specificity against the epitopes was tested with homologous and heterologous inhibition assays.
Of pSS sera 23% reacted against the 349–368aa peptide. Sera binding to unphosphorylated peptide reacted also with phos349–368aa. Although the same sera gave a positive reaction against both peptides, the optical density values received from antibodies to phos349–368aa were higher, indicating a higher concentration or stronger affinity. When phos349–368aa was used as soluble inhibitor, in homologous inhibition the reactivity was almost completely abolished (92%). In contrast, when the unphosphorylated peptide was used as inhibitor, the reactivity of sera against phos349–368aa was only partially reduced (35%), indicating that sera from these patients possess two distinct groups of antibodies: one against the unphosphorylated and one against the phosphorylated epitope.
The phosphorylation of the serine366 residue resulted in a significant increase in antibody binding on epitope 349–368aa of La/SSB. These observations might explain the increased antigenicity of La/SSB autoantigen in various pathological situations in which phosphorylation may occur.