An epitope on Ki antigen recognized by autoantibodies from lupus patients shows homology with the SV40 large T antigen nuclear localization signal

Significance of Autoantibodies to Ki/SL as Biomarkers for Systemic Lupus Erythematosus and Sicca Syndrome

Anti-Ki/SL antibodies were first described in 1981 and have been associated with systemic lupus erythematosus (SLE) and Sicca syndrome. Despite the long history, very little is known about this autoantibody system, and significant confusion persists. Anti-Ki/SL antibodies target a 32 kDa protein (also known as PSME3, HEL-S-283, PA28ƴ, REGƴ, proteasome activator subunit 3), which is part of the proteasome complex. Depending on the assay used and the cohort studied, the antibodies have been reported in approximately 20% of SLE patients with high disease specificity as compared to non-connective tissue disease controls. The aim of this review is to summarize the history and key publications, and to explore future direction of anti-Ki/SL antibodies.

Clinical and serological features of 35 patients with anti-Ki autoantibodies

The objective of this study was to analyse clinical and serological associations of anti-Ki antibodies. Thirty-five patients with anti-Ki antibodies, detected by CIE, selected from laboratory routine, were studied. All patients were affected by autoimmune diseases: SLE and pSS were the most frequent diagnoses. The cohort was constituted by 27 female and eight males. Main clinical features were skin involvement (60%), xerophtalmia (48.6%), Raynaud's phenomenon (43%), photosensitivity (34%), xerostomia (31.4%). CNS involvement was present in four (11.4%) and renal disease in seven cases (20%). ANA, anti-dsDNA and RF were detected in 100%, 60% and 34.5%. In SLE, anti-Ki was detected in 6% of cases, more frequently in males compared to other SLE patients without anti-Ki (P < 0.004). Nineteen anti-Ki positive patients affected by SLE showed more frequently malar rash and multiple autoantibody specificities compared to 16 anti-Ki positive patients with other diseases (P = 0.044 and P = 0.0003, respectively). Our study confirms a preferential occurrence of anti-Ki antibodies in patients with sicca and skin involvement. Malar rash and multiple ANA specificities were significantly associated with SLE compared to other diseases in our study. Anti-Ki were detected in 6% of patients with SLE with a significant prevalence in males.

Molecular dissection of the 11S REG (PA28) proteasome activators

The proteasome activators known as 11S REG or PA28 were discovered about 10 years ago. They are homo- or heteroheptameric rings that bind to the ends of 20S proteasomes and activate cleavage of peptides but not folded proteins. In this article, we focus on structural features of three homologous REG subunits (termed alpha, beta, gamma) that contribute to their oligomerization, proteasome binding and proteasome activation. We review a number of published studies on the biochemical properties of REGs and present new results in which N-terminal sequences and sequences flanking REG activation loops have been exchanged between homologs. Characterization of these chimeras and previously constructed C-terminal chimeras reveal that N-terminal and loop flanking sequences affect oligomerization, whereas C-terminal sequences are essential for proteasome binding. None of these regions is responsible for the broad activation specificity of REGs alpha/beta versus the narrow specificity of REGgamma. Rather, mutation in a single residue lining the channel through the REGgamma heptamer changes the activation property of the gamma homolog to match that of REGs alpha and beta.