Leucine Zipper mediated Homodimerization of Autoantigen L7 analyzed by Electrospray Ionization Mass Spectrometry and Yeast Two Hybrid Interactions

Abstract

The eucaryotic protein L7, originally isolated from rat liver cells, associates in the cytoplasm with the large subunit of ribosomes (1). Like other riboproteins, L7 has been identified as a potent autoantigen in systemic autoimmune diseases such as Systemic Lupus Erythematosus, Mixed Connective Tissue Disease and Progressive Systemic Scleroderma, while a defined biological function has remained as yet unknown (2). The N-terminal region of protein L7 contains a sequence motif which is similar to the “Basic-Region-Leucine-Zipper (BZIP)” domain of eucaryotic transcription factors (1). In the canonical BZIP-domain the leucine zipper region promotes homo-or heterodimerization through coiled coil formation, and the BZIP dimer interacts with specific target sequences on DNA (3,4). We have previously shown that the basic domain of protein L7 interacts specifically with as yet uncharacterized cognate sites on mRNAs thereby inhibiting their cell-free translation (5), and that L7 is capable of forming homodimers according to cross-linking and gel electrophoresis studies (1), thus suggesting that L7 interacts with mRNA as a dieter. Upon transfection into Jurkat T-lymphoma cells L7 selectively suppresses the translation of two nuclear proteins and induces apoptotic cell death (6).