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Developmental characterization of a Drosophila RNA-binding protein homologous to the human systemic lupus erythematosus-associated La/SS-B autoantigen.

Public Health Research Institute, New York, New York 10016.

ABSTRACT

Patients with humoral autoimmune diseases such as systemic lupus erythematosus and Sjögren's syndrome contain antibodies in their sera directed against certain normal cellular components such as the La/SS-B autoantigen, an RNA-binding protein believed to function as a putative processor of RNA polymerase III precursor transcripts. We have identified cDNA clones from the fruit fly Drosophila melanogaster that encode a protein displaying significant sequence homology with human La/SS-B. The fly protein (which we refer to as D-La) contains a putative ribonucleoprotein 1 (RNP1) and RNP2 RNA-binding domain. D-La also possesses a leucine zipper motif, suggesting that it may interact with itself or other proteins. Using gel retardation analysis, we show that D-La can bind RNA; in addition, we demonstrate the first reported DNA-binding activity associated with a La protein. Northern (RNA) blot analysis revealed a single 1,600-nucleotide transcript expressed throughout embryonic, larval, pupal, and adult development. Surprisingly, whole-mount in situ hybridization experiments revealed that D-La transcripts are not present in all ovarian tissues. In addition, early expression throughout the embryo is followed by a restricted pattern of mesodermal expression that is later confined to the visceral mesoderm, gonads, gut, and salivary glands. These results suggest that D-La may play a more specialized role during fly development as opposed to a rather general role inferred by its homology to La proteins from other organisms.

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