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these are generally also loosely included under the term 'ANA'.

Small cytoplasmic RNP (scRNP)

One of these cytoplasmic antigens is called La/SS-B. It functions as a termination factor for RNA polymerase III. Therefore, La/SS-B scRNPs are composed of an RNA polymerase III transcript to which the La/SS-B protein is bound (Figure 3). The RNA transcripts include 7S RNA, 5S RNA, tRNA, U6-RNA, the Y-RNAs as well as a number of viral RNAs.

Ro/SS-A scRNPs are composed of several proteins complexed with a subset of the La/SS-B associated RNAs, the so-called Y-RNAs (Figure 3). In human cells four different Y-RNAs have been identified, together with two different Ro/SS-A proteins, designated Ro52 and Ro60. The Ro60 protein is unusual in containing both an RNP consensus motif for binding RNA and a zinc finger motif for binding to DNA. The Ro52 protein contains multiple zinc fingers and a leucine zipper sequence, but no RNP consensus motif. Most probably, this protein is complexed to Y-RNPs via binding to the Ro60 protein. The 50 kDa La/SS-B protein contains, as expected, an RNP consensus motif.

About 30% of both Ro/SS-A and La/SS-B is localized in the nucleus of the cell, which explains the nuclear immunofluorescence of anti-Ro/SS-A and anti-La/SS-B. Contrarily, Y-RNAs are predominantly

cytoplasmic. Presumbly, Ro/SS-A and La/SS-B bind to newly synthesized Y-RNA in the nucleus, after which the mature complex, termed scRNP is transported to the cytoplasm.

No function has as yet been associated with the Ro/SS-A proteins.

rRNP

Ribosomal P-proteins are phosphorylated proteins that are present, often in multiple copies, on the ribo-some. PI (19 kDa) and P2 (17 kDa) interact with the eukaryotic elongation factors EF1 and EF2 and play a role in aminoacyl-tRNA binding and polypeptide synthesis. These proteins share a 22 amino acid sequence at the C-terminal that contains an epitope recognized by so-called anti-rRNP or anti-P antibodies.

Cytoplasmic enzymes

Aminoacyl-tRNA synthetases are a group of cytoplasmic enzymes that catalyze the binding of tRNA to their respective amino acids. A number of them have been identified as antigens for a special class of ANA.

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