Intracellular localization and nucleocytoplasmic transport of Ro RNP components

The Ro and La autoantigens, which were originally identified by sera from autoimmune patients, consist of RNA-protein complexes (RNPs), in which the protein components carry most of the autoantigenic determinants. In human cells, Ro RNPs consist of one of four small RNA molecules, termed hY1, hY3, hY4 and hY5 [35], associated with several proteins, Ro60, Ro52 and La [8, 35, 106] and possibly other, yet unidentified polypeptides. Controversial data have been published on the association of the protein calreticulin with Ro RNPs [53, 58, 75, 78], but recently in vitro evidence has been obtained that non-phosphorylated calreticulin is able to interact with hY RNAs directly [17]. The physiological significance of this interaction remains to be established. In addition to its association with hY RNAs, La is (in most cases transiently) associated with all other RNA polymerase III transcripts. The hY RNAs are the only known cellular RNAs stably bound by La. While the Ro RNPs were found to reside mainly in the cytoplasm, the other La RNPs are mainly nuclear. In this review recent progress made on the intracellular localization and transport of Ro RNP components and of assembled Ro RNPs will be discussed.