Molecular composition of Ro small ribonucleoprotein complexes in human cells. Intracellular localization of the 60- and 52-kD proteins.

Ro small ribonucleoprotein complexes (RoRNPs) are thought to comprise several proteins, including the 60-kD Ro and the 52-kD Ro proteins, and several small RNAs, designated Y RNAs. Although RoRNPs are fairly ubiquitous in nature, their precise composition remains unknown, their function has been elusive, and their intracellular localization has been controversial. We have analyzed HeLa cell extracts by glycerol density gradient fractionation in order to determine the distribution of the individual protein and RNA components of RoRNPs. We found that 52-kD Ro was not detectable in an RNP complex with the 60-kD protein under a variety of conditions. Pretreatment of cell extracts with ribonuclease affected gradient migration of the 60-kD but not the 52-kD protein, suggesting that the latter is not complexed with RNA. The migration of the hY RNAs in these gradients closely followed that of 60-kD and not 52-kD Ro. Immunofluorescence analysis of two different cell lines with monospecific antibodies against 52- and 60-kD proteins strongly suggests that these two proteins are not present on overlapping sets of structures in vivo. We conclude that the 52-kD Ro protein is not a detectable component of the RoRNP complex under these conditions despite its reactivity with Ro autoimmune antisera.