SPECIFIC SEQUENCES IN THE FRAGILE-X SYNDROME PROTEIN FMR1 AND THE FXRPROTEINS MEDIATE THEIR BINDING TO 60S RIBOSOMAL-SUBUNITS AND THE INTERACTIONS AMONG THEM

Fragile X syndrome, the most common form of hereditary mental retardat ion, usually results from lack of expression of the FMR1 gene. The FMR 1 protein is a cytoplasmic RNA-binding protein. The RNA-binding activi ty of FMR1 is an essential feature of FMR1, as fragile X syndrome can also result from the expression of mutant FMR1 protein that is impaire d in RNA binding. Recently, we described two novel cytoplasmic protein s; FXR1 and FXR2, which are both very similar in amino acid sequence t o FMR1 and,which also interact strongly with FMR1 and with each other. To understand the function of FMR1 and the FXR proteins, we carried o ut cell fractionation and sedimentation experiments with monoclonal an tibodies to these proteins to characterize the complexes they form. He re, we report that the FMR1 and FXR proteins are associated with ribos omes, predominantly with 60S large ribosomal subunits. The FXR protein s are associated with 60S ribosomal subunits even in cells that lack F MR1 and that are derived from a fragile X syndrome patient, indicating that FMR1 is not required for this association. We delineated the reg ions of FMR1 that mediate its binding to 60S ribosomal subunits and th e interactions among the FMR1-FXR family members. Both regions contain sequences predicted to have a high propensity to form coiled coil int eractions, and the sequences are highly evolutionarily conserved in th is protein family. The association of the FMR1, FXR1, and FXR2 protein s with ribosomes suggests they have functions in translation or mRNA s tability.