ESSENTIAL ROLE FOR KH DOMAINS IN RNA-BINDING - IMPAIRED RNA-BINDING BY A MUTATION IN THE KH DOMAIN OF FMR1 THAT CAUSES FRAGILE-X SYNDROME

The KH domain is an evolutionarily conserved sequence motif present in many RNA-binding proteins, including the pre-mRNA-binding (hnRNP) K p rotein and the fragile X mental retardation gene product (FMR1). We as sessed the role of KH domains in RNA binding by mutagenesis of KH doma ins in hnRNP K and FMR1. Conserved residues of all three hnRNP K KH do mains are required for its wild-type RNA binding. Interestingly, while fragile X syndrome is usually caused by lack of FMR1 expression, a pr eviously reported mutation in a highly conserved residue of one of its two KH domains (lle-304-->Asn) also results in mental retardation. We found that the binding of this mutant protein to RNA is severely impa ired. These results demonstrate an essential role for KH domains in RN A binding. Furthermore, they strengthen the connection between fragile X syndrome and loss of the RNA binding activity of FMR1.