THE KH DOMAIN PROTEIN ENCODED BY QUAKING FUNCTIONS AS A DIMER AND IS ESSENTIAL FOR NOTOCHORD DEVELOPMENT IN XENOPUS EMBRYOS

Mutations in the mouse indicate that quaking gene function is essentia l for both embryogenesis and for development of the nervous system. Re cent isolation of the mouse quaking gene identified a putative RNA-bin ding protein containing a single KH domain. We have previously isolate d the Xenopus homolog of quaking, Xqua, and shown that the sequence is highly conserved through evolution. Here, we report experimental data on the biochemical function of the quaking protein and its role durin g development. We demonstrate that the quaking protein expressed durin g early embryogenesis, pXqua(357), can bind RNA in vitro, and we have mapped the regions of the protein that are essential for RNA binding. We present evidence that pXqua can form homodimers and that dimerizati on may be required for RNA binding. Oocyte injection experiments show that pXqua(357) is located in both the nucleus and cytoplasm. In the X enopus embryo, Xqua is first expressed during gastrulation in the orga nizer region and its derivative, the notochord. In later stage embryos , Xqua is expressed in a number of mesodermal and neural tissues. We d emonstrate that disruption of normal Xqua function, by overexpression of a dominant inhibitory form of the protein, blocks notochord differe ntiation. Xqua function appears to be required for the accumulation of important mRNAs such as Xnot, Xbra, and gsc. These results indicate a n essential role for the quaking RNA-binding protein during early vert ebrate embryogenesis.