A proline-rich protein binds to the localization element of Xenopus Vg1 mRNA and to ligands involved in actin polymerization

A 340 nucleotide element within the 3' untranslated region of Vg1 mRNA dete rmines its localization to the vegetal cortex of Xenopus oocytes. To identi fy protein factors that bind to this region, we screened a cDNA expression library with an RNA probe containing this sequence. Five independent isolat es encoded a protein (designated Prrp for proline-rich RNA binding protein) having two RNP domains followed by multiple polyproline segments. Prrp and Vg1 mRNAs are co-localized to the vegetal cortex of stage IV oocytes, subs tantiating an interaction between the two in vivo, Prrp also associates wit h VegT mRNA, which like Vg1 mRNA uses the late localization pathway, but no t with Xcat-2 or Xwnt-11 mRNAs, which use the early pathway. The proline-ri ch domain of Prrp interacts with profilin, a protein that promotes actin po lymerization, Prrp can also associate with the EVH1 domain of Mena, another microfilament-associated protein. Since the anchoring of Vg1 mRNA to the v egetal cortex is actin dependent, one function of Prrp may be to facilitate local actin polymerization, representing a novel function for an RNA bindi ng protein.