CYTOPLASMIC RETENTION OF XENOPUS NUCLEAR FACTOR-7 BEFORE THE MID BLASTULA TRANSITION USES A UNIQUE ANCHORING MECHANISM INVOLVING A RETENTION DOMAIN AND SEVERAL PHOSPHORYLATION SITES

Xenopus nuclear factor 7 (xnf7) is a maternally expressed protein that belongs to the B-box zinc finger gene family consisting of transcript ion factors, protooncogenes, and ribonucleoproteins. Its function is r egulated by retention in the cytoplasm from oocyte maturation until th e mid blastula transition (MBT) when it reenters the nucleus. We defin ed a 22-amino acid cytoplasmic retention domain (CRD) in xnf7 that fun ctioned cooperatively with two phosphorylation sites within the xnf7 m olecule to retain the protein in the cytoplasm until the MBT. Deletion of this region or mutations in the phosphorylation sites resulted in the early entry of xnf7 into the nucleus. A mutation changing one of t he phosphorylation sites to a glutamic acid resulted in the prolonged retention of the xnf7 protein in the cytoplasm until stages 9-10, well past the MBT. Additionally, a mutant form of xnf7 possessing a second nuclear localization signal at the COOH terminus was retained in the cytoplasm. This suggests that retention of xnf7 was not due to the mas king of its NLS as is the case with NFkB and dorsal but was due to a n ovel anchoring mechanism in which the CRD interacts with an anchor pro tein. The CRD sequence is also found in another B-box zinc finger prot ein that is also retained in the cytoplasm until the MBT in the newt. Therefore, we believe that this may be an important mechanism whereby the function of a number of nuclear proteins is regulated during devel opment.