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 transcription factors, protooncogenes, and ribonucleoproteins. Its function is regulated by retention in the cytoplasm from oocyte maturation until the mid blastula transition (MBT) when it reenters the nucleus. We defined a 22-amino acid cytoplasmic retention domain (CRD) in xnf7 that functioned cooperatively with two phosphorylation sites within the xnf7 molecule 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 the 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 masking of its NLS as is the case with NFkB and dorsal but was due to a novel anchoring mechanism in which the CRD interacts with an anchor protein. The CRD sequence is also found in another B-box zinc finger protein 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 development.