Nuclear transport and phosphorylation of the RNA binding Xenopus zinc finger protein XFG 5-1.

XFG 5-1 is a Krüppel-type Xenopus zinc finger protein with specific RNA homopolymer binding activity in vitro. In the oocyte, the protein is distributed between nucleus and cytoplasm; the nuclear fraction, not the cytoplasm, contains phosphorylated isoform(s) of XFG 5-1. In vitro phosphorylation by use of oocyte/egg extracts or purified casein kinase II is specific to the amino-terminal portion of the protein. The carboxy-terminal zinc finger domain contains a signal sufficient for nuclear transport. Overexpression of either full length XFG 5-1 or of the carboxy-terminal portion alone, which maintains RNA binding and nuclear import activities, was achieved in Xenopus embryos by mRNA injection. This treatment did not result in impaired regulation of development, suggesting that XFG 5-1 functions in a way distinct from the mode of action exemplified in the Drosophila zinc finger protein Krüppel.