Identification of two novel zinc finger modules and nuclear localization signal in rat spermatidal protein TP2 by site-directed mutagenesis.

Spermatidal protein TP2, which appears transiently during stages 12-16 of mammalian spermiogenesis, is a DNA condensing zinc metalloprotein with a preference to GC-rich DNA. We have carried out a detailed site-directed mutagenesis analysis of rat spermatidal protein TP2 to delineate the amino acid residues involved in coordination with two atoms of zinc. Two zinc fingers modules have been identified involving 4 histidine and 4 cysteine residues, respectively. The modular structure of the two zinc fingers identified in TP2 define a new class of zinc finger proteins that do not fall into any of the known classes of zinc fingers. Transfection experiments with COS-7 cells using wild type and the two zinc finger pocket mutants have shown that TP2 preferentially localizes to nucleolus. The nuclear localization signal in TP2 was identified to be (87)GKVSKRKAV(95) present in the C-terminal third of TP2 as a part of an extended NoLS sequence.