DNA condensation by the rat spermatidal protein TP2 shows GC-rich sequence preference and is zinc dependent

Transition protein-2 (TP2), isolated from rat testes, was recently shown to be a zinc metalloprotein. We have now carried out a detailed analysis of the DNA condensing properties of TP2 with various polynucleotides using circular dichroism spectroscopy. The condensation of the alternating copolymers by TP2 (incubated with 10 microM ZnSO4), namely, poly(dG-dC).poly(dG-dC) and poly(dA-dT).poly(dA-dT), was severalfold higher than condensation of either of the homoduplexes poly(dG).poly-(dC) and poly(dA).poly(dT) or rat oligonucleosomal DNA. Between the two alternating copolymers, poly(dG-dC).poly(dG-dC) was condensed 3.2-fold more effectively than poly(dA-dT).poly(dA-dT). Preincubation of TP2 with 5 mM EDTA significantly reduced its DNA-condensing property. Interestingly, condensation of the alternating copolymer poly(dI-dC).poly(dI-dC) by TP2 was much less as compared to that of poly(dG-dC).poly(dG-dC). The V8 protease-derived N-terminal fragment (88 aa) condensed poly(dA-dT).poly(dA-dT) to a very small extent but did not have any effect on poly(dG-dC).poly-(dG-dC). The C-terminal fragment (28 aa) was able to condense poly(dA-dT).poly(dA-dT) more effectively than poly(dG-dC).poly(dG-dC). These results suggest that TP2 in its zinc-coordinated form condenses GC-rich polynucleotides much more effectively than other types of polynucleotides. Neither the N-terminal two-thirds of TP2 which is the zinc-binding domain nor the C-terminal basic domain are as effective as intact TP2 in bringing about condensation of DNA.