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

Abstract

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.