Architectural DNA-binding properties of the spermatidal transition proteins 1 and 2

Mammalian spermiogenesis is characterized by replacement of somatic histones by a set of basic nuclear transition proteins thought to be actively involved in the chromatin remodeling process. The two major transition proteins of the elongating spermatids, namely TP1 and TP2, were expressed and purified using a bacterial expression system. Both topoisomerase and ligase-mediated supercoiling assays demonstrated that TP1, as well as TP2, did not produce detectable changes in the twist and/or writhe of DNA molecules upon binding. Ligase-mediated circularization assay further demonstrated that neither of the transition proteins under study produced bends in linear DNA but that they both have the capacity to stimulate oligomerization of linear DNA fragments. We further established that the transition proteins are in vitro substrates for the Ca+2-phospholipid-dependent protein kinase (PKC) as well as the cAMP-dependent protein kinase (PKA). PKC phosphorylation was found to strongly weaken the DNA-condensing ability of TP2. These results suggest that the major transition proteins represent architectural factors able to stabilize DNA in a nonsupercoiled state, thereby promoting DNA condensation.