Sperm nuclear basic proteins of tunicates and the origin of protamines

Sperm nuclear basic proteins (SNBPs) are the chromosomal proteins that are found associated with DNA in sperm nuclei at the end of spermiogenesis. These highly specialized proteins can be classified into three major types: histone type (H-type), protamine-like type (PL-type), and protamine type (P-type). A hypothesis from early studies on the characterization of SNBPs proposed a mechanism for the vertical evolution of these proteins that involved an H1 → PL → P transition. However, the processes and mechanisms involved in such a transition were not understood. In particular, it was not clear how a molecular transition from a lysine-rich protein precursor (H1 histone) to the arginine-rich protamines might have taken place. In deuterostomes, the presence of SNBPs of the H-type in echinoderms and of protamines in the higher phylogenetic groups of vertebrates had long been known. The initial work on the characterization of tunicate SNBPs attempted to define the types and range of SNBPs that characterize this phylogenetically intermediate group. It was found that tunicate SNBPs belong to the PL-type. In this work we discuss how the study of SNBPs in the tunicates has been key to providing support to the H1 → PL → P transition. Most significantly, it was in tunicates that a potential molecular mechanism to explain the lysine-to-arginine transition was first reported.