Genome Organization by Vertebrate Sperm Nuclear Basic Proteins (SNBPs)

Abstract

DNA is tightly packed in the sperm nucleus through its association with specific chromosomal proteins that are heterogeneous in size and known as sperm nuclear basic proteins (SNBPs). Despite their structural diversity, SNBPs are evolutionarily related and can be classified into three major groups or types: histone (H-type); protamine (P-type), and protamine-like (PL-type). The three types are widespread amongst vertebrates. During spermiogenesis these proteins replace the somatic histones that are present at the onset of spermatogenesis. Mammals exhibit an increased level of complexity as transition proteins (TPs) temporarily bind to DNA before being replaced by protamines in the mature sperm. The proper sequential chromatin remodeling is dependent on global post-translational modifications (PTMs) of the chromosomal proteins involved, including acetylation and phosphorylation. A transient 20nm chromatin fiber that is independent of the SNBP type is often formed. The temporally and spatially organized compaction of chromatin during spermiogenesis may be important for the understanding of post-meiotic events such as gene expression, Huntington’s disease CAG expansion, and the remnant histones that are present in the mature sperm of certain mammals, including humans. Alterations in SNBP composition result in DNA damage and infertility, underscoring the importance of these proteins for male germline genome integrity.