Beyond DNA Compaction: An Emerging View of the Diverse Functions of Drosophila Sperm Nuclear Basic Proteins

Abstract

While sperm of different animals exhibit morphologically distinct features, the overall themes of sperm nuclei are surprisingly similar among species. Sperm nuclei are extremely condensed and transcriptionally inert, and it has long been realized that sperm DNA in such condensed nuclei interacts with a unique set of proteins known as sperm nuclear basic proteins (SNBPs). Collectively referred to as SNBPs, this class of proteins includes evolutionarily and biochemically distinct chromosomal proteins, from nucleosomal histones to sperm-specific chromatin proteins such as protamines. The prevailing idea has been that SNBPs as a whole allow for extreme nuclear condensation and that the condensed nucleus plays an important role in efficient migration of sperm towards eggs for successful fertilization. However, this idea remains debatable due to the lack of supporting experimental evidence. Alternatively, it may be that the histone-SNBP transition serves as a mechanism to erase the epigenetic memory on sperm chromosomes so that the zygote can initiate embryogenesis with developmentally totipotent paternal chromosomes. In addition, SNBPs themselves may play active roles in sperm formation and function. For my dissertation, I used a model organism, Drosophila melanogaster, to investigate how distinct aspects of spermatogenesis and embryogenesis are regulated by MST- HMG box proteins, a group of sperm-specific chromatin proteins that derived from HMG box proteins. Also, using mutations in an MST-HMG box protein that specifically affect paternal chromosome stability in the early embryo, I studied how embryos respond to aberrant DNA structures at developmental stages that have long been thought to lack cell cycle checkpoints. Lastly, I describe a proteomic-based approach to comprehensively discover previously unidentified proteins that are likely enriched in the sperm head, including one new member of MST-HMG box family.