Prtl99C Acts Together with Protamines and Safeguards Male Fertility in Drosophila

• Sperm chromatin is organized by several basic proteins and not just by protamines
• Prtl99C is pivotal for sperm chromatin compaction and male fertility in Drosophila
• Protamines cannot be replaced by Mst77F or Prtl99C
• Prtl99C and protamines act additively to form highly compact paternal chromatin

SummaryThe formation of motile spermatozoa involves the highly conserved formation of protamine-rich, tightly packed chromatin. However, genetic loss of protamine function in Drosophila and mice does not lead to significant decompaction of sperm chromatin. This indicates that other proteins act redundantly or together with protamines. Here, we identify Prtl99C as a Drosophila sperm chromatin-associated protein that is essential for male fertility. Whereas the loss of protamines results in modest elongation of sperm nuclei, knockdown of Prtl99C has a much stronger effect on sperm nuclei. Loss of protamines and Prtl99C indicates an additive effect of these proteins on chromatin compaction, in agreement with independent loading of these factors into sperm chromatin. These data reveal that at least three chromatin-binding proteins act together in chromatin reorganization to compact the paternal chromatin.

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