APOLLO, a testis-specific Drosophila ortholog of importin-4, mediates the loading of protamine-like protein Mst77F into sperm chromatin.

Nuclear DNA in metazoan sperm is packed with a family of small, positively charged proteins termed SNBPs (sperm nuclear basic proteins), including mammalian and Drosophila protamines. During spermiogenesis, somatic cell chromatin is taken apart and replaced with sperm chromatin in a multi-step process leading to an extraordinary condensation of the genome. During fertilization, the ova face a similarly challenging task of SNBP eviction and reassembly of nucleosome-based chromatin. Despite its importance for the animal life cycle, sperm chromatin metabolism, including the biochemical machinery mediating the mutual replacement of histones and SNBPs, remains poorly studied. In Drosophila, Mst77F is one of the first SNBPs loaded into the spermatocyte nuclei. It persists in mature spermatozoa and is essential for sperm compaction and male fertility. Here, by using in vitro biochemical assays, we identify chaperones that can mediate the eviction and loading of Mst77F on DNA, thus facilitating the interconversions of chromatin forms in the male gamete. Unlike NAP1 and TAP/p32 chaperones that disassemble Mst77F-DNA complexes, ARTEMIS and APOLLO, orthologs of mammalian importin-4, mediate the deposition of Mst77F on DNA or oligonucleosome templates, accompanied by the dissociation of histone-DNA complexes. In vivo, a mutation of testis-specific Apollo brings about a defect of Mst77F loading, abnormal sperm morphology and male infertility. We identify importin-4 ortholog APL as a critical component of sperm chromatin assembly apparatus in Drosophila. Unexpectedly, we discover that in addition to recognized roles in protein traffic, a nuclear transport receptor (importin-4) can function directly in chromatin remodeling as a dual, histone- and SNBP-specific chaperone.