Argonaute CSR-1 restricts holocentromere protein CENP-A/HCP-3 localization.

Chromosome segregation errors caused by centromere malfunction can lead to chromosome instability and aneuploidy. In Caenorhabditis elegans, the Argonaute protein CSR-1 is essential for proper chromosome segregation, though the specific mechanisms are not fully understood. Here we investigated how CSR-1 regulates centromere and kinetochore function in C. elegans embryos. We found that the depletion of CSR-1 results in defects in mitotic progression and chromosome positioning relative to the spindle pole. CSR-1 knockdown does not affect centromeric histone H3 variant CENP-A/HCP-3 mRNA and protein levels, but increases the localization of HCP-3 and some kinetochore proteins onto the mitotic chromosomes. Such elevation of chromatin HCP-3 localization depends on the CSR-1 RNAi pathway upstream factor EGO-1 and CSR-1's PIWI domain activity. Our results suggest that CSR-1 restricts HCP-3 level at the holocentromeres, prevents erroneous kinetochore assembly, and thereby promotes accurate chromosome segregation. Our work sheds light on CSR-1's role in regulating deposition of HCP-3 on chromatin and centromere function in the embryos.