Haspin is a protein kinase known to phosphorylate histone H3 at threonine 3 (H3T3ph) during mitosis. This histone mark (H3T3ph) aids in centromeric recruitment of Aurora B kinase, which is essential for regulation of kinetochore microtubule attachment, sister chromatid cohesion, the mitotic checkpoint and cytokinesis. However, haspin's functions outside of mitosis remain less well understood. Haspin was identified due to its high expression in mouse testis, but its role in this tissue has never been characterized (Tanaka et al. 1999). Its inhibition affects chromosome condensation in mouse oocytes and asymmetric histone segregation in the Drosophila germline (Nguyen et al. 2014, Xie et al. 2015), and haspin protein remains throughout interphase, when overexpression can halt progression through G1. Despite these various roles and cell types, only a single substrate of haspin, H3T3ph, has been identified. How this mark leads to the variety of haspin functions, or whether there are other substrates remains unclear. Further, the activity of haspin's atypical C-terminal kinase domain has been shown to be regulated by residues in the N-terminus in human and frog haspin (Ghenoiu et al. 2014), but the N-terminal domains are poorly conserved, so it is not clear if this auto-regulatory mechanism is common. Unlike most eukaryotes, which have a single haspin gene, C. elegans has had a dramatic expansion to form a haspin gene family that includes two genes,
hasp-1 and
hasp-2, with closest homology to mammalian haspin, and over 10 other paralogs with lower homology (Higgins 2001). This expansion of the gene family raises two interesting possibilities. First, some haspin-like genes in C. elegans may have roles independent of phosphorylation of H3T3ph. Second, C. elegans haspins may have undergone differentiation and sub-functionalization such that individual paralogs control subsets of haspin functions. We aim to understand the function and regulation of haspin kinases by systematically examining the localization and loss-of-function of haspin paralogs. We confirmed that an existing putative-null allele of
hasp-1 is sterile due to a germline proliferation defect, consistent with its expected role in mitotic progression. In addition, we have generated two loss-of-function alleles in
hasp-2 using CRISPR/Cas9.
hasp-2 homozygotes show no dramatic loss of oocyte or sperm fertility, and H3T3ph is still present on mitotic and meiotic nuclei. However, the mutants have a slight viability defect, which we are currently investigating further. The cause of viability defects in
hasp-2 mutants, and the extent to which
hasp-2 function is redundant with other haspin paralogs, may lead us to new roles for this important family of kinases.