MEL-28/ELYS and CENP-C coordinately control outer kinetochore assembly and meiotic chromosome-microtubule interactions

Abstract

During mitosis and meiosis in the majority of eukaryotes, centromeric chromatin comprised of CENP-A nucleosomes and their reader CENP-C recruits components of the outer kinetochore to build an interface with spindle microtubules.1,2 One exception is C.elegans oocyte meiosis, where outer kinetochore proteins form cup-like structures on chromosomes independently of centromeric chromatin.3 Here, we show that the nucleoporin MEL-28 (ortholog of human ELYS) and CENP-CHCP-4 act in parallel to recruit outer kinetochore components to oocyte meiotic chromosomes. Unexpectedly, co-inhibition of MEL-28 and CENP-CHCP-4 resulted in chromosomes being expelled from the meiotic spindle prior to anaphase onset, a more severe phenotype than what was observed following ablation of the outer kinetochore.4,5 This observation suggested that MEL-28 and the outer kinetochore independently link chromosomes to spindle microtubules. Consistent with this, the chromosome expulsion defect was observed following co-inhibition of MEL-28 and the microtubule-coupling KNL-1/MIS-12/NDC-80 (KMN) network of the outer kinetochore. Use of engineered mutants showed that MEL-28 acts in conjunction with the microtubule-binding NDC-80 complex to keep chromosomes within the oocyte meiotic spindle and that this function likely involves the Y-complex of nucleoporins that associate with MEL-28; by contrast, the ability to dock protein phosphatase 1, shared by MEL-28 and KNL-1, is not involved. These results highlight nuclear pore-independent functions for a conserved nucleoporin and explain two unusual features of oocyte meiotic chromosome segregation in C.elegans: centromeric chromatin-independent outer kinetochore assembly, and dispensability of the outer kinetochore for constraining chromosomes in the acentrosomal meiotic spindle.