What Can We Learn about Meiosis by Studying the PCH2 in Maize?

Meiotic mutant meiN2415 identified in maize by forward genetic approach exhibits defects in chiasma resolution and chromosome segregation at anaphase I (AI). Using Bulk Segregation Analysis, we mapped the mutation to chromosome 10. A point mutation resulting in a premature stop codon was discovered in the trip13 gene encoding conserved AAA-ATPase family protein THYROID RECEPTOR-INTERACTING PROTEIN13. Expression of mutated trip13 in E.coli revealed truncated protein of ~36 kDa (the wild type PCH2 is ~60 kDa). The maize TRIP13 is a homolog of the mouse TRIP13 and the budding yeast Pch2 (Pachytene checkpoint2). Although TRIP13/Pch2 is conserved in organisms that undergo meiosis, its role is controversial. The maize pch2 mutant differs significantly from rice and Arabidopsis pch2 mutants. Transmission electron microscopy revealed incomplete synapsis. Although synapsis is delayed and incomplete, bivalent formation is not impaired in meiN2415 mutant. Mutation severely affects chromosome segregation at AI resulting in chromosome bridges due to unresolved chiasmata. Surprisingly, immunostaining meiN2415 meiocytes with antibodies against two structural component of SC lateral elements ASY1 (ortholog of yeast Hop1p) and AFD1 (ortholog of REC8, one of components of cohesion complex) as well as ZYP1, a structural component of the SC central element and a functional ortholog of yeast Zip1p, appeared to be similar to that observed in fertile plants suggesting that maize PCH2 does not regulate the initial chromosomal recruitment of these proteins. Using BiFC technique, we detected transient protein-protein interaction between PCH2 and ZYP1 in tobacco leaves infiltrated with vectors harboring PCH2 and ZYP1 coding regions.