The Arabidopsis Cdk1/Cdk2 homolog CDKA ;1 controls chromosome axis assembly during plant meiosis

Abstract

Meiosis is key to sexual reproduction and genetic diversity. Here, we show that the Arabidopsis cyclin‐dependent kinase Cdk1/Cdk2 homolog CDKA;1 is an important regulator of meiosis needed for several aspects of meiosis such as chromosome synapsis. We identify the chromosome axis protein ASYNAPTIC 1 (ASY1), the Arabidopsis homolog of Hop1 (homolog pairing 1), essential for synaptonemal complex formation, as a target of CDKA;1. The phosphorylation of ASY1 is required for its recruitment to the chromosome axis via ASYNAPTIC 3 (ASY3), the Arabidopsis reductional division 1 (Red1) homolog, counteracting the disassembly activity of the AAA+ ATPase PACHYTENE CHECKPOINT 2 (PCH2). Furthermore, we have identified the closure motif in ASY1, typical for HORMA domain proteins, and provide evidence that the phosphorylation of ASY1 regulates the putative self‐polymerization of ASY1 along the chromosome axis. Hence, the phosphorylation of ASY1 by CDKA;1 appears to be a two‐pronged mechanism to initiate chromosome axis formation in meiosis.

Closure motif-mediated state changes of the HORMA protein ASY1

“…In asy1 mutants harboring the full length reporter, ASY1:GFP accumulated in the nuclei of male meiocytes during meiotic prophase I, as reported previously (18) (Fig. 1B, Table 1).…”

“…Previous experiments suggested that the closure motif is involved in the chromosome association of ASY1 (17,18,20). To test this, we introduced the previously generated deletion construct of ASY1 (PROASY1:ASY1 1-570 :GFP, designated ASY1 ∆closure :GFP) in which the closure motif comprising the last 25 amino acids (aa) of ASY1 (596 aa in total) was removed from a functional reporter construct (PROASY1:ASY1:GFP, called ASY1:GFP) (18), into asy1 mutants ( Fig.…”

“…If this were the case, the chromosomal removal of the wild-type version of ASY1 would also be likely affected by the presence of ASY1 ∆closure -NLS:GFP. To test this hypothesis, we introduced ASY1 ∆closure -NLS without the GFP tag into wild-type plants expressing a full-length version of the ASY1 functional reporter (PROASY1:ASY1:RFP, called ASY1:RFP) together with a ZYP1b reporter (PROZYP1b:ZYP1b:GFP, called ZYP1b:GFP), generated previously (18,22).…”

“…However, all meiotic HORMADs studied, including Hop1, HORMAD1/2 and ASY1 also contain a closure motif at their own C-terminus, and because of this, these HORMADs tend to fold into a HORMA domain-closure motif bound state called self-'closed' state (12,15,17,18). This self-closed state of HORMADs is expected to block the binding of their HORMA domain with the closure motifs of the linker proteins such as ASY3, raising the question of how this interaction is formed in vivo (15,17,18). It has been proposed that the conformational conversion from a self-closed to the 'unlocked' state might be mediated by the conserved AAA+ ATPase PCH2/TRIP13 protein (15,17,19).…”

“…The meiotic HORMADs undergo a highly dynamic assembly and disassembly process, which is essential for the homologous chromosome However, how the chromosomal assembly of meiotic HORMADs is controlled is much less clear. Recently, we have shown that PCH2 is also implicated in nuclear targeting of ASY1 during early prophase (18). However, nothing is currently known about how PCH2 controls this process.…”