Cyclin-Dependent Kinase Inhibitors in Maize Endosperm and Their Potential Role in Endoreduplication

Abstract

Two maize (Zea mays) cyclin-dependent kinase (CDK) inhibitors, Zeama;KRP;1 and Zeama;KRP;2, were characterized and shown to be expressed in developing endosperm. Similar to the CDK inhibitors in Arabidopsis (Arabidopsis thaliana) and tobacco (Nicotiana tabacum), the maize proteins contain a carboxy-terminal region related to the inhibitory domain of the mammalian Cip/Kip inhibitors. Zeama;KRP;1 is present in the endosperm between 7 and 21 d after pollination, a period that encompasses the onset of endoreduplication, while the Zeama;KRP;2 protein declines during this time. Nevertheless, Zeama;KRP;1 accounts for only part of the CDK inhibitory activity that peaks coincident with the endoreduplication phase of endosperm development. In vitro assays showed that Zeama;KRP;1 and Zeama;KRP;2 are able to inhibit endosperm Cdc2-related CKD activity that associates with p13Suc1. They were also shown to specifically inhibit cyclin A1;3- and cyclin D5;1-associated CDK activities, but not cyclin B1;3/CDK. Overexpression of Zeama;KRP;1 in maize embryonic calli that ectopically expressed the wheat dwarf virus RepA protein, which counteracts retinoblastoma-related protein function, led to an additional round of DNA replication without nuclear division.

Multiple Functions of Kip-Related Protein5 Connect Endoreduplication and Cell Elongation

“…In addition, although strong overexpression of KRPs inhibits endoreduplication, moderate overexpression of KRP2 promotes it (Verkest et al, 2005). Consistently, the expression of KRPs is high in endoreduplicating tissues such as maize (Zea mays) endosperm or tomato (Solanum lycopersicum) fruit (Coelho et al, 2005;Nafati et al, 2011). Together, these results suggest that KRPs could act to maintain differentiation in addition to their role in the inhibition of cell proliferation.…”

“…Several reports have established that KRPs bind CDKs of the CDK A type and B type (Lui et al, 2000;De Veylder et al, 2001;Jasinski et al, 2002;Zhou et al, 2002), suggesting that they could all function at the G1/S transition and at the G2/M transition. Consistently, CDK complexes containing cyclin D or A can be inhibited by KRPs in vitro (Coelho et al, 2005), and overexpression of KRPs can restore the growth defects induced by D-type cyclin overexpression (Jasinski et al, 2002;Schnittger et al, 2003;Zhou et al, 2003).…”

Evolution of networks and sequences in eukaryotic cell cycle control

“…Even more distantly related homologues, such as the KRP homologues of animal Kip-Cip Cdk inhibitors, show quite clear sequence similarity over key regions known from structural biology to be central to cyclin-Cdk inhibition by p27 and known from detailed mapping studies to be critical for Cdk inhibition by KRP [95][96][97]. Figure 4 shows sequence alignments of this critical region of KRP from maize [95] and moss with animal p27, along with a presumably best-case alignment of budding yeast Sic1 and the homologous Rum1 from fission yeast [99] to the same region, based on structural considerations and modelling [52]. The alignment between the plant and animal protein is obvious and almost sure to indicate monophyly; while Sic1 and Rum1 align with each other, they do not obviously align to the p27/KRP family any better than would be expected by chance (note that even the few possibly conserved residues between Sic1 and KRPs are not generally conserved in Rum1).…”

“…A sequence alignment of animal p27 from human (Homo sapiens) and C. elegans (nematode) [98], plant KRP1 [95] from Zea mays (maize) and P. patens (moss), along with a best-case alignment of Rum1 from fission yeast (Schizosaccharomyces pombe) and Sic1 from budding yeast (Saccharomyces cerevisiae) to the same region based on structural considerations and modelling [52,98,99]. The homology between the plant KRP1 and animal p27 is likely indicative of monophyly; it is not clear that the Sic1/Rum1 alignment to KRP1/p27 is any better than would be expected by chance.…”

Regulating DNA Replication in Plants

“…The other is a plantspecific class constituted by SIAMESE (SIM) and SIM-RELATED (SMR) inhibitors (Walker et al 2000;Churchman et al 2006). Overexpression of KRP1/ICK1 and KRP2/ICK2, which are preferentially expressed in endoreplicating cells, affects the switch to endocycles in Arabidopsis (Wang et al 2000;De Veylder et al 2001;Verkest et al 2005;Weinl et al 2005), maize (Coelho et al 2005), and tomato (Bisbis et al 2006), whereas endoreplication is repressed in the sim mutants (Churchman et al 2006). Interestingly, SIM interacts with the CDKA;1, the canonical PSTAIRE-type CDK, and with cyclin D, implicating this cyclin not only in the G 1 /S transition but also in control of endocycles.…”