The Structure of the Karrikin-Insensitive Protein (KAI2) in Arabidopsis thaliana

Abstract

KARRIKIN INSENSITIVE 2 (KAI2) is an α/β hydrolase involved in seed germination and seedling development. It is essential for plant responses to karrikins, a class of butenolide compounds derived from burnt plant material that are structurally similar to strigolactone plant hormones. The mechanistic basis for the function of KAI2 in plant development remains unclear. We have determined the crystal structure of Arabidopsis thaliana KAI2 in space groups P21 21 21 (a = 63.57 Å, b = 66.26 Å, c = 78.25 Å) and P21 (a = 50.20 Å, b = 56.04 Å, c = 52.43 Å, β = 116.12°) to 1.55 and 2.11 Å respectively. The catalytic residues are positioned within a large hydrophobic pocket similar to that of DAD2, a protein required for strigolactone response in Petunia hybrida. KAI2 possesses a second solvent-accessible pocket, adjacent to the active site cavity, which offers the possibility of allosteric regulation. The structure of KAI2 is consistent with its designation as a serine hydrolase, as well as previous data implicating the protein in karrikin and strigolactone signalling.

Strigolactone Hormones and Their Stereoisomers Signal through Two Related Receptor Proteins to Induce Different Physiological Responses in Arabidopsis

“…The pocket of D14 is reported to be larger than that of KAI2, but both could theoretically accommodate GR24 (Bythell-Douglas et al, 2013;Guo et al, 2013;Zhao et al, 2013), consistent with the observed biological activity. However, the position of an intact SL in the active site pocket of D14 and the exact binding of KAR in KAI2 remain unclear.…”

“…The KAR signaling pathway has been partially resolved by the identification of a paralog of AtD14 in Arabidopsis known as KARRIKIN INSENSITIVE2 (KAI2; Waters et al, 2012a) and SUPPRESSOR OF MAX2 1 (SMAX1), an Arabidopsis homolog of rice D53 (Stanga et al, 2013). Like AtD14, KAI2 possesses an essential conserved catalytic triad within a hydrophobic cavity (Waters et al, 2014), and the overall structures of both proteins are generally very similar (Bythell-Douglas et al, 2013;Kagiyama et al, 2013;Zhao et al, 2013). Nonetheless, there is a clear distinction between KAI2 and AtD14 functions: whereas the former mediates seed and seedling development, the latter is involved in postjuvenile development, most notably axillary or secondary bud outgrowth.…”

A Selaginella moellendorffii Ortholog of KARRIKIN INSENSITIVE2 Functions in Arabidopsis Development but Cannot Mediate Responses to Karrikins or Strigolactones

“…Gly-133 is located on a long, protruding loop that joins the a/b-fold hydrolase domain to the cap domain of KAI2 and related proteins (Supplemental Figure 1) (Hamiaux et al, 2012;Bythell-Douglas et al, 2013;Zhao et al, 2013). This residue is the first in a short GGF motif that is absolutely conserved among this family of a/b-fold hydrolases, from bacteria to plants (Supplemental Figure 2).…”

Destabilization of strigolactone receptor DWARF14 by binding of ligand and E3-ligase signaling effector DWARF3

“…It can hydrolyze the synthetic SL GR24, albeit with an extremely low turnover rate of only about one molecule GR24 for every one molecule of D14 every three minutes [24] or less [25], and this hydrolysis has been proposed to be required for SL signaling [3,26]. Several groups reported high-resolution crystal structures of apo-D14 and its orthologs [24][25][26][27] and of KAI2 in apo- [23,24,27,28] and karrikin-bound [23] states. In contrast, attempts to co-crystallize D14 in complex with GR24 only yielded structures of D14 covalently bound to a GR24 hydrolysis intermediate [24] or weakly bound to the butenolide ring (D ring) hydrolysis product [26].…”