Surprising New Member of the KNOTTED1-Like Family of Transcriptional Regulators Lacks a Homeodomain

Transcriptional regulation by DNA binding homeodomain proteins is key to development in plants, animals, and fungi. The three amino acid loop extension (TALE) family is a subgroup of homeodomain proteins that contain three extra amino acids between helix-1 and helix-2; these are represented in plants by the BEL-like (BELL) and KNOTTED1-like (KNOX) proteins (reviewed in Hake et al., 2004). The KNOX family in plants and the myeloid ecotropic viral integration site (MEIS) family in animals share both the TALE homeodomain and an N-terminal conserved MEINOX (MEIS-KNOX) domain. New work from Magnani and Hake (pages 875–887) shows the importance of protein domains other than the homeodomain. In their work, bioinformatics searches for Arabidopsis KNOX proteins revealed KNATM, which contains the conserved MEINOX domain, but no homeodomain. This protein defines a new KNOX family subgroup that is conserved in dicots.

Much as the understudy shines when the diva skips town, KNATM might reveal a principal role in development for the MEINOX domain. The lack of a homeodomain is intriguing: does KNATM sequester KNOX proteins away from DNA by heterodimer formation? Indeed, the human TALE protein Meis2 has an alternative splice form that does not include the homeodomain, and this form acts as a dominant-negative competitor with homeodomain proteins (Yang et al., 2000). In character with the starring role of KNOX proteins in development, KNATM affects patterning. KNATM is expressed in the proximal-lateral regions of organ primordia and the leaf hydathode (see figure). Within the cell, a GFP-KNATM fusion is found in both the cytoplasm and the nucleus. Plants overexpressing KNATM show multiple leaf pattern defects, including longer petioles, curled-down leaves, and shorter leaf lamina, as well as delayed bolting.

Figure 1.

KNATB-GUS fusion expression in a seedling (left) and a mature embryo with a close-up of a cotyledon hydathode (right).

Interactions with other key developmental players are also important for KNATM function. Overexpression of both KNATM and the BELL protein SAWTOOTH1 suppresses both overexpression phenotypes, suggesting an antagonistic interaction. By contrast, overexpression of KNATM and the KNOX protein BREVIPEDICELLUS (BP) gives an additive phenotype, suggesting a synergistic interaction. Yeast two-hybrid assays show that KNATM interacts with other plant BELL TALE proteins through the MEINOX domain and also with BP through an acidic coiled-coil domain, raising the possibility that KNATM might regulate transcription through interaction with a DNA binding protein. Indeed, KNATM can activate transcription when linked to a DNA binding domain. In summary, this novel and intriguing KNOX protein can dimerize with other KNOX proteins and possibly regulate transcription independently of the homeodomain. Many other intriguing questions remain for an encore. Bravo!