Prenylation of the Floral Transcription Factor APETALA1 Modulates Its Function

Abstract

The Arabidopsis MADS box transcription factor APETALA1 (AP1) was identified as a substrate for farnesyltransferase and shown to be farnesylated efficiently both in vitro and in vivo. AP1 regulates the transition from inflorescence shoot to floral meristems and the development of sepals and petals. AP1 fused to green fluorescent protein (GFP) retained transcription factor activity and directed the expected terminal flower phenotype when ectopically expressed in transgenic Arabidopsis. However, ap1mS, a farnesyl cysteine-acceptor mutant of AP1, as well as the GFP-ap1mS fusion protein failed to direct the development of compound terminal flowers but instead induced novel phenotypes when ectopically expressed in Arabidopsis. Similarly, compound terminal flowers did not develop in era1-2 transformants that ectopically expressed AP1. Together, the results demonstrate that AP1 is a target of farnesyltransferase and suggest that farnesylation alters the function and perhaps specificity of the transcription factor. INTRODUCTIONProtein prenylation has been reported for several proteins that participate in growth regulation and signal transduction, cell cycle regulation, nuclear architecture, and membrane trafficking. These proteins include the Ras super family of small GTP binding proteins, the ␥ subunit of trimeric GTP binding proteins, yeast a mating factor, cyclic GMP phosphodiesterase, and type I inositol-(1,4,5)-trisphosphate 5 Ј -phosphatase (Anderegg et al., 1988;Hancock et al., 1989;Vorburger et al., 1989;Finegold et al., 1990;Anant et al., 1992;De Smedt et al., 1996) . Prenylation results in the covalent attachment of either farnesyl diphosphate (FPP) or geranylgeranyl diphosphate (GGPP) to a conserved cysteine near the C terminus of proteins by farnesyltransferase (FTase) or geranylgeranyltransferase-I (GGTase-I). The enzymes recognize a conserved CaaX box sequence motif in which C is a cysteine residue, a is usually an aliphatic amino acid, and X is any amino acid modified by FTase or, with only a few exceptions, leucine modified by GGTase-I (Schafer and Rine, 1992).Recent studies have shown that plant FTase is structurally and functionally conserved (Yalovsky et al., 1997;Rodríguez-Concepción et al., 1999b) and is required in signal transduction by abscisic acid in stomatal guard cells (Pei et al., 1998). Lack of FTase in Arabidopsis affects the control of cell division in the shoot apical meristem, resulting in developmental alterations (Yalovsky et al., 2000, this issue). The FTase mutant era1-2 (for enhanced response to abscisic acid), in which the gene for the FTase ␤ subunit ( AtFTB ) is deleted (Cutler et al., 1996), has enlarged shoot apical and floral meristems that produce larger leaves and inflorescences and an increased number of floral organs than does the wild type. Moreover, era1-2 plants are late flowering, and flowers often fail to develop (Yalovsky et al., 2000, this issue). Treatment of tobacco tissue culture cells with FTase inhibitors causes cell cycle arrest (Morehead et al., 1995), indi...

Identification and characterization of two Bamboo (Phyllostachys praecox) AP1/SQUA-like MADS-box genes during floral transition

“…In addition to early Xowering, constitutive expression of some AP1/SQUA-like genes could produce the leaves curling or other leaves development alterations in Arabidopsis (Krizek and Meyerowitz 1996;Yalovsky et al 2000;Berbel et al 2001;Chen et al 2008). In our studies, overexpression of PpMADS1 and PpMADS2 also resulted in leaf development alterations in Arabidopsis (Fig.…”

“…Some reports provided evidences that these two conserved motifs seem to be important for the function of AP1/SQUA-like proteins. For example, the CFAT/A (euAP1) motif is regarded as a farnesylation signal and to be required for correct function of AP1 (Yalovsky et al 2000). However, some proteins lacking both motifs are produced by alternative splicing, and their functions seem to be unaVected (Kyozuka et al 1997;Skipper et al 2005).…”

“…Gu et al (1998) reported that ful mutant Arabidopsis had broader cauline leaves, suggesting that FUL regulated the transcription of genes required for cellular diVerentiation during leaf development. Yalovsky et al (2000) speculated that the nonfarnesylated AP1 protein may impair the normal leaf development resulting in curling of leaves. With regard to leaf development, we may postulate that PpMADS1 is close to FUL because its overexpression caused much smaller leaves than WT, while PpMADS2 is close to AP1 because its overexpression caused curling of leaves.…”

Constitutive expression of the SAP1 gene from willow (Salix discolor) causes early flowering in Arabidopsis thaliana

“…The I region is less conserved, and the C-terminus is the most variable which is involved in ternary complex formation and transcriptional activation (Egea-Cortines et al 1999). However, neither SAP1-1 nor SAP1-2 have the prenylation motif "CFAA" at the C-terminus, which are found in AP1 and SQUA (Rodríguez-Concepción et al 1999;Yalovsky et al 2000). On the other hand, it appears that the absence of the prenylation motif is common to many AP1/SQUA homologs.…”

“…On the other hand, it appears that the absence of the prenylation motif is common to many AP1/SQUA homologs. This motif plays an important role in the determination of the function and specificity of AP1 in A. thaliana (Yalovsky et al 2000). Instead of the prenylation motif "CFAA" found in AP1 and a few other AP1/SQUA homologs, SAP1-1 and SAP1-2 have a C-terminal amino acid motif "GYGA."…”