The plant hormone auxin (indole-3-acetic acid) plays a major role in plant development. Its involvement in stem elongation, root growth, differentiation, branching, fruit development, apical dominance, phototropism and gravitropism and other processes is well-known. These whole-plant physiological responses occur via signal transduction pathways that are still under investigation. The molecular mechanism of auxin responses has been shown to involve auxin-binding protein (ABP1). This protein mediates cell expansion and possibly also controls the cell cycle. In a review article published in the December 2001 issue of Trends in Plant Science (6:586-90), Candace Timpte (University of New Orleans) discusses the current status of knowledge about Auxin Binding Protein (ABP1).
In the beginning of the article, the author describes and illustrates the molecular structure of ABP1. The presence of auxin-binding protein activity in maize coleoptiles was reported twenty years ago. Maize ABP1 cDNA encodes a 201 amino-acid protein with a C-terminal KDEL ((Lys-Asp-Glu-Leu) sequence. This sequence acts as an endoplasm reticulum (ER) retention signal. The structure of ABP1 consists of a beta-barrel dimer with a free C terminal end, allowing it to interact with other proteins.
The author emphasizes the role of ABP1 in cell elongation and in embryonic development. ABP1 has been cloned in Arabidopsis, and disruption of this gene by a T-DNA insertion is a lethal event. Embryo arrest occurred at the globular stage and was correlated with defective cross wall orientation and failure of cells to elongate. Complementation of the knockout line with a functional copy of ABP1 rescued the mutant and allowed development to continue. These results suggest that ABP1 plays a significant role in early embryonic development. However, it is not known whether ABP1 establishes individual cell or embryo polarity or cell elongation. In ABP1 loss-of-function BY-2 tobacco cell lines, auxin-induced elongation was abolished. In contrast, over-expression of ABP1 in transgenic tobacco plants induced cell elongation throughout leaves, rather than just at leaf tips as in wild type plants. These results indicate that ABP1 may mediate cell expansion.
The author describes other possible functions of ABP1. Analysis of leaf cells in transgenic tobacco lines over-expressing ABP1 indicated the presence of twice as many nuclei in G2 phase compared to the wild-type, suggesting an involvement in cell cycle regulation. In another experiment, a tobacco BY2 cell line was generated that expressed an antibody to ABP1. This antibody binds ABP1 and blocks its activity in vivo. Cells from this line arrested at G1, further indicating that ABP1 might function in cell cycle checkpoint regulation.
A number of studies have been conducted on the involvement of ABP1 in an auxin-induced plasma membrane hyperpolarization response. Different versions of peptides corresponding to the C-terminus of ABP1 are capable of inducing hyperpolarization. Antibodies that bind various regions of ABP1 have also been used in hyperpolarization assays, and only those that bind to the C-terminus affect the response. The author also describes structural studies on ABP1. Recently, the protein has been crystallized and the structure has been solved to a resolution of 1.9 angstroms. The data indicate that native ABP1 consists of two glycosylated homodimers in asymmetric units, consistent with a previously predicted beta barrel structure.
The localization of APB1 has caused a great deal of debate, as the majority of ABP1 has been found within the ER (as would be expected due to the presence of an ER retention signal), but a hormone receptor would be expected to reside on the plasma membrane. Indeed, a number of studies have indicated that a portion of the ABP1 pool is localized to the plasma membrane, where it is involved in an auxin-mediated membrane hyperpolarization response. ABP1 is assumed to either bind to a plasma membrane docking protein or directly interact with an ion channel(s). However, no docking protein has yet been identified. Structural studies have indicated that conformational changes in ABP1 occur upon binding to auxin, and the author suggests that such changes might influence the ability of ABP1 to bind to a docking protein or to transmit the signal. With regards to the proportion of the ABP1 pool found at the ER and Golgi, the author suggests that these molecules might interact with a transmembrane protein partner to regulate the secretion of cell wall components necessary for cell expansion.
Recent studies have shown that overproduction of the plant heterotrimeric G-alpha protein induces cell division, mimicking the auxin-induced cell division response. This suggests that the auxin signaling pathway controlling cell division might involve heterotrimeric G-proteins.
Although there is ample evidence to show the involvement of ABP1 in plant development, the exact mechanisms of its function are unclear, and a number of challenges remain. The identity of a hypothesized docking protein has not yet been determined. There are a large number of hormone signals which orchestrate plant development, but the exact mechanism of their operation and how ABP1 fits into signaling cross-talk is not known. The localization of most ABP1 molecules in the ER and Golgi is at odds with its plasma membrane site of action, requiring further study. The author concludes that, in spite of ABP1 being a crucial component of auxin signaling, additional research is needed to address these and other outstanding questions.
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