Structures and mechanism of the plant PIN-FORMED auxin transporter

Abstract

Auxins are hormones that have central roles and control nearly all aspects of growth and development in plants1–3. The proteins in the PIN-FORMED (PIN) family (also known as the auxin efflux carrier family) are key participants in this process and control auxin export from the cytosol to the extracellular space4–9. Owing to a lack of structural and biochemical data, the molecular mechanism of PIN-mediated auxin transport is not understood. Here we present biophysical analysis together with three structures of Arabidopsis thaliana PIN8: two outward-facing conformations with and without auxin, and one inward-facing conformation bound to the herbicide naphthylphthalamic acid. The structure forms a homodimer, with each monomer divided into a transport and scaffold domain with a clearly defined auxin binding site. Next to the binding site, a proline–proline crossover is a pivot point for structural changes associated with transport, which we show to be independent of proton and ion gradients and probably driven by the negative charge of the auxin. The structures and biochemical data reveal an elevator-type transport mechanism reminiscent of bile acid/sodium symporters, bicarbonate/sodium symporters and sodium/proton antiporters. Our results provide a comprehensive molecular model for auxin recognition and transport by PINs, link and expand on a well-known conceptual framework for transport, and explain a central mechanism of polar auxin transport, a core feature of plant physiology, growth and development.

WAVY GROWTH Arabidopsis E3 ubiquitin ligases affect apical PIN sorting decisions

“…Morphogenesis and adaptive growth responses in plants are defined largely by directional, intercellular auxin transport and variations therein. Polar PIN protein localization at the plasma membrane (PM) establishes directionality of auxin flow to modulate developmental programs that rely on transiently formed auxin maxima and minima [1][2][3][4] . Cis-and trans-acting regulators of PIN sorting and polarity have been established in recent years, amongst which control by PINOID (PID)/WAG AGCVIII protein kinases appears unique, as they were found to control binary, apical-vs.basal switches in PIN polarity 5 .…”

A Physcomitrella PIN protein acts in spermatogenesis and sporophyte retention

“…Auxin plays crucial roles in plant development (Santner & Estelle, 2009; Weijers & Wagner, 2016), including Physcomitrella development (Decker et al ., 2006; Menand et al ., 2007; Fujita et al ., 2008; Coudert et al ., 2017; Nemec-Venza et al ., 2022) and biotechnology (Ruiz-Molina et al ., 2022). Although PIN proteins are central in auxin transport and action (Adamowski & Friml, 2015; Sauer & Kleine-Vehn, 2019; Ung et al ., 2022), only two of the three canonical Physcomitrella PIN proteins have been fully characterized; the similar and relatively highly expressed PpPINA and PpPINB. In contrast, little was known about the function of the more divergent and less expressed PpPINC.…”

“…Canonical PIN proteins share four highly conserved motifs in the hydrophilic loop and a strong conservation in the N’- and C’ terminal transmembrane regions, while noncanonical PINs are defined by a higher variability (Bennett et al ., 2014a). Structures and mechanism of auxin transport have recently been elucidated in great detail for the Arabidopsis thaliana PIN8 protein (Ung et al ., 2022). The Physcomitrella genome encodes four PIN genes, the three canonical PpPINA , PpPINB and PpPINC , and the noncanonical PpPIND , with PpPINA and PpPINB being the most similar to each other (Bennett et al ., 2014b).…”

New Wine in an Old Bottle: Utilizing Chemical Genetics to Dissect Apical Hook Development

“…Among them, Naphthylphthalamic acid (NPA), which associates with PINs and inhibit their functions, has been thoroughly studied and widely used in the study of auxin actions [ 65 , 66 ]. Very recently, the structure of PIN8, with or without NPA binding, has been resolved with single-particle cryo-EM; this further clarified the mode-of-actions of PINs in auxin transport and could possibly guide the design of more potent regulators [ 67 ]. Etiolated Arabidopsis seedlings treated with 10 mM NPA or 1-NOA fully abolished apical hook formation ( Figure 2 ).…”