USTC reveals the mechanism of auxin transport

Why are sunflowers always blooming with their “smile faces” to the Sun? This fascinating phenomenon is controlled by auxin existing in plants.

Auxin, a plant hormone, is involved in regulating almost every process of plant growth and development, such as embryonic development, tissue differentiation, phototropic, and gravitropic responses. A distinct characteristic of auxin is its directional cell-to-cell transport, namely polar auxin transport (PAT), in which the PIN-FORMED (PIN) family proteins play an important role. Specific PIN members are asymmetrically localized at the plasma membrane, and their polarity determines the directionality of auxin flow. However, their structures and transport mechanism remain to be uncovered.

In a recent study, a research team led by Prof. SUN Linfeng from the University of Science and Technology of China (USTC) of the Chinese Academy of Sciences and his collaborators investigated the PIN1 protein, a member of the auxin “transporter”. They revealed the high-resolution structures of the PIN1 protein, as well as the structures when PIN1 is bound to an inhibitor, N-1-naphthylphthalamic acid (NPA), and its substrate auxin, indole-3-acetic acid (IAA). The mechanism of PIN1 transporting auxin is also elucidated through functional analysis. This work was published in Nature.  

In this study, Prof. SUN’s team investigated the Arabidopsis PIN1, a canonical member of the PIN family, set up a new, radioisotope-based functional assay system, and studied the transport activity of PIN. The transport process is activated by kinases and inhibited by NPA. This operator-friendly assay system used mammalian HEK293F cells that are easier to culture and suitable for protein expression, providing a new method for studying auxin transport.

To solve the problem of unstable conformation and small molecular weight of PIN1 protein, the team synthesized and screened nanobodies targeting PIN1 protein using an in vitro nanobody synthesis platform. Eventually, they achieved a structure of PIN1 in complex with a nanobody at a resolution of 3.0 Å by cryo-electron microscopy single particle analysis. Hence, the researchers for the first time revealed the three-dimensional structure of the canonical member of PIN family.

Furthermore, the team analyzed the complex structure of PIN1 bound to IAA auxin or NPA inhibitor and proposed a model for PIN1-mediated auxin efflux and inhibition by NPA.

This research lays a crucial foundation for understanding the transport and regulation of auxin, as well as the design of herbicides and growth regulators targeting PIN for agricultural use.