The Nuts and Bolts of PIN Auxin Efflux Carriers

Abstract

The plant-specific proteins named PIN-FORMED (PIN) efflux carriers facilitate the direction of auxin flow and thus play a vital role in the establishment of local auxin maxima within plant tissues that subsequently guide plant ontogenesis. They are membrane integral proteins with two hydrophobic regions consisting of alpha-helices linked with a hydrophilic loop, which is usually longer for the plasma membrane-localized PINs. The hydrophilic loop harbors molecular cues important for the subcellular localization and thus auxin efflux function of those transporters. The three-dimensional structure of PIN has not been solved yet. However, there are scattered but substantial data concerning the functional characterization of amino acid strings that constitute these carriers. These sequences include motifs vital for vesicular trafficking, residues regulating membrane diffusion, cellular polar localization, and activity of PINs. Here, we summarize those bits of information striving to provide a reference to structural motifs that have been investigated experimentally hoping to stimulate the efforts toward unraveling of PIN structure-function connections.

Keywords: PIN efflux carriers; auxin transport; protein domains; sequence motifs; subcellular trafficking.

Figure 1

Schematic representation of Arabidopsis thaliana long and short PINs. (A) Topology of PIN1. Parts of hydrophilic loop marked with gray boxes indicate approximate arrangement of predicted conserved – C and variable – V regions. The positions of highly conserved, canonical motifs HC1–HC4 are indicated with solid lines and amino acid range indications. (B) Experimentally not verified topology model of PIN5 with 10 transmembrane domains separated by short hydrophilic region. Amino acid positions in italics indicate the predicted beginning and end of short HL. The tyrosine (Tyr) aligning within the putative tyrosine motif is indicated at position 207.

Figure 2

Schematic designation of known sequences and residues in a generalized model of Arabidopsis thaliana long PINs and their role during intracellular trafficking. (A) Symbols indicate the functional elements and their approximate location in the canonical PIN model, precise positions of those motifs for particular PINs are listed on the right. (B) Simplified illustration of PIN vesicular trafficking with the functional motifs indicated at those steps of the intracellular pathway in which they play most prominent roles. The ER export signal present in the long hydrophilic loop of the PIN protein is oriented into the cytoplasm and serves as a signal for further secretion. GNOM is a regulator of protein recycling from early endosomal compartments like TGN and its inhibition by BFA impairs exocytosis. The μ-adaptin motifs in the HL participate in interaction with clathrin machinery. PIN phosphorylation status controlled by kinases (PID, D6PK) and phosphatase (PP2A) is crucial for cycling, PM localization and activity of the auxin efflux carrier. Pin1At-facilitated isomerization of proline residues in the vicinity of phosphorylation sites affects PIN1 polar localization. Cysteines present in small cytosolic loops linking the helices play a role in PIN PM diffusion and trafficking. Ubiquitylation serves as a signal for vacuolar degradation of the carrier.