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.

Genome-wide identification and expression profiling of the PIN auxin transporter gene family in L. chinense

“…Functional site analysis within L. chinense PIN conserved motifs Through previous experimental veri cation and data analysis, several functional elements and sites that control PIN protein polarity, tra cking and activity have been identi ed [3,31,32]. Our multiple sequence alignments show that these elements reside for a large part within the highly conserved LcPIN sequence motifs ( Fig S2).…”

“…The structure domain and membership division of LcPIN protein PIN genes have been studied in detail in A. thaliana: they are mainly composed of two conserved hydrophobic loops in the N-and C-terminus and a variable hydrophobic loop in the middle [31]. By analyzing the transmembrane domain and conserved motifs in LcPIN proteins, we found that most of the transmembrane structures of LcPINs contain 5-9 transmembrane spirals except LcPIN6b.…”

“…Coordinated PIN-mediated auxin transport requires activation and polarity control via phosphorylation by protein kinases [38]. So far, the phosphorylation of PIN is known to be controlled by the following three protein families: AGC kinases, PROTEIN (MAP) KINASES (MPKs), and Ca2+/calmodulin-dependent protein kinase-related kinases (CRKs) [31]. We found a small number of phosphorylation sites associated with these kinases, such as S1~S3 (motif9) and T1~T3 (motif17, not all PINs have this motif), that are highly conserved (Additional le 3).…”

“…A more accurate classi cation for PIN genes was made based on the degree of HC1-HC4 sequence conservation, being: "Canonical PIN", "Noncanonical PIN". and "Semicanonical PIN" with PIN6 [31,41]. This changed classi cation is directly related to the presence or absence of certain motifs in the proteins.…”

Mutually opposing activity of PIN7 splicing isoforms is required for auxin-mediated tropic responses inArabidopsis thaliana

“…Rademacher and Offringa, 2012;Barbosa et al, 2018;Zwiewka et al, 2019). Also PIN 292 ubiquitination (on lysines) and controlled proteolytic degradation act in auxin-mediated 293 processes(Leitner et al, 2012).…”