PIN proteins

Family of transport proteins

This article is about auxin transport in plants. For Human gene Peptidyl-prolyl cis-trans isomerase NIMA-interacting 1, see PIN1.

Peptidyl-prolyl cis-trans isomerase Pin1

Identifiers
Organism| Arabidopsis thaliana
Symbol| PIN1
Entrez| 816316
PDB| 1J6Y
RefSeq (mRNA)| NM_127360.4
RefSeq (Prot)| NP_179395.1
UniProt| Q9SL42
Other data
Chromosome| 2: 7.84 - 7.84 Mb
| Search for

Structures| Swiss-model
Domains| InterPro

PIN proteins are integral membrane proteins in plants that transport the anionic form of the hormone auxin across membranes.[1][2] The discovery of the initial member of the PIN gene family, PIN1, occurred through the identification of the pin-formed1 (pin1) mutation in Arabidopsis thaliana. This mutation led to a stem that lacked almost all organs, including leaves and flowers.[3]

Most of the PIN proteins (e.g. PIN1/2/3/4/7 in the model plant Arabidopsis thaliana) localize at the plasma membrane (PM) where they serve as secondary active transporters involved in the efflux of auxin.[4] The PM-localized PIN proteins show asymmetrical localizations on the membrane and are, therefore, responsible for polar auxin transport. Some other members of the PIN family (e.g. PIN5 and 8 in Arabidopsis) localize mostly at the ER-membrane or have a dual PM and ER localization (e.g. PIN6 in Arabidopsis). These PIN proteins regulate the partitioning of auxin within the cell.

The PM-localized PIN proteins physically interact with a few members of the large PGP family of transporters that also work as auxin efflux carriers (PGP1 and PGP19 in Arabidopsis). These interactions result in a synergistic increase in auxin efflux.

The activity and localization of the PM-localized PIN proteins are regulated by several phosphorylations on their large cytosolic hydrophilic loop carried out by kinases of the AGC family (e.g. PID, WAG1, WAG2, PID2 in Arabidopsis) and the D6PK kinase.

## Maintenance of polarity

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PIN proteins in the plasma membrane organize into clusters of different sizes, each diffusing at varying rates . These clusters play crucial roles in signal transduction by amplifying signals, increasing sensitivity, and connecting with intracellular trafficking pathways like endocytosis.[5] Within these clusters, the agglomerations of auxin transporters are vital for maintaining their polarity, as they have lower mobility compared to dispersed proteins. PIN clustering and mobility depend on phosphoinositides, particularly PIN2's interaction with them, and enzymes like PIP5K1, shaping PIN cluster-like aggregates . Interestingly, PIN clusters don't align with REMORIN 1.2 but are affected by elevated salicylic acid levels or REM1.2, which induce hyperclustering of PIN2, influencing auxin distribution.[6] Moreover, connections between the plasma membrane, cell wall, and the composition of molecules like pectin and cellulose influence PIN clustering, impacting auxin transport.[7] Lastly, the microtubule cytoskeleton regulates PIN lateral diffusion, underscoring how cell wall chemistry and plasma membrane lipids control auxin transporter clustering, ultimately impacting Polar Auxin Transport (PAT).[8]

## Auxin feedback

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The impact of auxin on PIN polarity has been a subject of interest for many years, with various models suggesting that auxin's feedback occurs through PIN membrane cycling dynamics. However, recent advancements have challenged this hypothesis, with both natural and synthetic auxins promoting PIN2 endocytosis at low concentrations. The positive effect of auxin on PIN2 endocytosis may result in the retention of PIN2 polarity, which is potentially relevant for auxin regulation and its polar distribution.[9] Auxin-mediated re-arrangements of PIN polarity rely on changes in transcriptional gene expression activated by auxin signaling, with the auxin-responsive transcriptional activator WKY23 being a crucial factor required for this process. The receptor complex CAMEL–CAR, which phosphorylates PINs and regulates their polarity via subcellular trafficking, is required for the polarization of individual cells and represents a mechanism of auxin feedback on its transport machinery. Auxin induces a complex transcriptional mechanism that regulates many genes but also causes a fast, non-transcriptional response, targeting proteins like Myosin XI and its adaptor MadB2. This rapid auxin response plays a crucial role in multiple developmental processes.[10][11]

## References

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1. ^ Blilou I, Xu J, Wildwater M, Willemsen V, Paponov I, Friml J, et al. (January 2005). "The PIN auxin efflux facilitator network controls growth and patterning in Arabidopsis roots". Nature. 433 (7021): 39–44\. Bibcode:2005Natur.433...39B. doi:10.1038/nature03184. hdl:1874/21119. PMID 15635403. S2CID 4414301.
2. ^ Krecek P, Skupa P, Libus J, Naramoto S, Tejos R, Friml J, Zazímalová E (2009). "The PIN-FORMED (PIN) protein family of auxin transporters". Genome Biology. 10 (12): 249. doi:10.1186/gb-2009-10-12-249. PMC 2812941. PMID 20053306.
3. ^ Okada K, Ueda J, Komaki MK, Bell CJ, Shimura Y (July 1991). "Requirement of the Auxin Polar Transport System in Early Stages of Arabidopsis Floral Bud Formation". The Plant Cell. 3 (7): 677–684\. doi:10.1105/tpc.3.7.677. PMC 160035. PMID 12324609.
4. ^ Adamowski M, Friml J (January 2015). "PIN-dependent auxin transport: action, regulation, and evolution". The Plant Cell. 27 (1): 20–32\. doi:10.1105/tpc.114.134874. PMC 4330589. PMID 25604445.
5. ^ Jaillais Y, Ott T (April 2020). "The Nanoscale Organization of the Plasma Membrane and Its Importance in Signaling: A Proteolipid Perspective". Plant Physiology. 182 (4): 1682–1696\. doi:10.1104/pp.19.01349. PMC 7140965. PMID 31857424.
6. ^ Ke M, Ma Z, Wang D, Sun Y, Wen C, Huang D, et al. (January 2021). "Salicylic acid regulates PIN2 auxin transporter hyperclustering and root gravitropic growth via Remorin-dependent lipid nanodomain organisation in Arabidopsis thaliana". The New Phytologist. 229 (2): 963–978\. doi:10.1111/nph.16915. PMC 7821329. PMID 32901934.
7. ^ Feraru E, Feraru MI, Kleine-Vehn J, Martinière A, Mouille G, Vanneste S, et al. (February 2011). "PIN polarity maintenance by the cell wall in Arabidopsis". Current Biology. 21 (4): 338–343\. doi:10.1016/j.cub.2011.01.036. PMID 21315597. S2CID 2117627.
8. ^ Li H, von Wangenheim D, Zhang X, Tan S, Darwish-Miranda N, Naramoto S, et al. (January 2021). "Cellular requirements for PIN polar cargo clustering in Arabidopsis thaliana". The New Phytologist. 229 (1): 351–369\. doi:10.1111/nph.16887. PMC 7984064. PMID 32810889.
9. ^ Narasimhan M, Gallei M, Tan S, Johnson A, Verstraeten I, Li L, et al. (June 2021). "Systematic analysis of specific and nonspecific auxin effects on endocytosis and trafficking". Plant Physiology. 186 (2): 1122–1142\. doi:10.1093/plphys/kiab134. PMC 8195513. PMID 33734402.
10. ^ Marhava P (January 2022). "Recent developments in the understanding of PIN polarity". The New Phytologist. 233 (2): 624–630\. doi:10.1111/nph.17867. PMID 34882802. S2CID 245012549.
11. ^ Han H, Verstraeten I, Roosjen M, Mazur E, Rýdza N, Hajný J, et al. (2021). Rapid auxin-mediated phosphorylation of Myosin regulates trafficking and polarity in Arabidopsis (Report). doi:10.1101/2021.04.13.439603.