Membrane contact sites and cytoskeleton-membrane interactions in autophagy

Abstract

In Eukaryotes, organelle interactions occur at specialised contact sites between organelle membranes. Contact sites are regulated by specialised tethering proteins, which bring organelle membranes into close proximity, and facilitate functional crosstalk between compartments. While contact site proteins are well characterised in mammals and yeast, the regulators of plant contact site formation are only now beginning to emerge. Having unique subcellular structures, plants must also utilise unique mechanisms of organelle interaction to regulate plant-specific functions. The recently characterised NETWORKED proteins are the first dedicated family of plant-specific contact site proteins. Research into the NET proteins and their interacting partners continues to uncover plant-specific mechanisms of organelle interaction and the importance of these organelle contacts to plant life. Moreover, it is becoming increasingly apparent that organelle interactions are fundamental to autophagy in plants. Here, we will present recent developments in our understanding of the mechanisms of plant organelle interactions, their functions, and emerging roles in autophagy.

Keywords: actin; autophagy; chloroplast; contact site; cytoskeleton; endoplasmic reticulum; membrane; mitochondria; plasma membrane; vacuole.

Fig. 1

NETWORKED family proteins connect the actin cytoskeleton to specific membrane compartments. Depicted are the subcellular localisations of GFP fusions of representative NET proteins, analysed by confocal microscopy. (A) NET1A‐GFP predominantly localises to the transverse walls of root cell files, where it was found localised to plasmodesmata [5]. (B) NET2A‐GFP localises to punctae at the pollen tube plasma membrane [5]. (C) NET3B‐GFP binds actin to the ER [11]. (D). GFP‐NET3C localises to punctae at ER‐PM Contact Sites [5]. (E) NET4A‐GFP localises to punctae at the vacuole tonoplast [5]. Figure 1 has been reproduced from [5, 6, 11] (A), (B) and (E) have been reproduced from [5]; (C) has been reproduced from [11]; (D) has been reproduced from [6]. [Colour figure can be viewed at wileyonlinelibrary.com]

Fig. 2

Characterised mechanisms of organelle interactions and actin‐membrane contact in plant cells. EPCS are mediated by SYT1 and VAP27. VAP27 binds to PM‐localised NET3C to connect the PM, ER and actin. NET3C forms a complex with the microtubule‐binding proteins IQD2 and KLRC1 to link the ER, PM, microtubules and actin. Actin‐PM contact sites are also mediated by NET2 and the transmembrane receptor kinases, PRK4&5. NET1 binds actin and localises to the plasmodesmata [5]. NET3B binds actin to the ER. ER‐mitochondrial interactions are mediated by the interaction of VAP27 with the outer mitochondrial membrane proteins, TraB1a and TraB1b. Interactions between the ER and chloroplasts occur through a yet‐uncharacterised mechanism. NET4 binds actin at the vacuole tonoplast through interaction with tonoplast‐localised RabG3. [Colour figure can be viewed at wileyonlinelibrary.com]

Fig. 3

Potential roles of organelle interactions and actin‐membrane interplay in plant autophagy. (A) EPCS contribute to autophagosome biogenesis. The ARP2/3‐regulator, EH/Pan1 interacts with VAP27 and PM‐localised T‐PLATE components. Regulation of the cortical actin cytoskeleton and endocytic machinery functions to drive expansion of the ER‐derived phagophore. It is also possible that EPCS are important in lipid transfer from the PM to the phagophore membrane. Additionally, the SCAR‐WAVE protein, NAP1, promotes ARP2/3‐dependent actin dynamics at the ER to drive autophagosome biogenesis. (B) Organelle interactions with the ER may play a role in selective autophagy. The outer mitochondrial membrane proteins TraB1a and TraB1b bind VAP27 to mediate ER‐mitochondrial interactions, and mitophagy through interaction with ATG8. Tethering of mitochondria to the ER may promote their sequestration to the ER‐derived phagophore. ATI1 and ATI2 may also have a role in mediating interaction of the chloroplast and the ER or phagophore, and interact with ATG8 to mediate autophagic chloroplast degradation. (C) Actin‐Membrane interactions may function to target autophagosomes to the vacuole. NET4 localises to the tonoplast through interaction with RabG3, and may link actin transport networks to the tonoplast to promote autophagosome delivery. Regulation of actin‐driven force by NET4 may also promote fusion of autophagosomes and the tonoplast. [Colour figure can be viewed at wileyonlinelibrary.com]