Unconventional Transport Routes of Soluble and Membrane Proteins and Their Role in Developmental Biology

Abstract

Many proteins and cargoes in eukaryotic cells are secreted through the conventional secretory pathway that brings proteins and membranes from the endoplasmic reticulum to the plasma membrane, passing through various cell compartments, and then the extracellular space. The recent identification of an increasing number of leaderless secreted proteins bypassing the Golgi apparatus unveiled the existence of alternative protein secretion pathways. Moreover, other unconventional routes for secretion of soluble or transmembrane proteins with initial endoplasmic reticulum localization were identified. Furthermore, other proteins normally functioning in conventional membrane traffic or in the biogenesis of unique plant/fungi organelles or in plasmodesmata transport seem to be involved in unconventional secretory pathways. These alternative pathways are functionally related to biotic stress and development, and are becoming more and more important in cell biology studies in yeast, mammalian cells and in plants. The city of Lecce hosted specialists working on mammals, plants and microorganisms for the inaugural meeting on "Unconventional Protein and Membrane Traffic" (UPMT) during 4-7 October 2016. The main aim of the meeting was to include the highest number of topics, summarized in this report, related to the unconventional transport routes of protein and membranes.

Keywords: autophagy; exosomes; intercellular channels; leaderless proteins; protein secretion; trafficking mechanisms; unconventional secretion.

Figure 1

Schematic representation of secretory pathways within the eukaryotic cell. Some of the UPS pathways are indicated (see text for more details): (1) Leaderless proteins directly translocated across the PM, by means of non-vesicular (1a) and vesicular (1b) UPS pathways. Examples are proteins FGF2, Annexin A2, Galectin-3 and Interleukin-1β. (2) Soluble or transmembrane proteins with ER localization subsequently transported to the PM, or to the vacuole, or to the extra cellular space (by using or not using secretory lysosomes, dashed lines) bypassing the Golgi apparatus. In addition, the conventional secretory pathway is also indicated (3), but it should be considered that some proteins normally functioning in the conventional membrane traffic can have an additional unconventional role. MVB, multivesicular bodies. Modified from [7].

Figure 2

Some examples of unusual or unexplored intra- and intercellular pathways and organelle biogenesis. During the meeting the participants underlined that intra- and intercellular pathways, including organelle biogenesis and plasmodesmata, can represent other types of UPS pathways (see Section 4 and Section 6 in the text). An example of vesicular UPS mediated by a plant-specific double-membrane-bound compartment is the contentious exocyst-positive organelle (EXPO), which biogenesis seems to be mediated by a UPS pathway. Moreover, in eukaryotes RNA and proteins transported by intercellular channels regulate different processes in relation to trafficking mechanisms. In mammalian cells, another example is the unconventional transport of FoxO3A, a transcription factor working in normal conditions on genomic DNA in the nucleus, to mitochondria, while in plants chloroplast vesicle transport and thylakoid biogenesis is under investigation to identify protein factors involved in the formation of this unusual membrane system.

Figure 3

A picture showing Walter Nickel during his talk.

Figure 4

Examples of direct ER to vacuole (V) import routes bypassing the Golgi apparatus and the Trans Golgi Network (TGN) in plant cells. The scarcity of proteins with Golgi-modified glycans (red triangles) on the tonoplast may indicate that the major route (1) to the tonoplast bypasses the Golgi apparatus. Moreover, in seed endosperm, some storage proteins are directly transported to the protein storage vacuoles (2). Recently, it has been shown that a specific plant exocyst subcomplex, containing EXO70B1, participates in the direct ER to vacuole import of anthocyanins (3) which are synthesized by a multienzyme complex loosely associated to the ER.

Figure 5

UPS seems to be involved in regulation of plant sexual reproduction. (A) Simplified scheme, modified from [83], of compatible pollen response pathway in Brassica. Exocyst dependent secretion was shown to be important for the plant stigma receptivity. The octameric exocyst complex at the stigma–pollen grain contact sites mediates secretion of factors stimulating pollen hydration and germination, by non-conventional secretion of exosomes via multivesicular bodies (MVB) fusion with the PM. EXO70A1 is a subunit of the exocyst complex which functions as a tethering complex to mediate polar secretion. Specific pollen proteins are proposed to mediate the recognition of compatible pollen, but the corresponding receptor is still unknown; (B) Pollen tube-pistil signaling in A. thaliana. A hypothesis is that the translationally controlled tumor protein (TCTP), a leaderless secretory protein, probably released via exosomes, is involved in male–female recognition, indicating a putative role of UPS in pollen tube guidance towards the ovules for fertilization.