Endosomes

Overview "Endosomes"

endosomes

image : Electron micrograph of endosomes in human HeLa cells. Early endosomes (E - labeled for EGFR, 5 minutes after internalisation, and transferrin), late endosomes/MVBs (M) and lysosomes (L) are visible. Bar, 500 nm. Wikipedia

Endosomes are dynamic compartments with a variable biochemical composition, structure, and function [15, 30]. In mammalian cells, secretory vesicles from the Golgi and endocytic vesicles from the plasma membrane fuse with the early endosomes. Most of the recycling of plasma membrane proteins appears to occur in early endosomes and recycling endosomes, whereas the intermediate and late endosomes mostly sequester membrane proteins into internal vesicles. The internal vesicles in multivesicular endosomes, also called multivesicular bodies (MVBs) arise from invaginations of the limiting membrane and usually carry the membrane proteins targeted for degradation in the lysosome/vacuole [15, 23, 40].

In addition, late endosomes appear to recycle some membrane proteins back to the trans-Golgi network (TGN) before fusing with the lysosome, where the internal vesicles are degraded. Thus, endosomes are key sorting stations that contribute to regulate the protein composition of the plasma membrane, the TGN, and the vacuoles/lysosomes. Particular Rab GTPases, which are important regulators of vesicular trafficking [60], associate with specific endosomes and even with discrete domains within a single endosome. For this reason, Rab GTPase are frequently used as endosomal markers [46, 48, 49, 60]. In Arabidopsis, the three members of the RabF subfamily, i.e. ARA6/RABF1, RHA1/RABF2a and ARA7/RABF2b have been shown to localize to endosomal/prevacuolar compartments [16, 55].

In contrast to animal cells, different types of vacuoles and prevacuolar compartments can coexist in the same plant cell. This feature confers amazing complexity to the plant endomembrane transport system. Some of these pathways only occur in specific cell types; e.g. the trafficking of proteins to the protein storage vacuole normally occurs only in seed tissues. As in animal and yeast cells, plant endosome/prevacuolar compartments traffic both biosynthetic and endocytic cargo. Several recent studies using membrane styryl FM dyes such as FM4-64, filipin-labeled plant 3-β-hydroxysterols, fluid-phase endocytosis markers such as Lucifer Yellow, SNAREs, Rab GTPases and plasma membrane proteins fused to fluorescent proteins have unambiguously demonstrated the extraordinary dynamics of the endocytic pathway in plant cells [1, 4, 8, 10-12, 14, 16, 17, 19, 27, 29, 32, 43-45, 53-58].

Sorting of plasma membrane proteins into endocytic vesicles and subsequently into the internal vesicles of MVBs are crucial steps in the downregulation of receptors involved in development, signaling, and cell differentiation as well as in general turnover of plasma membrane proteins in eukaryots [23]. The endosomal invagination process is unique because, unlike most characterized vesiculation processes, the vesiculating membrane buds away from the cytoplasm. This invagination process requires the concentration of membrane proteins into specific membrane domains and the initiation of budding into the endosomal lumen. Approximately 18 proteins, named class E VPS (vacuolar protein sorting) proteins, have been found to be involved in the endosomal sorting/invagination process of yeast and animal cells [2]. Ubiquitination of membrane-bound receptors is now well established as a signal for sorting into internal vesicles of MVBs [15, 18]. When the ubiquitinated receptors reach the intermediate endosome, flat clathrin coats highly enriched in HRS (hepatocyte growth factor-regulated tyrosin kinase substrate) form on the endosomal membranes [31, 39] (Fig. 1). Three multisubunit complexes called ESCRT-I, -II, -III (endosomal sorting complexes required for transport) are also required for protein sorting into MVB internal vesicles in yeast and mammalian cells [2, 5, 23, 24].

Once the three ESCRTs are assembled onto the endosomal membrane, the AAA ATPase Vps4p (yeast)/SKD1 (mammals) is required for endosomal membrane invagination, presumably by releasing the ESCRTs from the membrane. The binding of Vps4p/SKD1 to the membrane is regulated by its ATPase cycle, being membrane-associated in its ATP-bound form and cytoplasmic in its ADP-bound form. Vps4p/SKD1 is a crucial player in the MVB pathway: when Vps4p/SKD1 is knocked out or a dominant negative (ATPase deficient) form of Vps4p/SKD1 is expressed, aberrant endosomes form, causing disruptions in the secretory and endocytic pathways [3, 28, 33]. Whereas there is some variation in the organization of the ESCRT complexes in different organisms [59], the function of Vps4p/SKD1 appears to be highly conserved in all eukaryotic cells. However, whereas yeast and mammalian cells are able to survive expressing ATPase deficient versions of Vps4p/SKD1, the expression of such a form of AtSKD1 in plant cells causes dramatic alterations in the endomembrane system and cell death, suggesting an essential role of the endosomal sorting machinery in plants [16]. Fluorescent fusion proteins of AtSKD1 and ELC, a Vps23p/TSG101 homolog part of the ESCRT machinery, have been shown to localize to endosomes in Arabidopsis [16, 50].

A number of important plant proteins continuously cycle between the plasma membrane and the endosomes in a mechanism called constitutive cycling. Examples include the PIN proteins, (putative auxin efflux regulators), PIP2 (a plasma membrane ATPase thought to be a water channel), BRI1 (brassinosteroid receptor), and KAT1 (a K+-channel) [7, 13, 14, 29, 37, 42, 43]. This mechanism appears to allow dynamic changes in the distribution of plasma membrane proteins. A key player in constitutive cycling appears to be GNOM, a GDP/GTP exchange factor for the ARF GTPases [51], localizes to early/recycling endosomes and appears to mediate the Brefeldin A-sensitive recycling of AtPIN1 to the plasma membrane [10] (Fig. 3).

Although there is no doubt about the complexity of the plant endosomal system, the identity of the plant early endosomes has remained elusive. Recent studies suggest that a TGN-related compartment received the endocytosed material from the plasma membrane, acting as a early endosome [26].

Endosomes are also important compartments in the trafficking from the Golgi to the vacuole [22, 25, 36, 52, 54]. In particular, MVBs act as prevacuolar compartments in developing seeds. They carry storage proteins and processing proteases and deposit them in protein storage vacuoles [35, 41].

The recycling of vacuolar cargo receptors back to the TGN also happens at the endosomes. The endosomal protein complex called retromer, that in mammals and yeast is responsible for the recycling of vacuolar cargo receptors, has been identified in Arabidopsis [20, 21, 34, 47]. Interestingly, the plant retromer does not only seem to be involved in the recycling of vacuolar cargo receptors, such as VSR1, but also in PIN protein cycling and PIN1 repolarization during plant development [21].

List of FP probes for endosomes and endosome related compartments

Diagram of the pathways that intersect endosomes in the endocytic pathway of animal cells. Examples of molecules that follow some of the pathways are shown, including receptors for EGF, transferrin, and lysosomal hydrolases. Recycling endosomes, and compartments and pathways found in more specialized cells, are not shown. Wikipedia

Early Endosomes and Late Endosomes​

Key Players in Endocytic Trafficking

 Eukaryotic cells internalize nutrients, receptors, and signaling molecules through endocytosis, a process that delivers cargo from the plasma membrane into a series of intracellular compartments known as endosomes. These organelles serve as dynamic sorting and trafficking stations that determine the fate of internalized material whether it’s recycled back to the cell surface, sent to the Golgi apparatus, or targeted for degradation.

🧬 What Are Endosomes?

Endosomes are membrane-bound organelles in the endocytic pathway that receive vesicles formed at the plasma membrane and guide cargo through a controlled maturation process. They are essential for cellular homeostasis, nutrient uptake, receptor regulation, and signal transduction.

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