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Aquaporin-2 is a water-channel membrane protein in kidney collecting duct cells. Vasopressin regulates how much of it is present in the apical cell membrane, changing the pathway available for water reabsorption. Aquaporin-2 supports passive water movement driven by the surrounding osmotic conditions.^[ref]

In isolated perfused rat collecting ducts, researchers measured water permeability and then fixed the same tubules for electron microscopy with antibody labelling. Vasopressin increased apical channel labelling and measured permeability; withdrawal reversed both. Coupling a functional measurement to localization supported regulated transfer of channel-bearing vesicles to and from the surface.^[ref]

A toy redistribution makes the distinction clear: of 100 channel units, move 20 from vesicles into a surface pool initially containing 20. Surface abundance doubles to 40 while total abundance stays 100. This illustrates how trafficking can produce a rapid response; it is not a measured fold change in channel number or water flux.

ESCRT components help sort membrane cargo and remodel membranes, including formation of internal vesicles in an endosome. Ubiquitin can be a cargo-selection signal, but recognition and the membrane-remodelling steps are distinct operations.

A glycoprotein is a protein carrying one or more covalently attached carbohydrate groups. Attachment occurs through glycosylation; the carbohydrate component is part of the modified molecule, rather than simply dissolved beside it. Protein sequence alone therefore need not specify the complete chemical form measured in a biological preparation.

Pmel17 illustrates why the distinction is functional. It is a glycoprotein made as a membrane protein, then processed and organized inside developing premelanosomes. Experiments followed its molecular processing and its localization to internal vesicles and striations.^[ref] Calling it a glycoprotein identifies a chemical class; explaining its function still requires knowing its processing, position and interactions with other components.

J. L. van Rijn is the first author of a 1984 study of how membrane composition and an accessory protein affect activation of prothrombin. The investigators varied phospholipid concentration and composition, with and without factor Va, and measured the action of factor Xa on bovine prothrombin. This tested an enzyme in a membrane-associated reaction system rather than assuming that its behavior in solution determines the whole process.^[ref]

For vesicles containing 20 mole percent phosphatidylglycerol and 80 mole percent phosphatidylcholine, the reported apparent Michaelis constant for prothrombin was 0.04 micromolar with factor Va, compared with 2.2 micromolar without it. The ratio is $2.2/0.04=55$. That comparison shows the scale of the change in an apparent kinetic parameter; it is not a claim that the reaction rate increases 55-fold at every substrate concentration. The experiment illustrates how a surface and an accessory protein can change the effective behavior of a biochemical reaction.^[ref]

Membrane fission separates a connected membrane into distinct components, for example by severing a vesicle neck. Bending a membrane into a bud without severing that neck is not yet fission.

Movement of proteins and lipids between cellular compartments by vesicles, tubules, membrane fusion and selective retrieval.

A multivesicular body is an endosome containing multiple small vesicles within its lumen. These internal vesicles form when regions of the boundary cell membrane bud inward and undergo separation. This reverses the familiar direction of a transport vesicle budding outward into the cytosol.

Membrane topology explains a useful consequence. A cytosol-facing portion of a membrane protein becomes enclosed inside an internal vesicle; subsequent delivery to a lysosome can expose the whole cargo to degradation. Sorting onto an internal vesicle can therefore remove a signalling receptor from the cytosolic machinery that it previously contacted.

A reconstitution experiment added purified ESCRT-III components to large artificial vesicles and observed formation and detachment of inward buds. This demonstrated a direct membrane-separation activity in a simplified system, rather than merely finding the proteins near cellular multivesicular bodies.^[ref]

Growth concentrated at a selected part of the biological cell surface. Spatial selection can be controlled at transport, vesicle docking or fusion rather than only by directed vesicle motion.

A premelanosome is an immature melanosome whose internal architecture is developing before the compartment becomes heavily filled with melanin. In a characteristic maturation sequence, an initially vesicular compartment develops internal striations and then accumulates pigment on the resulting scaffold. “Immature” therefore describes a stage of an organelle's development, rather than a different kind of pigment.

Pmel17 helps construct that scaffold. Joanne F. Berson and colleagues localized processed Pmel17 to internal vesicles and striations of premelanosomes. Expressing the protein in nonpigmented cells also generated striation-like structures in multivesicular bodies.^[ref] Building the scaffold can thus be distinguished experimentally from synthesizing the pigment that later occupies it.

S. Nielsen is the first author of a 1995 study connecting vasopressin-regulated water permeability to the movement of water-channel proteins. The investigators perfused isolated rat collecting ducts, measured their permeability to water, and used labeled electron microscopy to locate the channels in the same tubules. During vasopressin exposure, permeability and channel labeling at the exposed cell membrane increased; removing vasopressin reversed both changes.^[ref]

The study supported redistribution of channels from internal vesicles to the cell membrane, rather than treating a physiological response as evidence of new protein synthesis. The channel is known as aquaporin-2. For a conceptual example, a fixed pool of 100 channels can change from 20 at the surface to 40 at the surface by moving channels from internal stores. The total is unchanged even though the surface pool doubles. This distinction between total abundance, subcellular location and measured function makes trafficking a testable explanation for a rapid hormonal response.

Synaptobrevin is a membrane-anchored SNARE protein associated with secretory vesicles, including synaptic vesicles. Its cytosolic helix joins complementary helices from syntaxin and SNAP-25 to form a SNARE complex. When the partners initially occupy opposing lipid bilayers, their assembly helps bring those lipid bilayers close enough for membrane fusion. The protein helps couple the membranes mechanically; it is not a pore through which the neurotransmitter simply leaks. ^[ref]

Reconstituting synaptobrevin and its complementary SNARE proteins into separate liposomes made fusion measurable outside a cell. The ability to accumulate docked intermediates at low temperature and trigger fusion on warming also distinguished membrane docking from the subsequent merger. Living synapses add regulation that this minimal assay does not reproduce. ^[ref]

Syntaxin is a family of membrane-anchored SNARE proteins that contributes a helix to a membrane-fusion complex. In neuronal secretion, syntaxin and SNAP-25 on the target cell membrane assemble with synaptobrevin on a synaptic vesicle. Syntaxin contributes one helix, synaptobrevin one and SNAP-25 two, producing a four-helix SNARE complex from three protein chains. Assembly across opposing lipid bilayers couples molecular binding to membrane fusion. ^[ref]

The membrane context matters. Reconstituted complementary SNARE proteins in different liposomes can promote fusion, while a complex assembled after fusion has its membrane anchors on the same lipid bilayer. Detecting a syntaxin-containing complex therefore does not by itself show that it is still bridging a vesicle to its target. ^[ref]

Thomas Wollert investigates how proteins reshape cell membranes. In a 2009 study, Wollert and colleagues reconstructed membrane cutting using purified ESCRT-III components and large artificial vesicles. Fluorescence microscopy revealed the formation and detachment of smaller vesicles into their interiors. This reduced a process normally embedded in a complex biological cell to components that could be added and omitted separately.^[ref]

Three ESCRT-III components sufficed for detachment in this system. Additional machinery, including an ATPase, was needed to recycle the protein assembly and sustain further rounds. The distinction is illuminating: the minimum machinery for one cutting event need not be the minimum machinery for repeated operation. The purified system established sufficiency under its experimental conditions, without showing that all processes in living biological cells use exactly that minimal sequence. The paper credits Wollert with preparing artificial vesicles, purifying and labeling proteins, and performing microscopy.^[ref]

Ordinary vesicle budding and fusion preserve which side of a membrane a protein domain faces: a cytoplasmic domain remains cytoplasmic, and a luminal domain remains luminal. Invagination can enclose previously external space without transporting its contents through a bilayer. The endoplasmic reticulum lumen is continuous with the space between the two nuclear envelope membranes; it is not the chromosome-containing nucleoplasm. Any proposed evolutionary remodelling must account for these distinct spaces and the membrane connections between them.

Transcytosis carries material through a biological cell from one surface to another by combining endocytosis, intracellular vesicle transport and exocytosis. In an epithelial sheet, cargo can enter at the apical surface facing a cavity and leave at the basal or lateral surface facing underlying tissue. This route crosses the cell rather than passing through the gap between neighbouring cells.

An illuminating example is transfer of maternal antibodies across the intestine of a newborn rat. Richard Rodewald followed antibodies carrying an electron-dense ferritin label, and independently followed antibodies recognizable by a peroxidase reaction. Electron microscopy located tracer at the apical surface, in internal tubular vesicles and near release sites at the opposite surface. Experiments used isolated, ligated intestinal segments and control tracers to distinguish selective transport from leakage between cells. The observations establish a route across the epithelial cell, not merely disappearance from the intestinal cavity.^[ref]

An ATP-driven proton pump that acidifies many eukaryotic endomembrane compartments.

Hydrolysis of ATP drives proton movement and helps establish an acidic compartment. The resulting electrochemical gradient supports cargo processing and coupled transport; a low luminal pH is actively maintained rather than an intrinsic property of all membrane vesicles. ^[ref]

Vasopressin, also called antidiuretic hormone, is a peptide hormone made by neurons in the hypothalamus and released into blood through the posterior pituitary. An important action is to conserve water by increasing its reabsorption in the kidney.^[ref]

In the collecting duct, vasopressin regulates the abundance of aquaporin-2 water channels in the cell membrane facing the fluid that will become urine. More channels increase membrane permeability to water, allowing more water to return toward the blood by osmosis. This changes a route through the membrane rather than making the channel an active water pump.^[ref]

A revealing 1995 experiment perfused isolated rat collecting ducts and combined measured water flow with electron microscopy locating labeled channels. During vasopressin exposure, water permeability and channel abundance at the surface increased together. Withdrawal reversed both changes. Measuring function and channel location in the same tubules connected the physiological response to movement of channels from internal vesicles to the cell membrane.^[ref]

A vesicle is a small compartment enclosed by a lipid bilayer. Its vesicle lumen can carry soluble cargo, while its lipid bilayer carries lipids and membrane proteins. In a transport pathway, budding separates a carrier from one compartment and membrane fusion joins it to another. Orientation matters: fusion of a secretory vesicle with the cell membrane connects its vesicle lumen to the extracellular space while incorporating its membrane proteins into the surface. ^[ref]

A vesicle is therefore more than a freely diffusing package. Recognition, movement and membrane fusion require distinct interactions. In a reconstitution experiment, complementary SNARE proteins placed in separate liposomes brought opposing lipid bilayers together and promoted fusion; the experiment isolated one part of transport from the rest of a living cell. ^[ref]

Vesicle docking brings a transport vesicle into close, stable association with its target membrane before membrane fusion. Recognition factors and SNARE proteins help organize the contact. Docking and fusion are experimentally distinct: a carrier can remain attached without exchanging its internal contents with the target compartment. Reconstituted liposomes bearing appropriate SNARE proteins allowed such docked intermediates to be distinguished from subsequent fusion. ^[ref]

Merger of a vesicle membrane with a target membrane. The process connects the two enclosed aqueous spaces or releases cargo outside the cell, while also transferring membrane components to the target surface.