Cellular Transport

The most important function of the compartment membranes is to ensure the stability and organisation of the cellular environment. In particular it is impor-tant that the cellular environment is:

• electrochemically stable,

• biochemically well-organised,

• and effectively separated from the outside environment.

These properties are ensured by a number of transport mechanisms that, col-lectively, deal with all of these aspects.

Small Molecule Transport. The electrochemical stability of compartments is primarily maintained by the selective permeability of small molecules, which is implemented by a large class of transmembrane proteins.

Passive transport amounts to diffusion. Some gases may pass the membrane bysimple diffusion. Slightly larger molecules, i.e., various ions and even water, pass by facilitated diffusion throughmolecular channels.

Active transport requires a source of energy. Molecular pumps hydrolyse ATP, whilemolecular transporters use the power of electrochemical gradients.

Large Molecule Transport. The biochemical organisation of the cell is maintained by the selective transport of large molecules, e.g., proteins, from production site to deployment site, The central mechanism is that of protein targeting. Every translated protein has one or moresignal sequences, identify-ing the appropriate deployment site, embedded in its amino acid chain. Each such sequence is recognised by the transport machinery associated with the corresponding destination.

Non-secretory proteins are produced in the cytosol and subsequently trans-ported to the lumen or membrane of an organelle. Nuclear Localisation Sig-nals (NLSs), for example, are recognised by Nuclear Pore Complexes (NPCs) that facilitate transport from cytosol to nucleoplasm. Other signals direct pro-teins to the lumens or membranes of peroxisomes, or to the membranes or sub-compartments of mitochondria.

RNA molecules are produced in the nucleoplasm and, if appropriate, subse-quently transported to the cytosol. In order to be recognised and transported by the NPCs they must form ribonucleoprotein complexes with proteins that exhibitNuclear Export Signals(NESs).

The Secretory Pathway. Secretory proteinsare meant for deployment in, or on membranes in contact with, exoplasmic solutions. Such solutions are rich in entities that contain important metabolites but are potentially harmful. Thus, many secretory proteins are hydrolases, i.e., enzymes that break down organic compounds, and cannot be allowed to roam freely inside the cell.

Signal recognition particles recognise secretory proteins already during transla-tion, and immediately associate them with the rough ER surface. Here they are injected directly into either the membrane or the lumen of the rough ER by co-translational translocation.

Once folded into the proper conformation within the ER the proteins are pack-aged intoanterograde vesicles and moved forward to the Golgi complex. Here they are matured and sorted into vesicles according to their final destination, which might be either the lysosome or the cell surface. Facilitating proteins are continuously shipped back to the ER inretrograde vesicles.

Finally, once fully matured, the secretory proteins leave the Golgi complex in vesicles. Some go to the plasma membrane, where they are secreted into the exoplasm by exocytosis. Acid hydrolases, on the other hand, are deployed to the lysosome, where they are used to break down organic compounds.

In the latter case the vesicle is coated with a double protein coat. The ac-tual formation of the vesicle happens due to a coat of Clathrin particles, but underneath this coat there is another, comprised of adopter protein complexes (APs).

The Endocytic Pathway. The mechanism also facilitatesreceptor mediated endocytosis, which is used by the cell to selectively subsume particles from the exoplasm. The process is facilitated by specialised transmembranal receptor proteins whose exoplasmic domains are able to ligate specific particles.

Meanwhile, and independent of this,clathrinparticles continuously assemble on the cytosolic side of the plasma membrane - thereby forcing it to formclathrin coated pits that grow progressively deeper until released into the cytosol as separateclathrin coated vesicles.

The diffusing receptors tend to associate with clathrin coated pits because their intra-cellular domain binds to complementaryadaptin (AP) molecules exposed by the clathrin coat. Such associated receptors and the particles that they bind, if any, are internalised when the coated vesicle is formed.

Once internalised, coated vesicles shred their clathrin coat and become early endosomes. At this stage the subsumed particles are still ligated by the receptor proteins. This changes, however, when the early endosome merges with a late endosome. The acidic environment in this compartment makes the receptors separate from the ligated particles.

From the late endosomes the receptor proteins are recycled to the plasma mem-brane. The internalised particles, however, are transferred by vesicles to

lyso-Figure 2.8: LDL degradation pathway. Copyright 2004 from Molecular Cell Biology by Lodish et al [LBZ⁺99]. Reproduced by permission of W.H. Freeman and Company/Worth Publishers.

somes where they are broken down into useful metabolites,

CASE: The LDL Degradation Pathway. The best known example of this process is the LDL degradation pathway shown in Fig. 2.8. This is one mecha-nism, by which the cell acquires thecholesterol required for membrane synthe-sis. Transmembranal LDL receptors ligate LDL when the exoplasmic domain encounters the ApoB binding site exposed by LDL particles. The cholesterol is released when the tightly packed cholesteryl esters of the LDL particles are hydrolysed in the lysosome [AJL⁺02, LBZ⁺99].

Related Diseases. It happens that the gene encoding the transmembranal LDL receptor proteins somehow mutates. Sometimes these mutations are be-nign, and they cause no particular problems. It may also happen, however, that a mutation affects the coding of either the exoplasmic or the cytosolic binding domain in an adverse way. Either case leads to transcription of transmembranal receptor proteins that exhibit reduced affinity between the affected binding site and the corresponding binding sites of the ordinary ligands.

When such a defect affects the extra-cellular part of the receptor, its ability to bind LDL particles is reduced. In contrast, when the defect affects the intra-cellular part of the receptor protein, it can bind but not internalise LDL parti-cles. Both cases lead to abnormally high blood levels of LDL, which dramatically increases the risk of the cardiovascular diseaseatherosclerosis. The resulting dis-order is calledfamilial hypercholesterolemia and is hereditary, as it propagates with the mutated gene.