Heavy plasma membrane

Desensitization and lateral segregation of receptor and G-protein Subcellular fractionation studies have revealed two plasma membrane fractions in neutrophils, based on their densities a light plasma membrane fraction and a heavy plasma membrane fraction [156]. In unstimulated cells, the light plasma membrane fraction shows concentration of the N-formyl peptide receptor and Gj-proteins. In cells desensitized by... 

Just as the coupling of receptors and G-proteins in plasma membrane microdomains is regulated (see Section 9.1.2), evidence indicates that co-localization or segregation of G-proteins and effectors in membrane microdomains is a mechanism by which cell responsiveness to chemoattractant is regulated. Resting, unactivated neutrophils respond little to N-formyl peptides, however, when primed by a variety of chemical and physical conditions, these cells become very responsive [69, 166]. Studies show [166] that in unprimed cells N-formyl peptide receptors, phospholipase Cp2, and Lyn kinase predominate in the heavy plasma membrane fractions, whereas Q2 and Gj3 are in the light plasma membrane fractions. As a consequence of priming, Gj2 (but not Gjs) moves to... 

Biochemical characterization of clathrin-coated vesicles revealed that their major coat components are clathrin and various types of adaptor complexes. Clathrin assembles in triskelions that consist of three heavy chains of approximately 190 kDa and three light chains of 30 40 kDa. Four types of adaptor complexes have been identified to date, AP-1, AP-2, AP-3 and AP-4 (AP for adaptor protein). Whereas AP-1, AP-3 and AP-4 mediate sorting events at the TGN and/or endosomes, AP-2 is involved in endocytosis at the plasma membrane. Each adaptor complex is a hetero-tetrameric protein complex, and the term adaptin was extended to all subunits of these complexes. One complex is composed of two large adaptins (one each of y/a/S/s and [31-4, respectively, 90-130 kDa), one medium adaptin (pi -4, <50 kDa), and one small adaptin (ol-4, <20 kDa). In contrast to AP-1, AP-2 and AP-3, which interact directly with clathrin and are part of the clathrin-coated vesicles, AP-4 seems to be involved in budding of a certain type of non-clathrin-coated vesicles at the TGN. 

In neurons and non-neuronal cells, kinesin is associated with a variety of MBOs, ranging from synaptic vesicles to mitochondria to lysosomes. In addition to its role in fast axonal transport and related phenomena in non-neuronal cells, kinesin appears to be involved in constitutive cycling of membranes between the Golgi and endoplasmic reticulum. However, kinesin is not associated with all cellular membranes. For example, the nucleus, membranes of the Golgi complex and the plasma membrane all appear to lack kinesin. Kinesin interactions with membranes are thought to involve the light chains and carboxyl termini of heavy chains. However, neither this selectivity nor the molecular basis for binding of kinesin and other motors to membranes is well understood. 

Mercury can influence ion, water, and nonelectrolyte transport in different cells [ 14, 77]. The cell membrane is believed to be the first point of attack by heavy metals however, intracellular enzymes and metabolic processes may also be inhibited [70, 78, 79]. The attachment of heavy metals to ligands in or on the plasma membrane may result in changes in passive permeability or selective blockage of specific transport processes. Many membrane transport systems are known to be sensitive to sulphydryl-group modification [ 14, 80, 81]. 

As we saw in Chapter 7, zinc uptake in plants involves proteins of the ZIP family, some of which are root specific while others are found in both roots and shoots. The transport of zinc from the cytosol in many organisms is often associated with members of the cation diffusion facility (CDF) family. Although there are 12 predicted family members in Arabidopsis, only one, MTP1, has been characterized, which seems to function in the transport of Zn into the vacuole. Two members of the heavy metal ATPase (HMA) family, HMA2 and HMA4, have been shown to function in the transport of zinc out of the cells across the plasma membrane. 

Clathrin uptake can be inhibited by anticlathrin heavy chain antibodies. When applied to the cells, they lead to aggregation of clathrin in the cytoplasm and reduce the number of clathrin-coated pits on the plasma membrane (49). 

Other aquaporins with related sequences occur broadly. There are at least ten in mammals.359,3593 Plants, which must accommodate to heavy loss of water in hot dry weather, have aquaporins in both plasma membranes and tonoplasts.360 Bacteria also have aquaporins.356,361 A defect in aquaporin-2 of the kidney collecting duct leads to nephrogenic diabetes insipidus, in which the kidneys fail to concentrate urine in response to secretion of the hormone vasopressin.3553 362,363... 

The proteins whose abundances are altered in the plasma membrane in drug-resistant cells (Table 13.2) include those involved in increased amino acid uptake (4F2 cell-surface antigen heavy chain, large neutral amino acids transporter small subunit-1), reduced glucose uptake (stomatin, facilitated glucose transport... 

In order for a metal ion to reach its intracellular protein target, a number of complex barriers must be crossed. First, the metal existing in the extracellular environment must traverse the plasma membrane of the cell. The lipid bilayers of cellular membranes are generally impermeable to metals and cellular uptake of the ion requires the action of metal transport proteins. A host of membrane transporters reside at the cell surface, some of which are specific for certain ions (e.g. only copper or only zinc), while others are more promiscuous in their choice of metal ion substrate (e.g. can transport both copper and zinc). But all are designed to ensure that cells acquire proper levels of the essential heavy metal ions such as copper, zinc, iron, and manganese. 

Cystic fibrosis—a fatal, hereditary disease characterized by a heavy mucus buildup in the lungs—is caused by a defective plasma membrane protein. In persons with cystic fibrosis this transport protein, known as the sodium-potassium pump, abnormally transports sodium ions across the membrane without carrying the chloride ions that usually accompany them. Research is currently underway to correct through genetic engineering the faulty gene that codes for the plasma membrane protein. 

Synaptosomes can be subfractionated into a heavy membrane fraction that contains plasma membranes, synaptic vesicle clusters and most of the contaminating membranes, a light membrane fraction that is enriched in synaptic vesicles and devoid of measurable contamination by mitochondria or neuronal plasma membranes, and a... 

Since a malaria-infected red cell contains three membrane interfaces the plasma membrane of the red cell, the parasitophorous vacuolar membrane (PVM) and the parasite plasma membrane (PPM) studying membrane transport in such a system is not straightforward. Substrates have to traverse multiple membrane systems and a single membrane may contain multiple transporters for a particular substrate. To complicate the matter further, when we (and others) embarked on such studies not only were we usually unaware of the complexities of the transport systems that might be encountered, we were also unappreciative of the technical difficulties as well as possible artefacts that might result from isolating parasites and using heavy infections in unnatural hosts or from in vitro cultures. 

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