Circular dichroism and fluorescence studies on a cation channel forming plasma membrane proteolipid

The MAL Family of Proteins: Normal Function, Expression in Cancer, and Potential Use as Cancer Biomarkers

The MAL family of integral membrane proteins consists of MAL, MAL2, MALL, PLLP, CMTM8, MYADM, and MYADML2. The best characterized members are elements of the machinery that controls specialized pathways of membrane traffic and cell signaling. This review aims to help answer the following questions about the MAL-family genes: (i) is their expression regulated in cancer and, if so, how? (ii) What role do they play in cancer? (iii) Might they have biomedical applications? Analysis of large-scale gene expression datasets indicated altered levels of MAL-family transcripts in specific cancer types. A comprehensive literature search provides evidence of MAL-family gene dysregulation and protein function repurposing in cancer. For MAL, and probably for other genes of the family, dysregulation is primarily a consequence of gene methylation, although copy number alterations also contribute to varying degrees. The scrutiny of the two sources of information, datasets and published studies, reveals potential prognostic applications of MAL-family members as cancer biomarkers-for instance, MAL2 in breast cancer, MAL2 and MALL in pancreatic cancer, and MAL and MYADM in lung cancer-and other biomedical uses. The availability of validated antibodies to some MAL-family proteins sanctions their use as cancer biomarkers in routine clinical practice.

MALL, a membrane-tetra-spanning proteolipid overexpressed in cancer, is present in membraneless nuclear biomolecular condensates

Proteolipids are proteins with unusual lipid-like properties. It has long been established that PLP and plasmolipin, which are two unrelated membrane-tetra-spanning myelin proteolipids, can be converted in vitro into a water-soluble form with a distinct conformation, raising the question of whether these, or other similar proteolipids, can adopt two different conformations in the cell to adapt their structure to distinct environments. Here, we show that MALL, another proteolipid with a membrane-tetra-spanning structure, distributes in membranes outside the nucleus and, within the nucleus, in membrane-less, liquid-like PML body biomolecular condensates. Detection of MALL in one or other environment was strictly dependent on the method of cell fixation used, suggesting that MALL adopts different conformations depending on its physical environment —lipidic or aqueous— in the cell. The acquisition of the condensate-compatible conformation requires PML expression. Excess MALL perturbed the distribution of the inner nuclear membrane proteins emerin and LAP2β, and that of the DNA-binding protein BAF, leading to the formation of aberrant nuclei. This effect, which is consistent with studies identifying overexpressed MALL as an unfavorable prognostic factor in cancer, could contribute to cell malignancy. Our study establishes a link between proteolipids, membranes and biomolecular condensates, with potential biomedical implications.

Plasmolipin and Its Role in Cell Processes

The mechanisms involved in the origin and development of malignant and neurodegenerative diseases are an important area of modern biomedicine. A crucial task is to identify new molecular markers that are associated with rearrangements of intracellular signaling and can be used for prognosis and the development of effective treatment approaches. The proteolipid plasmolipin (PLLP) is a possible marker. PLLP is a main component of the myelin sheath and plays an important role in the development and normal function of the nervous system. PLLP is involved in intracellular transport, lipid raft formation, and Notch signaling. PLLP is presumably involved in various disorders, such as cancer, schizophrenia, Alzheimer's disease, and type 2 diabetes mellitus. PLLP and its homologs were identified as possible virus entry receptors. The review summarizes the data on the PLLP structure, normal functions, and role in diseases.

Solubility of hydrophobic surfactant proteins in organic solvent/water mixtures. Structural studies on SP-B and SP-C in aqueous organic solvents and lipids

The solubility of hydrophobic pulmonary surfactant proteins in different organic solvents and organic solvent/water combinations has been analyzed. Three organic solvents have been selected: methanol (MetOH), acetonitrile (ACN) and trifluoroethanol (TFE). Porcine SP-B showed very similar calculated secondary structure when dissolved in methanol, 60% ACN or 70% TFE and reconstituted in lysophosphatidylcholine (LPC) micelles or dipalmitoylphosphatidylcholine (DPPC) vesicles, as deduced from circular dichroism studies. SP-B was calculated to possess around 45% of alpha-helix in all these systems. The fluorescence emission spectrum of SP-B has been also characterized in aqueous solvents and lipids. It always showed a splitting of the tryptophan contribution into two components with different emission maxima. SP-C had a very different structure in 80% ACN or 70% TFE. While alpha-helix was the main secondary structure of SP-C in ACN/water mixtures--around 50%--, it had almost exclusively beta-structure when dissolved in 70% TFE. The CD spectrum of SP-C in TFE showed dependence on the protein concentration, suggesting that protein-protein interactions could be important in this beta-conformation. SP-C reconstituted in LPC micelles or DPPC vesicles had a CD spectrum qualitatively similar to that one in aqueous ACN, with a dominant alpha-helical structure. The alpha-helical content of SP-C in micelles of LPC and vesicles of DPPC, 60 and 70%, respectively, was calculated to be higher than the alpha-helical content of the protein dissolved in any aqueous organic solvent.

Identification of membrane-bound carbonic anhydrase in white matter coated vesicles: the fate of carbonic anhydrase and other white matter coated vesicle proteins in triethyl tin-induced leukoencephalopathy

We have extended our studies on the content of white matter derived coated vesicles (WMCVs) to show that they are enriched in membrane-bound carbonic anhydrase. Within the myelin complex membrane-bound carbonic anhydrase is concentrated in the periaxolemmal domain; however, this protein is enriched almost sevenfold in the bilayer of coated vesicles even relative to this myelin membrane region. These data suggest that some vesicles are derived from a site at which this enzyme is highly localized. The enrichment observed for membrane-bound carbonic anhydrase is unique since other periaxolemmal proteins such as CNPase and plasmolipin are only present in equal amounts in periaxolemmal-myelin fractions and WMCVs. Based on their known localization, the presence of CNPase coupled with the absence of MAG in WMCVs suggest that these vesicles are derived from the paranodal region. The identification in WMCVs of periaxolemmal-myelin proteins associated with ion and fluid movement, such as carbonic anhydrase, Na+,K+ ATPase, and the putative K+ channel protein plasmolipin, prompted us to examine the status of these vesicles in triethyl tin (TET)-induced myelin edema. Coated vesicles and other membrane fractions were isolated from whole brains of control and TET-treated rats. Whole brains were used so we could compare the effects of TET on WMCV proteins with the effect on proteins enriched in gray matter coated vesicles. The results indicated that TET had no detectable effect on compact or periaxolemmal-myelin, however, Western blot analysis showed that WMCV proteins, such as carbonic anhydrase, CNPase, and plasmolipin, were virtually absent or greatly diminished from the whole brain coated vesicle fraction.(ABSTRACT TRUNCATED AT 250 WORDS)

Identification of plasmolipin as a major constituent of white matter clathrin-coated vesicles

We have isolated and characterized coated vesicles from bovine white matter and compared them to those isolated from gray matter. The virtual absence of synaptic vesicle antigens in the white matter coated vesicles indicates they are distinct from those found in gray matter and from vesicles derived from synaptic membranes. The white matter coated vesicles also lack compact myelin components, e.g., the myelin proteolipid, galactocerebroside, and sulfatides, as well as the periaxolemmal myelin marker myelin-associated glycoprotein. On the other hand, these vesicles contain 2',3'-cyclic nucleotide phosphohydrolase. The vesicles also contain high levels of plasmolipin, a protein present in myelin and oligodendrocytes. Plasmolipin was found to be four to five times higher in white matter coated vesicles than in gray matter coated vesicles. Based on western blot quantitation, the concentration of plasmolipin in white matter coated vesicles is 3-4% of the vesicle bilayer protein. These studies indicate that a significant proportion of coated vesicles from white matter may be derived from unique membrane domains of the myelin complex or oligodendroglial membrane, which are enriched in plasmolipin.

Expression of plasmolipin in the developing rat brain

Plasmolipin is an hydrophobic plasma membrane proteolipid present in both kidney and brain. The protein consists of two subunits of 17-18.5 kD, which together form K+ selective voltage-dependent channels. In this report, we define the embryonic and postnatal expression of plasmolipin in the developing rat brain. Plasmolipin was found to be essentially restricted to the postnatal period increasing eight-fold between the first to fourth week after birth. A fetal plasmolipin immunoreactive protein (FPIP) was identified in embryonic brain and also during the early postnatal development of the cerebellum. The expression of FPIP was biphasic with an initial transient increase between E15-E20 followed by a decrease in its levels. FPIP was not detected in the developed rat CNS. FPIP was found in a variety of dividing and immature cells including cultured astrocytes and embryonic neurons, neuroblastoma cells, and rat thymus. In contrast, plasmolipin was restricted to oligodendrocytes of the neural cells tested and to renal tubular epithelial cells.

The phylogenic expression of plasmolipin in the vertebrate nervous system

Plasmolipin is a plasma membrane proteolipid is a major myelin membrane component (Cochary et al., 1990). In this study we report the phylogenic expression of plasmolipin in the vertebrate nervous system. Using Western blot analysis with polyclonal antibodies, we have analyzed membrane fractions, including myelin, from elasmobranchs, teleosts, amphibians, reptiles, birds and mammals. On the basis of immune detection, plasmolipin appears to be restricted to the mammalian nervous system. Comparison of the central and peripheral nervous systems of mammals showed only minor differences in the level of plasmolipin in these two regions. Within mammals, little quantitative differences were observed when rat, human and bovine membrane fractions were compared. The late evolutionary expression of plasmolipin which results in its restriction to mammals makes it unique among the (major) myelin proteins. The potential physiologic significance of these data are discussed.

Expression of plasmolipin in oligodendrocytes

Plasmolipin is a plasma membrane proteolipid which has recently been described as a component of myelin (Cochary et al.: Journal of Neurochemistry 55:602-610, 1990). The present study reports the expression and localization of plasmolipin in primary glial cultures and secondary oligodendrocyte cultures. Double-label immunofluorescence showed that plasmolipin was expressed by galactocerebroside (GC)-positive oligodendrocytes, but was absent from astrocytes, characterized by their positive staining for glial fibrillary acidic protein (GFAP). At 1 week in culture plasmolipin staining was relatively weak in the cell body of some of the GC-positive cells. During the following 3 weeks in culture plasmolipin staining of oligodendrocytes gradually increased and was present in the cell body, its plasma membrane, and all the processes. However, the plasmolipin antibodies did not stain regions of the flat membrane sheets. Western blot analysis of homogenates from primary glial cultures showed that plasmolipin levels gradually increased during the first 5 weeks in culture. We conclude that the presence of plasmolipin in myelin is a result of its expression by oligodendrocytes.

Physiological and pharmacological regulation of biological calcification

Biological calcification is a highly regulated process which occurs in diverse species of microorganisms, plants, and animals. Calcification provides tissues with structural rigidity to function in support and protection, supplies the organism with a reservoir for physiologically important ions, and also serves in a variety of specialized functions. In the vertebrate skeleton, hydroxyapatite crystals are laid down on a backbone of type I collagen, with the process being controlled by a wide range of noncollagenous proteins present in the local surroundings. In bone, cells of the osteoblast lineage are responsible for the synthesis of the bone matrix and many of these regulatory proteins. Osteoclasts, on the other hand, are continually resorbing bone to both produce changes in bone shape and maintain skeletal integrity, and to establish the ionic environment needed by the organism. The proliferation, differentiation, and activity of these cells is regulated by a number of growth factors and hormones. While much has already been discovered over the past few years about the involvement of various regulators in the process of mineralization, the identification and functional characterization of these factors remains an area of intense investigation. As with any complex, biological system that is in a finely tuned equilibrium under normal conditions, problems can occur. An imbalance in the processes of formation and resorption can lead to calcification disorders, and the resultant diseases of the skeletal system have a major impact on human health. A number of pharmacological agents have been, and are being, investigated for their therapeutic potential to correct these defects.(ABSTRACT TRUNCATED AT 250 WORDS)

Presence of the plasma membrane proteolipid (plasmolipin) in myelin

Plasma membrane proteolipid (plasmolipin), which was originally isolated from kidney membranes, has also been shown to be present in brain. In this study, we examined the distribution of plasmolipin in brain regions, myelin, and oligodendroglial membranes. Immunoblot analysis of different brain regions revealed that plasmolipin levels were higher in regions rich in white matter. Plasmolipin was also detected in myelin, myelin subfractions, and oligodendroglial membranes. Immunocytochemical analysis of the cerebellum revealed that plasmolipin was localized in the myelinated tracts. Plasmolipin levels in myelin were enriched during five successive cycles of myelin purification, similar to the enrichment of myelin proteolipid apoprotein (PLP) and myelin basic protein (MBP). In contrast, levels of Na+,K(+)-ATPase and a 70-kDa protein were decreased. When myelin or white matter was extracted with chloroform/methanol, it contained, in addition to PLP, a significant amount of plasmolipin. Quantitative immunoblot analysis suggested that plasmolipin constitutes in the range of 2.2-4.8% of total myelin protein. Plasmolipin, purified from kidney membranes, was detected by silver stain on gels at 18 kDa and did not show immunological cross-reactivity with either PLP or MBP. Thus, it is concluded that plasmolipin is present in myelin, possibly as a component of the oligodendroglial plasma membrane, but is structurally and immunologically different from the previously characterized myelin proteolipids.

Secondary structure of detergent-solubilized phospholamban, a phosphorylatable, oligomeric protein of cardiac sarcoplasmic reticulum

The structure of phospholamban, a 30-kDa oligomeric protein integral to cardiac sarcoplasmic reticulum, was probed using ultraviolet absorbance and circular dichroism spectroscopy. Purified phospholamban was examined in three detergents: octyl glucoside, n-dodecyloctaethylene glycol monoether (C12E8) and sodium dodecyl sulfate (SDS). Ultraviolet absorption spectra of phospholamban reflected its aromatic amino acid content: absorption peaks at 275-277 nm and 253, 259, 265 and 268 nm were attributed to phospholamban's one tyrosine and two phenylalanines, respectively. Phospholamban phosphorylated at serine 16 by the catalytic subunit of cAMP-dependent protein kinase exhibited no absorbance changes when examined in C12E8 or SDS. Circular dichroism spectroscopy at 250-190 nm demonstrated that phospholamban possesses a very high content of alpha-helix in all three detergents and is unusually resistant to denaturation. Dissociation of phospholamban subunits by boiling in SDS increased the helical content, suggesting that the highly ordered structure is not dependent upon oligomeric interactions. The purified COOH-terminal tryptic fragment of phospholamban, containing residues 26-52 and comprising the hydrophobic, putative membrane-spanning domain, also exhibited a circular dichroism spectrum characteristic of alpha-helix. Circular dichroism spectra of phosphorylated and dephosphorylated phospholamban were very similar, indicating that phosphorylation does not alter phospholamban secondary structure significantly. The results are consistent with a two-domain model of phospholamban in which each domain contains a helix and phosphorylation may act to rotate one domain relative to the other.

Role of lipids in calcification of cartilage

This review addresses the role of lipids and membranes in biologic calcification and examines their regulation during endochondral ossification. The close association of lipids with mineral deposition has been well established. Early observations indicated that lipids, particularly phospholipids, can not be totally extracted from calcified tissues until the tissues are decalcified. Phospholipids associated with mineral are also enriched in extracellular membrane vesicles, called matrix vesicles. Numerous studies indicate that mineral deposits in calcifying cartilage are first seen in these phosphatidylserine and alkaline phosphatase enriched vesicles and that the process of endochondral calcification of epiphyseal growth plate is possibly mediated by them. Matrix vesicles, and the phospholipids present in them, appear to be involved in initial formation of calcium hydroxyapatite crystals via the interaction of calcium and phosphate ions with phosphatidylserine to form phospholipid:Ca:Pi complexes (CPLX). CPLX is present in tissues which are undergoing initial mineral deposition but are absent from nonmineralizing tissues. Evidence suggests that CPLX resides in the interior of matrix vesicles where the earliest mineral crystals are formed in association with the vesicle membrane. More recently, it has been determined that specific membrane proteins, called proteolipids, participate in CPLX formation and hydroxyapatite deposition, in part by structuring phosphatidylserine in an appropriate conformation. Phosphatidylserine involvement in the initiation of mineralization has been extensively investigated because of its extremely high binding affinity for Ca2+. In addition to structuring a specific phospholipid environment, proteolipids may also act as ionophores, promoting export of protons and import of calcium and phosphate, both requirements of biologic calcification.(ABSTRACT TRUNCATED AT 250 WORDS)

Identification of the plasma membrane proteolipid protein as a constituent of brain coated vesicles and synaptic plasma membrane

We have analyzed brain coated vesicles and synaptic plasma membrane for the presence of the plasma membrane proteolipid protein. Coated vesicles were isolated from calf brain gray matter with a final purification on Sephacryl S-1000 and reisolated twice by chromatography to ensure homogeneity. Fractions were analyzed by gel electrophoresis, immunoblotting for clathrin heavy chain, and by electron microscopy. Using an immunoblotting assay we were able to demonstrate the presence of the plasma membrane proteolipid protein in these coated vesicles at a significant level (i.e., approximately 1% of the bilayer protein of these vesicles). Reisolation of coated vesicles did not diminish the concentration of the protein in this fraction. Removal of the clathrin coat proteins or exposure of the coated vesicles to 0.1 M Na2CO3 showed that the plasma membrane proteolipid protein is not removed during uncoating and lysis but is intrinsic to the membrane bilayer of these vesicles. These studies demonstrate that plasma membrane proteolipid protein represents a significant amount of the bilayer protein of coated vesicles, suggesting that these vesicles may be a transport vehicle for the intracellular movement of the plasma membrane proteolipid protein. Isolation of synaptic plasma membranes proteolipid adult rat brain and estimation of the plasma membrane proteolipid protein content using the immunoblotting method confirmed earlier studies that show this protein is present in this membrane fraction at high levels as well (approximately 1-2%). The level of this protein in the synaptic plasma membrane suggests that the synaptic plasma membrane is one major site to which these vesicles may be targeted or from which the protein is being retrieved.

The synthetic precursor specific region of pre-pro-parathyroid hormone forms ion channels in lipid bilayers

We have used the chemically synthesized sequence of pre-pro-parathyroid hormone and several of its analogues to test the notion that the capacity of amphipathic peptides to aggregate in membranes and form ion-permeable channels correlates with their ability to function as signal sequences for secreted proteins. We found that pre-pro-parathyroid hormone (the signal sequence and pro-region of parathyroid hormone (M], as well as some of its analogues, forms aggregates of monomers which are ion-permeable. The ion-permeable aggregates (2-3 monomers) formed by (M) are voltage-dependent and are more permeable for cations than for anions. The compounds which formed ion channels in bilayers also acted as potential signal sequences. We conclude that the ability of peptides to form ion-permeable pathways in bilayers may be correlated to their ability to function as signal peptides.

Ion-translocating properties of calcifiable proteolipids

De novo formation of calcium hydroxyapatite in biological systems occurs on membrane surfaces through specific interactions of Ca, Pi, phospholipids, calcifiable proteolipids, and ion flux to and from the nucleating site. This paper reports an in vitro model demonstrating an ion transport function for calcifiable proteolipid. Bacterionema matruchotii proteolipid was incubated with a radiolabeled H+-channel inhibitor, 14C-dicyclohexyl-carbodiimide, and binding characterized by displacement studies with DCCD or ethyldimethylaminopropylcarbodiimide. A carboxyl binding site was suggested by displacement of DCCD by the nucleophile, glycine ethyl ester. The displacement studies indicated that proteolipid bound DCCD via carboxyl group interaction in a hydrophobic region of the protein. SDS-polyacrylamide gel electrophoresis showed that all label was associated with a single band of 8500 Mr. No non-specific binding of 14C-DCCD to phospholipids occurred, since all bound label was associated with protein following Sephadex LH-20 chromatography of crude proteolipid. Phospholipid liposomes were prepared containing bacteriorhodopsin and proteolipid or proteolipid-14C-DCCD, via cholate dialysis. Transmembrane pH changes established by the bacteriorhodopsin H+ pump were measured in the presence and absence of added proteolipid. Proteolipid had an effect similar to those of uncouplers such as tetraphenylboron. Both the rate and extent of proton translocation increased following addition of proteolipid to BR-liposomes. 14C-DCCD abolished the proteolipid-augmented ion transport. When tetraphenylboron was used to abolish the transmembrane electrical potential, calcifiable proteolipid did not augment proton transport.(ABSTRACT TRUNCATED AT 250 WORDS)

Expression of a plasma membrane proteolipid during differentiation of neuronal and glial cells in primary culture

Plasma membrane proteolipid protein (PM-PLP) synthesis was examined in embryonic rat neurons and neonatal rat glial cells during differentiation in culture. Glial cultures were treated with 1 mM N6, O2, dibutyryl cyclic adenosine monophosphate (dbcAMP) following confluency to induce differentiation, which resulted in the elaboration of long cellular processes. However, no changes in the biosynthetic level of PM-PLP was observed during the differentiation of these cells. Neurons differentiated spontaneously in culture, forming cellular aggregates immediately following plating and elaborating a network of neurites over 7 days. The differentiation of neurons was accompanied by a seven-fold increase in PM-PLP synthesis with increases in biosynthetic increase in PM-PLP synthesis with increases in biosynthetic rate observed between days 1 and 3 and between days 3 and 7 in culture. Ultrastructural examination of neurons indicated that the Golgi apparatus was also developing during this period of time, with an increase in both the number of lamellae and generation of vesicles. The transport of PM-PLP to the plasma membrane was therefore examined in neurons at day 7 in culture by pulse labeling experiments with monensin and colchicine. Monensin (1 microM) was found to inhibit the appearance of radiolabeled PM-PLP in the plasma membrane by 63%, indicating that a functional Golgi apparatus is required for transport of PM-PLP to its target membrane. Colchicine (125 microM) also inhibited the appearance of newly synthesized PM-PLP in the plasma membrane by greater than 40%, suggesting that microtubules may also be required for PM-PLP transport to the plasma membrane.

Characterization and biosynthesis of the plasma membrane proteolipid protein in neural tissue

In this study we have characterized, in brain, the expression of a plasma membrane proteolipid protein (PM-PLP) complex that can form cation-selective channels in lipid bilayers. We isolated PLP fractions from synaptic plasma membrane and glial microsomes and found a high degree of similarity in both size and amino acid composition to the complex we had previously isolated from kidney. Antibodies specific to the kidney PM-PLP were prepared, and, on the basis of immunoblot and immunoprecipitation studies, the PM-PLP complex isolated from neural membranes was shown to be immunologically related to the kidney PM-PLP. These proteolipid proteins exhibited a molecular weight of approximately 14K and contained a high percentage of hydrophobic amino acids with an apparent absence of cysteine. The biogenesis of PM-PLP in brain was studied by in vitro translation of free and bound polysomes and total RNA in a rabbit reticulocyte lysate followed by immunoprecipitation of the translation products. From these studies it is concluded that the PM-PLP complex is synthesized on the rough endoplasmic reticulum. On the basis of the identical electrophoretic mobility of material isolated from plasma membranes and material immunoprecipitated after translation of bound polysomes and isolated RNA, it appears that the PM-PLP does not undergo detectable posttranslational processing between its site of synthesis and its incorporation into the plasma membrane.

Chronic experimental allergic encephalomyelitis in guinea pigs: immunologic studies on the two major myelin proteins

Humoral and cell-mediated immunity to the two major myelin proteins, basic protein (MBP) and proteolipid protein, have been investigated during the course of chronic experimental allergic encephalomyelitis (EAE) induced in guinea pigs with whole neural tissue. A positive proliferative response to MBP was observed at 10 and 13 days postimmunization, but was not detectable at subsequent stages. Serum antibodies to MBP first appeared during the chronic stages of the disease. A proliferative response to proteolipid apoprotein was not detected during any stage of chronic EAE. Guinea pigs immunized with proteolipid alone, however, showed a proliferative response. The data suggest that MBP is one of the antigens involved in the induction of the acute episode of chronic EAE, but its role in later stages and that of proteolipid protein remain unknown.