Protein into phospholipid monolayers

A much more complex behaviour is observed for the process of penetration of various proteins into phospholipid monolayers. This behaviour depends strongly on the protein and the solution properties although some common features are observed. Fainerman et al. [116] studied the P-lactoglobulin penetration dynamics into DPPC monolayers. For a (i-lactoglobulin bulk concentration of 510 mol/l and molar areas of the lipid larger than the critical value, A > A, first order phase transitions are observed. Thus, two-dimensional condensed phase are formed although at these molar area values the pure DPPC monolayer exists only in the fluid-like state and does not form any domains. The first-order phase transition in the DPPC monolayer becomes visible by the characteristic break point in the dynamic surface pressure curve Fl(t) (see Fig. 4.50). 

Generally, the recrystaUization of S-layer protein into coherent monolayer on phospholipid films was demonstrated to depend on (1) the phase state of the hpid film, (2) the nature of the lipid head group (size, polarity, and charge), and (3) the ionic content and pH of the subphase [122,138] (Table 6). 

In vivo, the folding process may be supported by a periplasmic chaperone called Skp. Skp is a 17 kDa protein associated with the plasma membrane that, together with peptidyl prolyl isomerases and disulfideexchanging enzymes, helps folding freshly synthesized proteins in the periplasm (Schafer et al., 1999). Skp binds to partially unfolded polypeptides. Depending on the presence of phospholipids, lipopolysaccharides, and bivalent cations, Skp exists in two conformations, one of which is protease-sensitive (DeCock et al., 1999). Moreover, it was shown that Skp binds to unfolded periplasmic proteins and inserts into phospholipid monolayers, corroborating its putative role as helper in folding and membrane insertion. 

Our results on the penetration of CTAB into lecithin monolayers and those of protein penetration into phospholipid monolayers are similar. Kimelberg and Papahadjopoulos (7) found increased penetration or All when the concentration of protein in the substrate increased. In our study, this effect was noticed when the concentration of CTAB in the bulk was increased (Figure 6) to 2 X 10 4M. Then even for initial lecithin pressures of 33 dynes/cm a penetration" of about 6 dynes/cm is observed. According to the discussion of Equation 8, it seems that this increased penetration may be related to the simultaneous reorientation of the alkyl chains of CTAB, followed by penetration inside the hydro-phobic region of the lipid monolayer and enhanced attraction. 

By using a surface radioactivity technique, the penetration of the hydrophobic and flexible 1-14C-acetyl--casein and the rigid and globular 1-14C-acetyl-lysozyme molecules into phospholipid monolayers in different physical states was monitored. The adsorption of ff-casein to lecithin mono-layers is described by a model in which it is assumed that the protein condenses the lecithin molecules so that the degree of penetration is a function of the lateral compressibility of the phospholipid monolayer. The interaction of ff-casein with phospholipid monolayers is dominated by the hydrophobicity of the macromolecule, but lysozyme tends to accumulate mostly beneath phospholipid monolayers in this situation, electrostatic interactions between the lipid and protein are important. 

It can be seen that Eq. (2.157) is just the ordinary Langmuir equation in its generalised form (2.40) which follow rigorously from the analysis of chemical potentials of the components of a mixed monolayer. It was demonstrated that Pethica s equation provides the description of quite complicated systems, including the penetration of a soluble protein into the monolayer of insoluble phospholipids able to form 2D aggregates [155]. In another paper, the case of mixed layers composed of a soluble and a 2D aggregating insoluble surfactant is considered [156]. 

There are at least nine apoproteins associated with the lipoproteins, as well as several enzymes and a cholesteryl ester transfer protein. There are two major types of apoproteins. Two apoproteins (apo B100 and apo B48) are tightly integrated into the phospholipid monolayer. The other seven proteins are less tightly associated with the phospholipid and exchange among the lipoproteins. The apoproteins have three major functions. (1) They are important structural components. (2) Some of the apoproteins modulate the ac-... 

These results show that the ability of these signal peptides to interact with phospholipid monolayers indeed correlates with their in vivo activity. The pressure increases due to the functional signal peptides (8—11 dyn/cm) are in the same range as those caused by proteins known to insert into monolayers (Bougis et al., 1981). In contrast, prothrombin, which binds only to the membrane surface, causes a pressure increase in a phospholipid monolayer of 1.9-2.3 dyn/cm (Mayer et al., 1983). These values are almost identical to those obtained for perturbation of the monolayer by the deletion-mutant signal peptide. 

Penetration of Human Serum Albumin into Phosphatidylserine Monolayers. We have tried to analyze quantitatively the results of Kimelberg and Papahadjopoulos (7) of protein penetration in phospholipid monolayers. These authors did not measure the surface density of the penetrating protein. They studied the system phosphatidylserine monolayer—human serum albumin (HSA) at different pH values. When the pH was 7.4, the protein and the lipid repelled each other. When the... 

TiTuch of our understanding of the phase behavior of insoluble - monolayers of lipids at the air-water interface is derived from Adam s studies of fatty acid monolayers (I). It is now clear that the phase behavior of phospholipid monolayers (2) parallels that of the fatty acids we make use of these structure variations in our study of the interactions of phosphatidylcholine (lecithin) monolayers with proteins. Because of the biological significance of the interfacial behavior of lipids and proteins, there is a long history of studies on such systems. When Adam was studying lipid monolayers, other noted contemporary surface chemists were studying protein monolayers (3) and the interactions of proteins with lipid monolayers (4). The latter interaction has been studied by many so-called 4 penetration experiments where the protein is injected into the substrate below insoluble lipid monolayers that are spread on the... 

The theory which describes the penetration of a soluble surfactant into a monolayer formed by molecules possessing equal partial molar area (mixtures of homologues), was extended recently to include the actual process of protein penetration into 2D aggregating phospholipid monolayer [155, 157]. This extension was based on the concept of independent segments of the protein molecules, occupying an area equal to that of the phospholipid molecule. In the theoretical models, various mechanisms for the effect of the soluble surfactant on the aggregation of the insoluble component can be considered ... 

The studies on the mode of interaction of prothrombin with phospholipid monolayers, using complementary methods of surface measurement are reviewed. They were investigated at air-water and Hg-water interfaces respectively by radioactivity and electrochemistry. A process more complex than a simple adsorption could be detected. Indeed, the variation of the differential capacity of a mercury electrode in direct contact with phospholipid monolayer, induced by the interaction with prothrombin could be interpreted as a model of its penetration into the layer this was confirmed by the study of the dynamic properties of the direct adsorption of this protein at the e-lectrode, followed in part by the reduction of S-S bridges at the electrode. It could be also concluded that prothrombin resists complete unfolding at these interfaces. 

FIG. 14 Schematic illustration of an archaeal cell envelope structure (a) composed of the cytoplasmic membrane with associated and integral membrane proteins and an S-layer lattice, integrated into the cytoplasmic membrane, (b) Using this supramolecular construction principle, biomimetic membranes can be generated. The cytoplasmic membrane is replaced by a phospholipid or tetraether hpid monolayer, and bacterial S-layer proteins are crystallized to form a coherent lattice on the lipid film. Subsequently, integral model membrane proteins can be reconstituted in the composite S-layer-supported lipid membrane. (Modified from Ref. 124.)... 

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