Translocation, protein insertion membranes

Fig. 1. Model for membrane protein insertion into the ER membrane. In stage I, an N-terminal signal peptide (not shown) has already initiated translocation across the membrane.

Chloroplasts in higher plants have three membranes the outer and inner envelope membranes and the thylakoid membrane. Very little is known about membrane protein assembly into the two envelope membranes (Soil and Tien, 1998). The thylakoid has been better studied and in fact appears to use mechanisms very similar to those found in E. coli for membrane protein insertion (Dalbey and Robinson, 1999). Thus, SRP, SecA, SecYEG, YidC, and Tat homologues are all present in the thylakoid membrane or in the stroma (the Tat system was first identified in thylakoids, in fact). In contrast to E. coli, however, there are thylakoid proteins that appear to insert spontaneously into the membrane, insofar as no requirement for any of the known translocation machineries has been detected (Mant et al, 2001). 

Nilsson, I., Witt, S., Kiefer, H., Mingarro, I., and von Heijne, G. (2000). Distant downstream sequence determinants can control N-tail translocation during protein insertion into the endoplasmic reticulum membrane./. Biol. Chem. 275, 6207-6213. 

Other adhesins of E. coli as antigen 43, AIDA-I, TibA and intimin of enter-opathogenic and enterohemorrhagic E. coli are true afimbrial adhesins i.e. they are integral outer membrane proteins. However, also intimin seems to be involved in invasion of host cells [62, 63], Intimin, which is actually a whole family of adhesins, is the only example of an adhesion that uses a protein (Tir translocated intimin receptor) in the host cell membrane, that is a bacterial protein inserted into the host by the bacterial type 3 protein secretion system [64],... 

The mechanism for translocating bacterial proteins across the Inner membrane shares several key features with the translocation of proteins into the ER of eukaryotic cells. First, translocated proteins usually contain an N-termlnal hydrophobic signal sequence, which is cleaved by a signal peptidase. Second, bacterial proteins pass through the Inner membrane In a channel, or translocon, composed of proteins that are structurally similar to the eukaryotic Sec61 complex. Third, bacterial cells express two proteins, Ffh and its receptor (FtsY), that are homologs of the SRP and SRP receptor, respectively. In bacteria, however, these latter proteins appear to function mainly In the insertion of hydrophobic membrane proteins Into the Inner membrane. Indeed, all bacterial proteins that are translocated across the inner membrane do so only after their synthesis In the cytosol is completed but before they are folded Into their final conformation. 

Table 9.1 provides the values of membrane resistance (/ ), capacitance (Cm), and thickness d) of artificial BLMs and natural cell membranes [11,18]. The resistance of artificial membranes is much higher than that of biological membranes. This results from the presence of translocators such as peptides and proteins in the cell membranes. The resistance of artificial membranes can however be reduced to the levels of natural cell membranes when ion translocators are inserted. Specific capacitance (C ) is the primary criterion to distinguish between solventless BLMs and black lipid films. Table 9.1 exhibits that the specific capacitance of the solventless BLMs (about 0.9 /itF cm ) approaches the values measured for natural cell membranes, and is almost twice the magnitude observed for black lipid membranes. These values of specific capacitance can be used to estimate the hydrocarbon thickness, d, of membranes using the equation... 

Recent studies by Katz et al. (1977) have served to demonstrate a likely method for membrane glycoprotein synthesis—specifically, the insertion of the G protein of vesicular stomatitus virus (VSV) into pancreatic endoplasmic reticulum (PER). Their results are consistent with a scheme of membrane glycoprotein synthesis which postulates that the growing protein is extruded across the ER membrane with the amino terminus extruded first and is then translocated across the membrane. Asymmetric insertion, as well as glycosylation, requires the presence of membranes during protein synthesis, and glycosylation is restricted to the luminal surface of the ER. 

The phosphatidylcholine in bile is synthesised in the endoplasmic reticulum of the hepatocyte and must be transported to the canalicular membrane. One possibility involves the nonspecific phosphatidylcholine transfer protein but a mouse null for this protein did not show reduced phosphatidylcholine secretion into bile and there was no compensatory increase in other phospholipids transfer proteins. However, the plasma membrane would receive a ready supply of phospholipid by insertion of vesicles, and the MDR3 protein translocates this molecule from the inner leafiet to the outer surface where there is contact with bile acids, as suggested by Smit and colleagues. The role of this transporter is shown in Figure 2.2. 

To function, Ras must be attached to the plasma membrane. Translocation from the cytoplasm to membrane requires a series of posttranslational modifications that begin with farnesylation of the cysteine residue, the fourth amino acid residue from the C terminus of the protein, by famesyl protein transferase (FPTase) (64). Attachment of the hydrophobic 15-carbon lipid farnesyl group allows Ras molecule insertion into the plasma membrane and is crucial for Ras signaling activity and transformation properties. As farnesylation is required for oncogenic Ras function, FPTase inhibitors (FTIs) are obvious candidate antineoplastic agents. Several drugs that inhibit Ras farnesylation are at various stages of clinical development (65). 

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