Inner membrane proteins

The PemB cellular localisation was determined both in E. chrysanthenu and in an E. coli recombinant strain by Western blot of the cell fractions with a PemB-antiserum. No PemB was detected in the culture supernatant and only trace amounts were found in the soluble cell fractions - periplasm and cytoplasm (Figure 2). PemB was found mostly in the total membrane fraction from which it could be completely extracted by Triton X-100/Mg2+ and partially extracted by Sarkosyl (Figure 2). This behaviour is typical of inner membrane proteins, but since some exceptions have been noticed it does not positively indicate the PemB localisation (15). We performed cell membrane fractionation in sucrose density gradient centrifugation both by sedimentation and flotation, using several markers of inner and outer membrane vesicles. PemB was found in the outer membrane vesicles (data not shown). 

Whether or not they are lipoproteins, both periplasmic proteins and outer membrane proteins translocate across the inner membrane thus there should be some cellular mechanisms that sort them. Unlike inner membrane proteins, outer membrane proteins do not have characteristic hydrophobic transmembrane segments as such, most, if not all, of them are thought to be composed of ft strands. Moreover, it has been suggested that such conformation may be the determinant of the integration into the outer membrane in other words, these proteins may be spontaneously integrated into the outer membrane. If this assumption is correct, the outer membrane proteins must fold at the periplasm. Another possibility is that the outer membrane proteins are integrated at certain sites where the inner and outer membranes are contacted. This issue has not been solved, but a recent experiment supports the periplasmic folding (Eppens et al., 1997). 

Most of the arguments described in the sections on bacterial signal peptides and membrane proteins seem to be valid for the eukaryotic systems, as well as the translocation phenomena across the ER membrane (Sakaguchi, 1997). They seem to be also true for the translocation system across the mitochondrial inner membrane protein into the intermembrane space and the system across the thylakoid membrane in chloroplasts. Although the TAT-dependent pathway has not been found in the ER, it exists on the thylakoid membrane (and possibly on the inner membrane of mitochondria). 

The inner envelope membrane proteins have a cleavable N-terminal transit peptide, as well as some hydrophobic domain (s) in their mature portion. There are two possibilities on the role of this hydrophobic domain it may work as an N-terminal signal peptide after the translocation into the stroma and the subsequent cleavage of the transit peptide. Alternatively, it may work as a stop-transfer signal. One more important question is how the distinction is made between the outer membrane proteins, the inner membrane proteins, and the thylakoid membrane proteins. It is still an enigma. 

Gafvelin, G., and von Heijne G. (1994). Topological frustration in multispanning E. coli inner membrane proteins. Cell 77, 401-412. 

Qi, H.-Y., and Bernstein, H. (1999). SecAis required for the insertion of inner membrane proteins targeted by the Eschericha coli signal recognition particle. J. Biol. Chem. 274, 8993-8997. 

Stuart, R., andNeupert, W. (1996). Topogenesis of inner membrane proteins of mitochondria. Trends Biochem. Sci. 21, 261-267. 

The two complexes that import cytosolic proteins through the mitochondrial inner membrane, TIM22 and TIM23 split the import pathways of hydrophobic inner membrane proteins from that of presequence-containing preproteins (Fig. 1). 

Maarse AC, Blom J, Keil P, Pfanner N, Meijer M (1994) Identification of the essential yeast protein MIM17, an integral mitochondrial inner membrane protein involved in protein import. FEBS Lett 349 215-221... 

Drury, L.S. Buxton, R.S. Identification and sequencing of the Escherichia coli cet gene which codes for an inner membrane protein, mutation of which causes tolerance to colicin E2. Mol. Microbiol., 2, 109-119 (1988)... 

The inner membrane is impermeable to many substances. Neutral molecules of <150 Da can penetrate die membranes, but the permeability for all other materials including small ions such as H+, K+, Na+, and Cl- is tightly controlled. The ratio of phospholipid to protein in the inner membrane is 0.27, and cardiolipin makes up 20% of the phospholipid present. Cholesterol is absent. Ubiquinone and other components of die respiratory chain are all found in die inner membrane. Proteins account for 75% of the mass of die membrane. 

Fig. 12. Coiled coils arising between helices that are part of different folds, (a) Soluble proteins, (b) Example of a membrane protein. Inner membrane proteins are Q-helical proteins with an up-and-down topology their helices therefore favor low crossing angles and frequently show mixtures of knobs-into-holes and ridges-into-...

Ruiz, N., Cronenberg, L.S., Kahne, D., Silhavy, TJ. Identification of two inner-membrane proteins required for the transport of lipopolysaccharide to the outer membrane of Escherichia coli. Proc Natl Acad Sci USA 105 (2008) 5537-5542. 

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