Extrinsic membrane proteins

Mnrata, N., and Miyao, M., 1985. Extrinsic membrane proteins in the photosynthedc oxygen-evolving complex. Trends in Biochemical Sciences 10 122-124. 

The interaction of an extrinsic membrane protein with a lipid bilayer can also be investigated by energy transfer. The interaction of cytochrome c has attracted much attention, and in an early study by Shaklai et al.(()5> the number of binding sites per red cell was determined. It was shown that an equation analogous to the Stem-Volmer relationship could be derived ... 

Once synthesized several factors influence the particular leaflet of the membrane lipid bilayer where the lipids reside. One is static interactions with intrinsic and extrinsic membrane proteins which, by virtue of their mechanism of biosynthesis, are also asymmetric with respect to the membrane. The interaction of the cytoplasmic protein, spectrin with the erythrocye membrane has been the subject of a number of studies. Coupling of spectrin to the transmembrane proteins, band 3 and glycophorin 3 via ankyrin and protein 4.1, respectively, has been well documented (van Doit et al, 1998). Interaction of spectrin with membrane lipids is still somewhat conjectural but recent studies have characterized such interactions more precisely. O Toole et al. (2000) have used a fluorescine derivative of phosphatidylethanolamine to investigate the binding affinity of specttin to lipid bilayers comprised of phosphatidylcholine or a binary mixture of phosphatidylcholine and phosphatidylserine. They concluded on the basis... 

The Mg2+-activated ATPase (or ATP synthase) is made up of two parts. The Fj component is the catalytic, Mg2+-binding, extrinsic membrane protein composed of five different subunits, a, (3, y, S and e. The F0 component is an intrinsic membrane complex that contains three subunits, a, b and c, and mediates proton translocation. The F, protein is bound to the membrane through interaction with F0. The complexity of the F,F0 enzyme has presented many difficulties. Hie greatest advances have been made for the bacterial enzymes, notably for thermophiles, Escherichia coli and Rhodospirillum rubrum, where progress has been made in the purification of subunits and their reconstitution into membranes, and the identification of binding sites for Mg2+ and nucleotides on the Fi subunits.300 FiF0 preparations can be incorporated into liposomes and display H+ translocation, ATP-P, exchange and ATP synthesis.301... 

Clathrin is most likely an extrinsic membrane protein on the cytosol side. It participates in receptor-mediated endocytosis processes. 

Fig. 3 Structural model of the cell membrane. The membrane is composed of a bimolecular leaflet of phospholipid with the polar head groups facing the extracellular and cytosolic compartments and the acyl groups in the middle of the bilayer. Integral membrane proteins are embedded in the lipid bilayer. Integral proteins are glycosylated on the exterior surface and may be phosphorylated on the cytoplasmic surface. Extrinsic membrane proteins, peripheral proteins, are linked to the cytosolic surface of the intrinsic proteins by electrostatic interactions. (From Ref. l)...

The correlation of the extent to which Tris inactivates photos)mthetic O2 evolution with the loss of 2/3 of the Mn bound to thylakoid membranes provided the first evidence that the oxygen-evolving complex (OEC) contained four bound Mn (1). Due to the linear dependence of activity and bound Mn, Tris was h) othesized to cause the concerted release of all four Mn from each reaction center. Later experiments which quantitated functional Mn after the removal of adventitious Mn by washing thylakoids with 50 mM CaCl2 confirmed the ratio of 4 Mn/OEC (2). However, in these experiments the amount of Mn released during inactivation by Tris or NH2OH was variable with the conditions of the extraction. This partial extraction of Mn was interpreted to result from a loss of one to three Mn/OEC in all of the reaction centers rather than the loss of 4 Mn/OEC in 1/4 to 3/4 of the reaction centers, respectively. Tris and NH2OH are now known to remove the extrinsic membrane proteins associated with the OEC (3,4). In the absence of these proteins, the mM concentrations of Ca " " used to remove adventitious Mn from thylakoids also activate the OEC and inhibit the loss of Mn from the enzyme (5-7). 

Upon Triton X-114/water phase partitioning of photosystem I particles, most of the proteins that were released by the high salt wash (5.0, 8.2, 17.2 and 19.3 kDa), all partitioned to the aqueous phase (fig. 2 lane 1). This is in line with the washing experiment and confirms that these photosystem I proteins are hydrophilic and consequently extrinsic membrane proteins. In this experiment we were not able to resolve the 10.5 kDa polypeptide. 

As shown in Figure 2D, the y subunit of CFj was not present in the purified p subunit preparation. The 35 kDa protein that is observed is apparently an extrinsic membrane protein that is unrelated to the P subunit but which is solubilized from the membrane at high salt concentrations. It is possible that this protein is the extrinsic 33 kDa protein (P33) of photosystem II which is known to be removed by washes in 1 M Ca02 (39). 

The third class is that of the peripheral or extrinsic membrane proteins. They reside entirely in the (aqueous) extracellular or intracellular space. They are physically bound to the manbrane by specific and/or nonspecific interactions. Most membrane-linked enzymes are such extrinsic proteins. 

C-Phycocyanin, a biliprotein isolated from blue-green and red algae [10], was tested in the chloroplast extract BLM system [4]. This protein has been identified as an extrinsic membrane protein in structures known as phycobilisomes [10]. The well established function of this protein is as an accessory energy transfer pigment for Photosystem II [11]. 

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