Protein translocation, mechanism

These macromolecules include histones, ribosomal proteins and ribosomal subunits, ttansctiption factors, and mRNA molecules. The transport is bidirectional and occurs through the nucleat pote complexes (NPCs). These are complex stmctures with a mass approximately 30 times that of a ribosome and are composed of about 100 diffetent proteins. The diameter of an NPC is approximately 9 run but can increase up to ap-ptoximately 28 nm. Molecules smaller than about 40 kDa can pass through the channel of the NPC by diffusion, but special translocation mechanisms exist fot latget molecules. These mechanisms are under intensive investigation, but some important features have already emerged. 

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). 

Like the Tom and Tim systems on mitochondrial outer and inner membranes, chloroplasts use the Toe and Tic systems on their outer and inner envelope membranes. Although there may not be a direct correspondence between both subunits, their functions for protein translocation appear quite similar. Thus, most of the sorting mechanisms within the envelope membranes are recognized as variations of the general sorting pathway to the stroma. 

Heat-shock proteins (Hsps) are proteins expressed virtually in all organisms as a response of exposure to a stress, such as elevated temperature (fever), protein degradation, mechanical or chemical stress. As chaperone proteins they are concerned with the intracellular folding and refolding, assembly and translocation of damaged proteins. 

Blobel discovered the mechanism for protein translocation across the endoplasmic reticulum membrane—the signal hypothesis. 

Considering the highly processive mechanism of the protein degradation by the proteasome, a question naturally arises what is a mechanism behind such translocation rates Let us discuss one of the possible translocation mechanisms. In [52] we assume that the proteasome has a fluctuationally driven transport mechanism and we show that such a mechanism generally results in a nonmonotonous translocation rate. Since the proteasome has a symmetric structure, three ingredients are required for fluctuationally driven translocation the anisotropy of the proteasome-protein interaction potential, thermal noise in the interaction centers, and the energy input. Under the assumption that the protein potential is asymmetric and periodic, and that the energy input is modeled with a periodic force or colored noise, one can even obtain nonmonotonous translocation rates analytically [52]. Here we... 

Voos, W., H. Martin, T. Krimmer, and N. Pfanner. 1999. Mechanisms of protein translocation into mitochondria. Biochim Biophys Acta 1422 235-54. 

Membrane phospholipids are synthesized on the cytoplasmic side of SER membrane. Because the polar head groups of phospholipid molecules make transport across the hydrophobic core of a membrane an unlikely event, a translocation mechanism is used to transfer phospholipids across the membrane to ensure balanced growth. Choline-containing phospholipids are found in high concentration on the lumenal side of ER membrane because a prominent phospholipid translocator protein called flippase preferentially transfers this class of molecule. 

Hall BS, Hill K, McKenna M, Ogbechi J, High S, Willis AE, Simmonds RE (2014) The pathogenic mechanism of the Mycobacterium ulcerans virulence factor, mycolactone, depends on blockade of protein translocation into the ER. PLoS Path 10 el004061... 

---