Cell Biology

                Translocon (translocator, translocation channel):

A complex of proteins associated with the translocation of polypeptide across membranes. In eukaryotes:  transports polypeptides with a targeting signal sequence into the luminal space of ER from the cytosol. The same complex is also used to integrate proteins into the membrane itself.

In prokaryotes: a similar protein complex transports polypeptides across the plasma membrane or integrates membrane proteins.  Bacterial pathogens can also assemble other translocons in their host membranes, allowing them to export virulence factor into their target cells.

SEC 62/63

Translocation protein SEC63 homolog is a protein that in humans is encoded by the SEC63 gene.Translocation protein SEC62 is a protein that in humans is encoded by the SEC62 gene.

The Sec61 complex is the central component of the protein translocation apparatus of the endoplasmic reticulum (ER) membrane. The protein encoded by this gene and SEC62 protein are found to be associated with ribosome-free SEC61 complex. It is speculated that Sec61-Sec62-Sec63 perform post-translational protein translocation into the ER. The Sec61-Sec62-Sec63 complex might also perform the backward transport of ER proteins that are subject to the ubiquitin-proteasome-dependent degradation pathway. The encoded protein is an integral membrane protein located in the rough ER.

Mutations of this gene have been linked with autosomal dominant polycystic liver disease.

The mammalian Sec61 complex consisting of Sec61α, Sec61β, and Sec61γ has been identified as a crucial membrane component involved in the signal recognition particle (SRP)1-dependent co-translational protein translocation across the endoplasmic reticulum (ER) membrane. The Sec61 complex forms the hydrophilic pore in the membrane through which the nascent polypeptide is translocated.

Signal recognition particle (SRP) receptor

Signal recognition particle (SRP) receptor, also called docking protein, and is a dimer composed of 2 different subunits that are associated exclusively with the rough ER in mammalian cells.

Its main function is to identify the SRP units. SRP (signal recognition particle) is a molecule that helps the ribosome-mRNA-polypeptide complexes to settle down on the membrane of the endoplasmic reticulum.

The eukaryotic SRP receptor is a heterodimer of SR-alpha (70 kDa) and SR-beta (25 kDa), both of which contain a GTP-binding domain,while the prokaryotic SRP receptor comprises only the monomeric loosely membrane-associated SR-alpha.

Binding immunoglobulin protein (BiP)

Binding immunoglobulin protein (BiP) also known as 78 kDa glucose-regulated protein (GRP-78) or heat shock 70 kDa protein 5 (HSPA5) is a protein that in humans is encoded by the HSPA5 gene.

BiP is a HSP70 molecular chaperone located in the lumen of the endoplasmic reticulum (ER) that binds newly synthesized proteins as they are translocated into the ER, and maintains them in a state competent for subsequent folding and oligomerization. BiP is also an essential component of the translocation machinery, as well as playing a role in retrograde transport across the ER membrane of aberrant proteins destined for degradation by the proteasome. BiP is an abundant protein under all growth conditions, but its synthesis is markedly induced under conditions that lead to the accumulation of unfolded polypeptides in the ER.