Question

Write a short review on two (2) approaches that has been used to study the mechanism of protein translocation. Maximum of 2 pages and include not more than 10 references.

Answer 1

Ans.- Protein Translocation :- It is a process by which proteins move between cellular compartments i.e.,cell organelles. Short amino-acid sequences are  known as signal peptides or signal sequences.These sequences can direct its localisation, although translocation also occurs in the absence of these signal sequences.

In prokaryotes, signal peptides direct the newly synthesized protein to the SecYEG protein-conducting channel or translocon which is present in the plasma membrane. In eukaryotes the signal peptide directs the newly synthesized protein to the Sec61 channel also known as translocon which is present in the endoplasmic reticulum.

Günter Blobel was awarded the 1999 Nobel prize on the translocation of proteins across membranes.He discovered that many proteins have a signal sequence, that is, a short amino acid sequence at one end that functions like a postal code for the target organelle.If the synthesized proteins belong in a different organelle ,i. e.,except ribosome, they can be transported there in either of two ways depending on the protein:

(1) Co-translational translocation (translocation during the process of translation)

(2) Post-translational translocation (translocation after the process of translation is complete).

(1) Co-translational translocation:-This process begins with the N-terminal signal peptide of the protein being recognized by a signal recognition particle (SRP).The ribosome provides the motive power that pushes the growing peptide into the ER lumen.Most proteins that are secretory, membrane-bound, or reside in the endoplasmic reticulum (ER), golgi or endosomes use the co-translational translocation pathway.Within the ER, the protein is first covered by a chaperone protein to protect it from the high concentration of other proteins in the ER, giving it time to fold correctly. Once folded, the protein is modified as needed (for example, by glycosylation), then transported to the Golgi for further processing and goes to its target organelles.In secretory proteins and type I transmembrane proteins, the signal sequence is immediately cleaved from the nascent polypeptide once it has been translocated into the membrane of the ER (eukaryotes) or plasma membrane (prokaryotes) by signal peptidase. The signal sequence of type II membrane proteins and some polytopic membrane proteins are not cleaved off and therefore are referred to as signal anchor sequences. However, many complex multi-transmembrane proteins contain structural aspects that do not fit the model. Seven transmembrane G-protein coupled receptors (which represent about 5% of the genes in humans) mostly do not have an amino-terminal signal sequence.

(2) Post-translational translocation:- During posttranslational translocation, additional proteins are necessary to ensure that the peptide moves unidirectionally into the ER membrane.

In prokaryotes this requires certain cofactors such as SecA and SecB. This pathway is facilitated by Sec62 and Sec63, two membrane-bound proteins. The Sec63 complex is embedded in the ER membrane. The Sec63 complex causes hydrolysis of ATP, which allows chaperone proteins to bind to an exposed peptide chain and slide the polypeptide into the ER lumen.A cytosolic complex of molecular chaperones and ATP (+ATP) facilitate this pathway of protein translocation. The translocon consists of the Sec61α,β,γ proteins and Sec63p. Other proteins, Sec62p, Sec71p, and Sec72p, are also required for efficient protein translocation. Kar2p is the yeast homologue of GRP78 and is required to pull the substrate protein into the ER lumen.The signal peptide (SP) at the amino terminus of substrate proteins is cleaved by signal peptidase (SPase). Once in the lumen the polypeptide chain can be folded properly. This occurs in only unfolded proteins that are in the cytosol.

In addition, proteins targeted to other destinations, such as mitochondria, chloroplasts, or peroxisomes, use specialized post-translational pathways. Also, proteins targeted for the nucleus are translocated post-translation.