Question

Push, don't pull. What is the energy source that powers the cotranslational movement of proteins across the endoplasmic reticulum?

Short answer

The energy comes from GTP hydrolysis during protein synthesis.

Step-by-step solution

Understanding Cotranslational Movement

Cotranslational movement refers to the process where proteins are moved across the endoplasmic reticulum (ER) membrane while being synthesized by ribosomes. This process combines translation (protein synthesis) and membrane translocation.

Identifying Key Players

During cotranslational translocation, the ribosome, the protein translocation channel (translocon), and a signal sequence on the nascent protein are crucial. As the ribosome synthesizes the protein, the nascent chain is directed into the translocon.

Source of Energy

The movement of proteins during cotranslational translocation is largely powered by the ribosome itself. The energy for this process primarily comes from GTP hydrolysis, which is required for several steps of protein synthesis.

Role of GTP in the Process

GTP (guanosine triphosphate) provides the energy for the initiation, elongation, and termination phases of protein synthesis. Specifically, it is used by various translation factors like the elongation factors that facilitate the movement of tRNA and mRNA through the ribosome during protein synthesis.

Integration with the ER Translocon

As the protein is synthesized, the ribosome docks to the translocon on the ER membrane. The energy from GTP hydrolysis during protein synthesis provides the force that pushes the growing polypeptide chain through the translocon into the ER lumen.

Key concepts

Endoplasmic Reticulum

The endoplasmic reticulum (ER) is a vital cellular structure responsible for producing and processing proteins and lipids. It plays a crucial role in the cotranslational translocation of proteins, where proteins are synthesized by ribosomes and simultaneously moved into the ER. There are two types of ER:
- **Rough ER:** Characterized by the presence of ribosomes on its surface, which gives it a "rough" appearance. It is primarily involved in protein synthesis and processing.
- **Smooth ER:** Lacks ribosomes and is associated with lipid synthesis and detoxification processes.

During cotranslational translocation, proteins destined for secretion or for specific organelles, are translated on ribosomes bound to the rough ER. These proteins enter the ER, where they are folded and modified before moving to their final destinations.

GTP Hydrolysis

GTP hydrolysis is a crucial biochemical reaction that provides energy necessary for various cellular processes. During cotranslational translocation, GTP (guanosine triphosphate) is broken down into GDP (guanosine diphosphate) and inorganic phosphate, releasing energy in the process.

This energy is crucial for:

• Driving the conformational changes in proteins and ensuring their proper folding.
• Facilitating the actions of translation factors, which assist in moving the newly formed proteins across the endoplasmic reticulum during synthesis.
• Supporting the elongation of peptide chains and the movement of ribosomes along the mRNA.

Without GTP hydrolysis, the essential processes of protein synthesis and translocation would be inefficient and cannot occur as required.

Protein Synthesis

Protein synthesis is the biological process of building proteins from amino acids in cells, which occurs in two main stages: transcription and translation. Translation is the critical phase directly connected to cotranslational translocation.

As ribosomes read the mRNA sequence, they synthesize polypeptides by linking amino acids in the order dictated by mRNA. This chain of amino acids folds into a specific three-dimensional shape, forming a functional protein.

Durng cotranslational translocation, protein synthesis is tightly integrated with the translocation across the ER membrane. As the ribosome synthesizes the amino acids into a polypeptide chain, it simultaneously pushes the growing chain into the ER, utilizing the energy from GTP hydrolysis to facilitate this movement.

Ribosomes

Ribosomes are macromolecular structures that form the site of protein synthesis in cells. Composed of RNA and proteins, ribosomes are either free-floating in the cytosol or attached to the endoplasmic reticulum (specifically the rough ER).

Key functions of ribosomes include:

• Decoding mRNA to build polypeptide chains.
• Ensuring that amino acids are added in the correct sequence dictated by the mRNA template.
• Initiating the cotranslational translocation process by binding to the ER and interacting with the translocon to enable protein translocation into the ER.

During cotranslational translocation, ribosomes play a dual role. They are responsible for the synthesis of the protein and for driving the translocation of nascent proteins into the ER through the use of GTP-powered energy mechanisms.