Protein Targeting to the Endoplasmic Reticulum

Introduction to Protein Targeting

Protein targeting refers to the cellular mechanisms that direct newly synthesized proteins to their correct destinations within the cell. This process is essential for proper cellular function, as proteins must reach specific organelles or membranes to perform their roles. The Endoplasmic Reticulum (ER) is a major site for protein targeting, especially for proteins destined for secretion, membrane insertion, or residence within the endomembrane system.

• Default Pathway: Proteins without targeting sequences remain in the cytoplasm.

• Nuclear Targeting: Larger proteins destined for the nucleus possess a Nuclear Localization Signal (NLS) and are imported via nuclear pores (gated transport).

• ER Targeting: Proteins targeted for the ER contain specific signal sequences and are translocated across the ER membrane into a distinct compartment (transmembrane transport).

Overview of Cellular Transport Pathways

Proteins and other molecules are transported between cellular compartments by several mechanisms. The main types of transport are:

• Gated Transport: Movement through selective pores (e.g., nuclear pores).

• Transmembrane Transport: Direct passage across membranes (e.g., ER import).

• Vesicular Transport: Transfer via membrane-bound vesicles (e.g., Golgi to plasma membrane).

These pathways ensure that proteins reach the nucleus, mitochondria, peroxisomes, plastids, ER, Golgi, lysosomes, endosomes, and the cell surface as needed.

Endoplasmic Reticulum (ER) Structure and Function

ER Structure

The ER is an extensive network of membrane tubules and sheets that extends from the nuclear envelope to the cell periphery. It is divided into two main types:

• Rough ER (RER): Studded with ribosomes; primarily involved in protein synthesis and processing.

• Smooth ER (SER): Lacks ribosomes; involved in lipid synthesis, detoxification, and calcium storage.

During cell division, the ER network breaks down into vesicles for distribution to daughter cells.

ER Functions

• Protein Synthesis: Synthesis of proteins for the endomembrane system and secretion.

• Lipid Synthesis: Production of membrane lipids.

• Storage: Storage of ions and enzymes in some cell types.

• Detoxification: Detoxifying enzymes, especially in liver cells.

Mechanisms of Protein Targeting to the ER

Signal Sequences and Targeting

Proteins destined for the ER possess an ER signal sequence, typically a stretch of hydrophobic amino acids at the N-terminus (N-terminal signal peptide or start transfer sequence). This sequence is necessary and sufficient for ER targeting.

• Co-translational Translocation: The signal sequence directs the ribosome to the ER membrane during protein synthesis.

• Signal Peptidase: Cleaves the N-terminal signal sequence after translocation, allowing the mature protein to fold in the ER lumen.

• Chaperones: Assist in proper protein folding within the ER.

Signal Recognition Particle (SRP) Pathway

The Signal Recognition Particle (SRP) is a ribonucleoprotein that recognizes the ER signal sequence as it emerges from the ribosome. The SRP binds to both the signal sequence and the ribosome, temporarily halting translation.

• SRP-ribosome complex docks at the SRP receptor on the ER membrane.

• The ribosome associates with the translocon channel, allowing the nascent polypeptide to enter the ER lumen.

• Translation resumes, and the protein is translocated into the ER.

• GTP binding and hydrolysis regulate SRP and SRP receptor interactions.

Experimental Evidence: Microsomes and Protease Protection

Microsomes are vesicles derived from fragmented ER membranes. Experiments using microsomes and proteases demonstrate that proteins are protected during synthesis, confirming co-translational translocation.

• Proteins synthesized in the presence of microsomes are not degraded by proteases unless detergent is added, indicating successful translocation into the ER.

Transmembrane Protein Topology

Single-Pass Transmembrane Proteins

Single-pass transmembrane proteins are inserted into the ER membrane via specific sequences:

• Stop Transfer Sequence: A hydrophobic stretch that halts translocation and anchors the protein in the membrane.

• Type I Proteins: N-terminal domain in ER lumen, C-terminal in cytoplasm.

• Type II Proteins: Internal start transfer (signal anchor) sequence; orientation depends on positively charged residues near the transmembrane domain.

Multipass Transmembrane Proteins

Multipass proteins use multiple start and stop transfer sequences to achieve complex topologies. The order and orientation of these topogenic sequences determine the final arrangement of transmembrane domains.

• Each hydrophobic alpha-helix acts as a signal for membrane insertion.

• Internal signals are not cleaved and become transmembrane domains.

Post-Translational Translocation

In some eukaryotes (e.g., yeast), certain proteins are translocated into the ER post-translationally. This process also uses the translocon channel and may involve chaperones to maintain the protein in an unfolded state for import.

Protein Glycosylation in the ER

N-linked Glycosylation

Most proteins imported into the ER undergo N-linked glycosylation, which is critical for protein folding and quality control.

• Oligosaccharyltransferase: An integral membrane enzyme that adds preformed oligosaccharides to the asparagine residue in the sequence Asn-X-Ser.

• Glycosylation occurs co-translationally in the ER lumen.

• Oligosaccharides are initially built on a lipid carrier (dolichol) and transferred to the protein during import.

Lipid-Anchored Proteins

Some proteins are anchored to membranes via glycosyl-phosphatidyl-inositol (GPI) anchors. GPI anchoring occurs in the ER, where an enzyme cleaves the protein and attaches it to the GPI anchor. These proteins face the lumen or extracellular space and can be rapidly released by removing the anchor.

Protein Folding and Quality Control in the ER

Chaperones and Disulfide Bond Formation

Chaperone proteins, such as BiP (a member of the Hsp70 family), assist in folding newly imported proteins. Protein Disulfide Isomerase (PDI) catalyzes the formation and rearrangement of disulfide bonds, stabilizing protein structure in the oxidizing environment of the ER lumen.

Quality Control and ER-Associated Degradation (ERAD)

Proteins that fail to fold correctly are recognized and targeted for degradation:

• ERAD: Misfolded proteins are retrotranslocated to the cytoplasm, polyubiquitinated, and degraded by the proteasome.

• Accumulation of misfolded proteins triggers the unfolded protein response (UPR), increasing transcription of chaperones and degradation machinery.

Summary Table: Types of Protein Transport

Key Equations and Sequences

• N-linked Glycosylation Signal: Asn-X-Ser (where X is any amino acid except proline)

• Protein Import:

Additional info: These notes expand on the original slides and images by providing definitions, mechanisms, and context for each process, ensuring a self-contained study guide suitable for Anatomy & Physiology students.