Protein translocation II

What are the main types of membrane proteins based on their insertion mechanisms?

Membrane proteins can be classified as single TM segments and polytopic membrane proteins.

What determines the topology of prokaryotic transmembrane segments?

The topology is determined by the 'positive inside rule'.

What role does the multipass-translocon play in membrane protein biogenesis?

It is required for the biogenesis of many multi-spanning transmembrane proteins.

What is the significance of the first transmembrane (TM) segment for membrane proteins?

The insertion of the first TM segment is decisive for the protein’s topology.

What is the function of the PAT/BOS/GEL complex?

It organizes transmembrane protein assembly at the back side of the Sec61 complex.

How do tail-anchored proteins get inserted into the membrane?

They are inserted via a dedicated post-translational pathway called the GET pathway.

What is the EMC complex's role in membrane protein insertion?

The EMC complex is involved in the recognition and topology of transmembrane proteins (TMPs) with signal-anchor sequences and assists in tail-anchored protein insertion.

Which membrane-protein insertase family is conserved and assists in multispanning TM protein insertion?

The YidC family of membrane-protein insertases.

Which pathway is used for insertion of less hydrophobic tail-anchored proteins?

The EMC pathway is used for less hydrophobic tail-anchored proteins.

What types of membrane fractions are isolated during the analysis of digitonin-solubilized membranes?

Ribosome-bound (pellet) and ribosome-free (supernatant) fractions.

Membrane proteins

Proteins that are partially or fully embedded in cellular membranes, classified based on their topology.

Topology

The arrangement of a membrane protein within the lipid bilayer, which is influenced by hydrophobicity and specific rules such as the 'positive inside rule.'

Hydrophobicity

A property of a molecule that describes its tendency to repel water; significant for the partitioning of single TM segments into the lipid bilayer.

Positive inside rule

A principle governing the orientation of prokaryotic transmembrane segments, indicating that positively charged residues are more likely to be located on the cytoplasmic side.

Polytopic membrane proteins

Complex proteins that span the membrane multiple times and require multilayered mechanisms and additional factors for biogenesis.

Multipass-translocon

A protein complex necessary for the biogenesis of multi-spanning transmembrane proteins.

PAT/BOS/GEL complex

A complex that organizes the assembly of multipass transmembrane proteins at the near side of the Sec61 complex during their insertion.

The transition from OST to PAT/BOS/GEL occurs when 2 TM segments are already inserted into the membrane and a 3rd. TM is emerging out of the ribosome.

YidC family

A conserved family of membrane-protein insertases that assist in the insertion of multi-pass transmembrane proteins.

Tail-anchored proteins

Proteins with a single hydrophobic tail that requires a dedicated post-translational pathway for proper insertion into the membrane.

GET pathway

The pathway responsible for the insertion of tail-anchored proteins into the membrane.

EMC complex

A complex involved in the recognition, topology, and post-translational insertion of tail-anchored and some multipass transmembrane proteins.

Translocon

A channel-forming protein complex such as Sec61 that facilitates the insertion of polypeptides into the membrane.

Signal sequence

A short peptide present at the beginning of a protein that directs the transport of the protein to various cellular destinations.

What are the main types of membrane proteins based on their insertion mechanisms?

• Co-translational inserted proteins:

  • Typ I and Typ II single spanning proteins

  • multipass membrane proteins

• Post-translational inserted proteins:

  • tail-anchored proteins

  • small proteins

• Special cases:

  • beta porrins (found in the outer membrane of mitochondria, chloroplasts and Gram-negative bacteria) → get inserted by SAM and BAM

  • Lipid-Anchored Proteins:

    • Covalently attached to lipid molecules, which anchor them to the membrane.

    • Example: GPI-anchored proteins in eukaryotes

What determines the topology of prokaryotic transmembrane segments?

The topology is determined by the 'positive inside rule'.

What role does the multipass-translocon play in membrane protein biogenesis?

It is required for the biogenesis of many multi-spanning transmembrane proteins.

What is the significance of the first transmembrane (TM) segment for membrane proteins?

The insertion of the first TM segment is decisive for the protein’s topology.

What is the function of the PAT/BOS/GEL complex?

It organizes transmembrane protein assembly at the back side of the Sec61 complex.

How do tail-anchored proteins get inserted into the membrane?

They are inserted via a dedicated post-translational pathway called the GET pathway.

What is the EMC complex's role in membrane protein insertion?

The EMC complex is involved in the recognition and topology of transmembrane proteins (TMPs) with signal-anchor sequences and assists in tail-anchored protein insertion.

Which membrane-protein insertase family is conserved and assists in multispanning TM protein insertion?

The YidC family of membrane-protein insertases.

Which pathway is used for insertion of less hydrophobic tail-anchored proteins?

The EMC pathway is used for less hydrophobic tail-anchored proteins.

What types of membrane fractions are isolated during the analysis of digitonin-solubilized membranes?

Ribosome-bound (pellet) and ribosome-free (supernatant) fractions.

Membrane proteins

Proteins that are partially or fully embedded in cellular membranes, classified based on their topology.

Topology

The arrangement of a membrane protein within the lipid bilayer, which is influenced by hydrophobicity and specific rules such as the 'positive inside rule.'

Hydrophobicity

A property of a molecule that describes its tendency to repel water; significant for the partitioning of single TM segments into the lipid bilayer.

Positive inside rule

A principle governing the orientation of prokaryotic transmembrane segments, indicating that positively charged residues are more likely to be located on the cytoplasmic side.

Polytopic membrane proteins

Complex proteins that span the membrane multiple times and require multilayered mechanisms and additional factors for biogenesis.

Multipass-translocon

A protein complex necessary for the biogenesis of multi-spanning transmembrane proteins.

PAT/BOS/GEL complex

A complex that organizes the assembly of transmembrane proteins at the rear side of the Sec61 complex during their insertion.

YidC family

A conserved family of membrane-protein insertases that assist in the insertion of multi-pass transmembrane proteins.

Tail-anchored proteins

Proteins with a single hydrophobic tail that requires a dedicated post-translational pathway for proper insertion into the membrane.

GET pathway

The pathway responsible for the insertion of tail-anchored proteins into the membrane.

EMC complex

A complex involved in the recognition, topology, and post-translational insertion of tail-anchored and some multipass transmembrane proteins.

Translocon

A channel-forming protein complex such as Sec61 that facilitates the insertion of polypeptides into the membrane.

Signal sequence

A short peptide present at the beginning of a protein that directs the transport of the protein to various cellular destinations.

Ribosome-bound fraction

The portion of the membrane that contains ribosomes attached to nascent polypeptides, isolated during the study of protein insertion mechanisms.

Ribosome-free fraction

The portion of the membrane that does not contain any ribosomes, often used as a control in experiments involving membrane protein isolation.