Transmembrane Domain Partitioning Regulates Kinetics of Protein Trafficking Through the Secretory Pathway

Core Concepts

Partitioning of transmembrane proteins between ordered and disordered membrane domains regulates their kinetics of transport through the secretory pathway, with raft affinity facilitating faster exit from the Golgi but not the ER.

Abstract

The study investigates the role of lipid-driven membrane domains, known as lipid rafts, in regulating the kinetics of protein trafficking through the secretory pathway. The authors used a panel of minimal transmembrane domain (TMD) probes with defined preferences for raft and non-raft domains to dissect the effects of raft affinity on protein transport.
Key findings:

ER exit kinetics are primarily determined by the presence of a ΦxΦxΦ cytosolic sorting motif, rather than TMD-encoded raft affinity.
In contrast, Golgi exit kinetics are highly dependent on raft affinity, with raft-preferring probes exiting the Golgi ~2.5-fold faster than non-raft probes.
A kinetic model supports the role of partitioning between coexisting raft and non-raft Golgi membrane domains in facilitating cargo export.
Microscopy experiments reveal segregation of raft and non-raft probes within the Golgi, which is disrupted by inhibition of raft lipid synthesis.
These observations suggest that lipid-driven membrane organization plays an important role in regulating the kinetics of protein trafficking through the secretory pathway, particularly at the level of Golgi export.

Stats

Raft partition coefficient (Kp,raft) of LAT-TMD is ~4-fold higher than allL-TMD.
ER exit half-time (t1/2) of full-length LAT is ~4-fold faster than LAT-TMD.
Golgi exit t1/2 of LAT is ~2.5-fold faster than LAT-allL.

Quotes

"Partitioning to an ER-exit compartment (ERex) (analogous to a cellular ER exit site) allows ER efflux with first-order rate constant ka."
"Golgi exit kinetics are highly dependent on raft affinity, with raft preferring probes exiting Golgi ∼2.5-fold faster than probes with minimal raft affinity."

Key Insights Distilled From

Partitioning to ordered membrane domains regulates the kinetics of secretory traffic

by Castello-Ser... at www.biorxiv.org 04-19-2023

https://www.biorxiv.org/content/10.1101/2023.04.18.537395v2

Deeper Inquiries

How do the mechanisms of raft-mediated sorting in the Golgi relate to other known Golgi sorting pathways, such as those involving clathrin and adaptor proteins?

The mechanisms of raft-mediated sorting in the Golgi complement and interact with other known Golgi sorting pathways, such as those involving clathrin and adaptor proteins. Clathrin-mediated sorting typically involves the recognition of cargo proteins by clathrin adaptor proteins, leading to the formation of clathrin-coated vesicles that transport cargo between Golgi compartments or to the plasma membrane. In contrast, raft-mediated sorting relies on the affinity of certain membrane proteins for lipid rafts, which are specialized membrane microdomains enriched in cholesterol and sphingolipids.
Interestingly, some cargo proteins may utilize both pathways simultaneously or sequentially. For example, a cargo protein may first be sorted into clathrin-coated vesicles for transport within the Golgi stack and then further sorted into raft-enriched vesicles for delivery to the plasma membrane. The interplay between these pathways allows for the precise and efficient sorting of diverse cargo proteins to their respective destinations within the cell.

How do the potential physiological implications of raft-dependent Golgi export kinetics for the trafficking of different classes of membrane proteins?

The potential physiological implications of raft-dependent Golgi export kinetics for the trafficking of different classes of membrane proteins are significant. Raft-dependent Golgi export kinetics can impact the efficiency and specificity of protein trafficking to the plasma membrane and other organelles.
For membrane proteins that preferentially associate with lipid rafts, rapid Golgi exit facilitated by raft domains ensures timely delivery to the plasma membrane, where these proteins may play crucial roles in cell signaling, adhesion, and other cellular processes. On the other hand, proteins with minimal raft affinity may have slower Golgi exit kinetics, leading to their accumulation in the Golgi or redirection to alternative trafficking pathways.
Understanding the raft-dependent Golgi export kinetics of different classes of membrane proteins can provide insights into how cells regulate the spatial and temporal distribution of proteins within the cell. This knowledge is essential for maintaining cellular homeostasis, responding to extracellular signals, and carrying out specialized functions in various tissues and organs.

Could the principles of raft-mediated sorting uncovered in this study be extended to understand the organization and dynamics of other membrane-bound organelles beyond the Golgi?

The principles of raft-mediated sorting uncovered in this study could indeed be extended to understand the organization and dynamics of other membrane-bound organelles beyond the Golgi. Lipid rafts are not exclusive to the Golgi but are also present in other cellular membranes, including the plasma membrane, endosomes, and lysosomes.
By applying similar methodologies and concepts used in this study to other organelles, researchers can investigate how lipid rafts influence the sorting, trafficking, and localization of membrane proteins in diverse cellular compartments. For example, studying the role of lipid rafts in endosomal sorting pathways could provide insights into the mechanisms of receptor recycling, lysosomal targeting, and intracellular signaling.
Furthermore, understanding the organization and dynamics of lipid rafts in different organelles can shed light on how cells maintain membrane integrity, regulate membrane protein composition, and respond to environmental cues. This knowledge is crucial for unraveling the complex network of intracellular membrane trafficking and signaling pathways that govern cellular function.