Imagine protein X, destined to span the plasma membrane....

Imagine protein $X$, destined to span the plasma membrane. Assume that the mRNA carrying the genetic message for protein $X$ has already been translated by ribosomes in a cell culture. If you fractionate the cells (see Figure 6.4 ), in which fraction would you find protein X? Explain by describing its transit through the cell.

Key Concepts

Subcellular Fractionation

Subcellular fractionation is a technique used to separate cellular components based on differences in size and density. This allows for the isolation of organelles such as the ER, Golgi apparatus, and plasma membrane, making it possible to determine where specific proteins, like membrane-spanning proteins, are localized during their transit.

Secretory Pathway

The secretory pathway encompasses the cellular process by which proteins are directed from their site of synthesis to their final destination, such as the plasma membrane or extracellular space. This process involves targeting, translocation into the endoplasmic reticulum, processing in the Golgi apparatus, and ultimately transport via vesicles.

Endoplasmic Reticulum

The endoplasmic reticulum, particularly the rough ER, is an organelle where proteins destined for membranes or secretion are initially synthesized and inserted. Ribosomes dock onto the rough ER, where they facilitate co-translational translocation, allowing the nascent polypeptide to be properly inserted or translocated into the ER lumen.

Co-translational Translocation

Co-translational translocation is the process by which a nascent protein is simultaneously synthesized by a ribosome and inserted into or translocated across the ER membrane. This ensures that membrane proteins acquire the correct orientation and begin necessary post-translational modifications early in their synthesis.

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