protein targeting

Definition

Target type: biologicalprocess

The process of targeting specific proteins to particular regions of the cell, typically membrane-bounded subcellular organelles. Usually requires an organelle specific protein sequence motif. [GOC:ma]

Protein targeting is the process by which newly synthesized proteins are delivered to their correct destinations within the cell. This process is crucial for maintaining cellular function and is essential for many biological processes, including cell signaling, metabolism, and organelle biogenesis. The targeting process is highly selective, ensuring that each protein reaches its intended location and performs its specific function.

The first step in protein targeting is the synthesis of the protein on ribosomes. Ribosomes can be either free in the cytoplasm or attached to the endoplasmic reticulum (ER). The location of ribosome binding determines the initial targeting pathway.

For proteins destined for the cytoplasm, nucleus, mitochondria, or peroxisomes, ribosomes remain free in the cytoplasm. These proteins are synthesized and then either remain in the cytoplasm or are further transported to their specific destinations. For example, proteins with nuclear localization signals (NLS) will be imported into the nucleus.

Proteins targeted to the ER, Golgi apparatus, lysosomes, plasma membrane, or secretion are synthesized on ribosomes that bind to the ER. The signal recognition particle (SRP) recognizes a signal sequence on the nascent polypeptide chain and binds to it, halting translation. The SRP-ribosome complex then interacts with the SRP receptor on the ER membrane, initiating protein translocation.

During translocation, the nascent polypeptide chain crosses the ER membrane through a protein translocator. The signal sequence is cleaved off by a signal peptidase, and the protein folds into its correct conformation within the ER lumen.

After folding, proteins can be further modified in the ER, such as glycosylation. They then move to the Golgi apparatus, a series of flattened membrane-bound sacs, for further processing and sorting.

Proteins destined for the lysosomes are tagged with mannose-6-phosphate (M6P) in the Golgi. M6P receptors bind these proteins and deliver them to the lysosomes.

Proteins targeted for the plasma membrane are sorted in the trans-Golgi network and packaged into transport vesicles. These vesicles bud off from the Golgi and fuse with the plasma membrane, delivering the protein to the cell surface.

Proteins destined for secretion are packaged into secretory vesicles. These vesicles fuse with the plasma membrane and release their contents into the extracellular space.

The entire process of protein targeting is highly regulated and involves the interplay of numerous proteins and factors. This complex machinery ensures that each protein reaches its correct destination, enabling the cell to function efficiently.'
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Proteins (7)

Compounds (15)