Re: How are proteins targeted to there destination in the cell?

Dear Rebecca,

Protein sorting is certainly the primary process that targets proteins to their correct destinations within cells. These destinations are typically subcellular compartments, often within organelles such as mitochondria & chloroplasts. This process is summarized in the introductory section of the Protein Sorting: Organelle Biogenesis and Protein Secretion chapter from the online Molecular Cell Biology textbook.

The answer to your other question regarding the mechanisms that operate to target these proteins to their destinations within the cell is just a bit more complicated, but actually fairly well-understood. As explained in the Protein Sorting: Organelle Biogenesis and Protein Secretion section of Molecular Cell Biology:

"These proteins remain in the cytosol unless they contain a specific signal sequence that directs them to the mitochondrion, chloroplast, peroxisome, or nucleus. Many of these proteins are subsequently sorted further to reach their correct destinations within these organelles; such sorting events depend on yet other signal sequences within the protein. Each sorting event involves binding of a signal sequence to one or more receptor proteins on the surface or interior of the organelle."

This fairly complex process is illustrated beautifully in Figure 17-1. Overview of sorting of nuclear-encoded proteins in eukaryotic cells. Molecular Cell Biology also includes a very helpful Table 17-1. Properties of Uptake-Targeting Signal Sequences That Direct Proteins from the Cytosol to Organelles, describing the individual signal sequences in more detail.

Finally, there are also individual sections providing more thorough explanations of protein uptake-targeting by mitochondria & chloroplasts, peroxisomes, and the nucleus, if you're interested. These sections include details about protein sorting to more specific destinations within mitochondria & chloroplasts, as well.

I hope that this info. helps to answer your excellent questions. Thanks very much,

Jeff Buzby, Ph.D.
CHOC Research Institute