38  Addressing of proteins

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A protein contains on average approximately 1,000 letters or amino acids. After a protein has been produced in a cell, it is necessary to transport it to the location where it is to be used. For this purpose, each individual protein is given a complex address. Random development or assignment of this address is not imaginable. Moreover, incorrectly addressed proteins are in many cases not only useless, they can even be damaging.

Proteins are not produced at the location they are finally needed.  There are a great number of incorrect locations where a newly-formed protein could be transported, however only very few locations, and frequently only one single location, fulfils the correct purpose.

But how do proteins find the correct destination?
Newly-formed chains of amino acids contain a section in which the address is given. This address is the location where they are to be used. Each section is normally attached to the end of the longer chain representing the protein. Every correctly folded protein will fit at a certain location and must be properly addressed accordingly. A protein at an incorrect location is, however, much more dangerous than an incorrectly delivered letter, because it can cause a disease (1).
For a cell to function, it is not only necessary to produce the correct proteins, but also to solve the complex problem of precise addressing (2). To clarify the problem, during every minute of our human existence, it is necessary to produce not only one or two, but rather millions of properly addressed proteins in our body which then must be transported and properly built in. 

It is unrealistic to believe that such sequences could occur in an incremental process controlled by chance.

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(1) John Travis, Zip Code plan for proteins wins Nobel, Science News 156, 16 October, 1999, page 246.
(2) Guenter Blobel, Britannica Biography Collection.

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