recap

14.6 recap

Signal sequences in polypeptides direct them to their appropriate destinations inside or outside the cell. Many polypeptides are modified after translation.

learning outcomes

You should be able to:

  • Outline the steps by which a newly synthesized protein is moved into the nucleus or exported outside of the cell.

  • Justify the claim that posttranslational modification of a protein is ultimately directed by the DNA sequence of the gene encoding the protein.

Question 1

How do signal sequences determine where a protein will go after it is made?

Signal sequences are translated to regions in a protein that bind to recognition molecules and/or receptors associated with a particular destination in the cell.

Question 2

The genetic code determines the amino acid sequence of a protein. Is there a “code” that determines posttranslational modifications such as phosphorylation or glycosylation? How might this relate to the genetic code?

Post-translational modifications are made to specific amino acids at particular locations on a protein. The locations are determined by the three-dimensional shape of the protein, allowing an enzyme for the modification to bind to the amino acid at that location. So overall, the amino acid sequence of the protein determines which amino acids will be modified posttranslationally, and the amino acid sequence is determined by the genetic code and DNA sequence for the gene.

All of the processes we have just described result in a functional protein, but only if the amino acid sequence of that protein is correct. If the sequence is not correct, cellular dysfunction may result. Changes in the DNA—mutations—are a major source of errors in amino acid sequences. This is the subject of the next chapter.