Question

Design/interpret experiments to investigate the presence and function of protein targeting sequences (“zip codes”)

Can you give some possible examples?

Answer 1

Zip codes or signal sequence determine where the protein is supposed to go. The ribosome secrete proteins all have Zip codes in the N-terminal. Zip codes have 6-12 hydrophobic amino acids are cleaved by signal protease in ER.

Experiment
1. Degrade or cleave the zip code sequence changed which leads to stay in the cytoplasm.

Crude extract of cells with ribosomes and tRNA but no mRNA placed into test tube.

1. mRNA was added into the tube & the ribosome read it and made a proteins (addition of microsomes to pre-synthesized protein-no import (added mRNA first)
2. addition of microsomes before synthesis to see if the protein could get into the microsome after it was synthesized-import and cleavage of N terminal sequence (mRNA added after)
this showed that the protein must be in the membrane when it is being made to get into the ER.