Need questions 16-21 answered 16. What are the three mechanisms used to import proteins to specific organelles? 17. How are proteins directed to specific sites in the cell? 18. Describe an experiment we discussed which showed that signal sequences are both necessary and sufficient. 19. Describe how nuclear proteins are transported into the nucleus. 20. Describe how proteins are translocated into the lumen of the mitochondria. 21. Describe how proteins are directed to the ER? How are they translocated into the ER? What two kinds of proteins are transferred into the ER? What are the potential destinations for proteins that enter the ER?.