Protein Sorting By: Jarod, Tyler, and Tu
Definition Organizing of proteins
Organelles Nucleus Ribosomes Endoplasmic Reticulum Golgi Apparatus/Vesicles
How do they know where to go? Amino Acid Sequence (15-60) Signal Sequences Usually removed when destination is reached Can vary greatly; hydrophobicity/placement of charged amino acids
Importing Proteins Transport through selective nuclear pores Transport through membranes by protein translocators Transport through transport vesicles
Nuclear Pores Protein requires nuclear localization signal Nuclear transport receptors usher the protein in Proteins remain folded during importation
Mitochondria and Chloroplasts Protein translocators The protein is unfolded upon entry/signal sequence is removed Chaperone proteins assist in translocation
Into the ER Water soluble proteins completely translocate Transmembrane proteins are partially translocated The protein is not fully synthesized Polyribosomes are required for ER signal sequences
Into INTO the ER SRP and SRP receptor are required Signal sequence is sliced off by signal peptidase Protein is then released into the ER lumen
Into the ER Membrane Deals with transmembrane proteins Single membrane-spanning segment Double membrane-spanning segment Arrangement of the signal sequence
Quiz Time
Is a cell able to create organelles from scratch? Why or why not? No. Cells require information and materials contained in the organelle itself.
What is the name of the protein that pulls outside proteins through a membrane? They re called chaperone proteins.
Where do proteins enter the ER from? and what is the function of ER? They enter from the cytosol. The ER is used to fold proteins in the cell.
What are nuclear pores? Nuclear pores are gates that allow molecules to enter or leave the nucleus
What three ways do proteins enter membranes? 1: Through nuclear pores 2: Transport across membranes 3: Transport by vesicles
How many amino acid sequences are there in a signal sequence and what happen when it reached its destination? There are about 15 to 60 amino acid sequences, and will usually removed when destination is reached
Why do eucaryotic cells require a nucleus a separate compartment when procaryotic cells can manage perfectly well without it? Because most RNA transcripts have to be spliced before they can be translated; which the nuclear envelope separates the transcription and translation processes in space and time: a primary RNA transcript is held in the nucleus until it is properly processed to form an mrna, and only then is it allowed to leave the nucleus so that ribosomes can translate it.
Explain how an mrna molecule can remain attached to the ER membrane while individual ribosomes translating it are released and rejoin the cytosolic pool of ribosomes after each round of translation. By the ribosomes translating it; which the mrna is continuously moved through the ribosome to finish the translation dissociate from the 3 end of the mrna and from the ER membrane, but the mrna itself remains and recruited from the cytosolic pool, that have attached to the 5 end of the mrna and are still translating the mrna.
What arrangement of the signal sequences would enable the insertion of a multipass protein with an odd number of transmembrane segments It would need a cleaved signal sequence, followed by an interned stop-transfer sequence, followed by pairs of start and stop-transfer sequences.
BONUS
Describe what happen in this picture
The internal signal sequence functions as a membrane anchor, because there is no stop-transfer sequence, however, the C- terminal end of the protein continues to be translocated into the ER lumen. The resulting protein therefore has its N- terminal domain in the cytosol, followed by a single transmembrane segment that initiates translocation of the N- terminal domain of the protein until translocation is stopped by the stop-transfer sequence. A cytosolic domain is synthesized until the start-transfer sequence initiates translocation again. Finally, the C-terminal domain of the protein is translocated into the lumen of the ER, and the resulting protein therefore spans the membrane twice.which both N-terminal and C- terminal domains are in the ER lumen, and a loop domain between the two transmembrane regions is exposed in the cytosol.