The common pathway of entry into the endoplasmic reticulum (ER) of secretory, lysosomal, and plasma membrane proteins is best explained by which of the following?Show answer
(A) Binding of their mRNAs to a special class of ribosomes attached to the ER
(B) Addition of a common sorting signal to each type of protein after completion of synthesis
(C) Addition of oligosaccharides to all three types of proteins
(D) Presence of a signal sequence that targets each type of protein to the ER during synthesis
(E) Presence of a zinc finger-binding domain in these three types of proteins
ER plays important role in proteins' conformation maturation, transportation and translocation, which means their distribution in membrane bilayer.
Proteins from ER move out of the cell, into cell membrane and into cell's organelles membranes (e.g. lysosomes). Some of those proteins are expressed into cytoplasm after maturation (which includes proper folding and binding of oligosaccharides).
There are no special ribosomes, that are bound constantly to ER membrane and wait for mRNA to translate into protein. After gene (DNA) transcription of gene, mRNA is expressed into cytoplasm where ribosomes catch them. Actually, there are several binding sites on one mRNA so several ribosomes produce proteins simultaneously on one strand.
Main idea of that process is called "signal hypothesis", which says that there should be some signal that provide binding ribosome to ER and producing protein into ER, but not into cytoplasm. That signal is in first few amino acids of newly synthesized protein. After being exposed, this signal is recognized by special molecule – Signal Recognition Particle (SRP) which move ribosome with mRNA to SRP binding receptor on membrane of ER. This is very excited process which are uncovered in the MBoC clearly. Also prof. Schekman's lectures are very usefull.
Right answer is D. Gain more information in sources mentioned, because this is very interesting process, indeed.
Proteins from ER move out of the cell, into cell membrane and into cell's organelles membranes (e.g. lysosomes). Some of those proteins are expressed into cytoplasm after maturation (which includes proper folding and binding of oligosaccharides).
There are no special ribosomes, that are bound constantly to ER membrane and wait for mRNA to translate into protein. After gene (DNA) transcription of gene, mRNA is expressed into cytoplasm where ribosomes catch them. Actually, there are several binding sites on one mRNA so several ribosomes produce proteins simultaneously on one strand.
Main idea of that process is called "signal hypothesis", which says that there should be some signal that provide binding ribosome to ER and producing protein into ER, but not into cytoplasm. That signal is in first few amino acids of newly synthesized protein. After being exposed, this signal is recognized by special molecule – Signal Recognition Particle (SRP) which move ribosome with mRNA to SRP binding receptor on membrane of ER. This is very excited process which are uncovered in the MBoC clearly. Also prof. Schekman's lectures are very usefull.
Right answer is D. Gain more information in sources mentioned, because this is very interesting process, indeed.
