Nucleo-Cytoplasmic Exchange

ID #2006

Unless an mRNA has a signal recognition particle it wont be synthesized on the ER membrane, right? And if a protein is synthesized by a free ribosome, it cannot go into the nucleus, right? If these are true, then how can a protein synthesized without an SRP be localized to the nucleus?

mRNAs don't have signal recognition particles. mRNAs have codons, which get translated into amino acids, some of which act as a signal peptide, which in turn can be bound by a signal recognition particle. But once we've made that connection, then the answer is yes, a protein which is not bound by an SRP will not be translated at the ER membrane, it will be translated out in the cytoplasm on a free cytosolic ribosome along with every other protein in the cell. That's the key to this question -- the fact that all proteins that you might try to detect are being synthesized by free cytosolic ribosomes. Some of them were proteins that were "supposed" to go to the ER/Golgi/Lysosome/Plasma Membrane/Outside but can't, and others are unaffected by the loss of SRP and will still be able to go wherever they were supposed to, like the Cytoplasm/Nucleus/Mitochondria/Chloroplast/Peroxisome.

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