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You got it. They'll most often have a nuclear localization sequence - a "tag" of a short length of amino acids that tell the cell "this one goes to the nucleus."

https://en.wikipedia.org/wiki/Nuclear_localization_sequence

All translation begins in cytosol, by a cytosolic (free) ribosome. Remember, the rough endoplasmic reticulum is studded with ribosomes. A protein has a specific amino acid sequence that signals the ribosome where that protein needs to move. If that signal is reached during translation of the protein, this is called co-translational protein translocation (a mouthful, I know.) This typically pertains to proteins that are involved in cell membrane structure or proteins that need to be secreted. The ribosomes move to the RER with the incomplete protein, and feed it into the RER.

For proteins that stay inside the cell, they are shuttled to where they need to go because of the recognition of their targeting sequence. This is post-translational translocation. Specifically, for nuclear import (which is very selective), a special tag called a nuclear localization signal (NLS) is attached to the protein so it can pass through the nuclear pores.

Here’s a nice paper on this: https://www.ncbi.nlm.nih.gov/books/NBK26932/ “…histones, DNA and RNA polymerases, gene regulatory proteins, and RNA-processing proteins—are selectively imported into the nuclear compartment from the cytosol, where they are made.”

I’d love to answer any questions you may have! EDITS: Spelling, structure of paragraphs

Membrane proteins of the inner nuclear membrane (INM) was part of my research project, once upon a time. The nuclear pore complex has eight sided symmetry, with a central channel and eight peripheral routes, probably for membrane proteins. Every mitotic cycle, the nuclear membrane kind of melts into the er and is reformed around the daughter nuclei, probably directed by ranGTP gradient and proteins of the INM.

It is also worth pointing out that we do this on purpose and can modify the proteins we insert into genomes to contain an NLS (Nuclear Localization Sequence).

A protein I work with has a plasma membrane localization sequence, but we delete it because it's unnecessary for our studies 👍

There are a whole host of these signal peptides routing proteins all over the cell: https://en.wikipedia.org/wiki/Signal_peptide

Oh wow, that’s really interesting. Do you have any readings about that?

Thank you for answering a 3am thought of this bio student! And for also answering my next question of how the cell 'knows' where to send it!

Thank you! I knew about nuclear pores and knew that they were selective so this makes sense! I'll check out the paper, thanks again 👍

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Rough endoplasmic reticulum is studded with ribosomes and synthesises the ribosomes, Golgi apparatus transports the ribosomes which synthesises proteins, and smooth endoplasmic reticulum transports the proteins to the nucleus. I think. I recently learnt this and think that's what happens