Cellular processes work with “addresses” through microtubules and transport molecules
“Nuclear import signals” are targeting sequences rich in positively charged amino acids which constitute “addresses” for important locations in the cell.
As an example, motor proteins provide transport for RNA molecules through microtubules.
References:
Bray, Wetware (2009), p. 229
Major locations of the cell have something analogous to a postal ad-dress. Proteins destined to work in the nucleus, for example, contain distinctive sequences of amino acids rich in positively charged amino acids known as "nuclear import signals." These signatures are recognized by receptors in the nuclear membrane and the molecules conveyed forthwith into the nucleus. Other targeting sequences exist for organelles such as mitochondria and for the many kinds of internal membrane found in eukaryotic cells.
RNA molecules provide perhaps the most remarkable example. Messenger RNA molecules (those that code for a protein) are made in the nucleus. They then move into the cytoplasm, where they are directed to specific locations according to sequences of bases at one end of the molecule. These sequences do not code for amino acids and are not used to make protein; instead, they act as ZIP codes. Their precision can be amazing, some RNAs finding their way to the ends of growing nerve cells, or to the synapses of a large pyramidal cell.
Evidently something more than simple diffusion must be responsi-ble. A molecule the size of RNA would take weeks to diffuse from one end of a large nerve cell to the other. What in fact happens is that motor proteins seize the RNA molecules and then carry them along micro-tubules. Specific adapter proteins are needed to recognize the RNA, read its destination, and then attach it to a suitable motor protein heading in the desired direction. Once the RNA cargo has reached the correct region of the cell, other proteins cause it to detach from the motor. It begins its appointed task of making a protein.