Cell is a unit of life and is like a small country, both in its organization and in its self-regulating behavior. It has various structurally and functionally distinct compartments (organelles), such as nucleus, mitochondria, endoplasmic reticulum, etc..

Functional diversity of Organelles is due to differences in the kinds and arrangements of protein molecules which participate in the function of the organelle.

Protein is like an individual being which has own lifetime just as we do;
synthesis (birth), modification (education), intracellular transport (job hunting), function and its regulation (work and rest), degradation (die) . All these processes (fate of a protein) are determined by amino acid sequences of the protein.

Intracellular transport of proteins

Proteins synthesized on ribosomes are transported through specific amino acid sequences (targeting signal) to their destinations (workplaces: organelles)

Determination of targeting signal

Amino acid sequence responsible for a protein to target to the proper organelle was analyzed, using deletion, fusion and site-directed mutations of the protein.