Calculating populations of subcellular compartments using density matrix formalism

in our earlier paper (Ref. [1]) we developed an integrated probabilistic sy stem for predicting the subcellular localization of proteins and estimating the relative populations of the various compartments in yeast. To justify our formulas we show here that there is a one-to-one correspondence between our previous calculations and the prediction of a state of a many-particle quantum system. The equivalence between these two types of predictions can be easily established if one maps the probability of finding a particular protein in a certain subcellular compartment to the probability of measurin g the corresponding quantum particle in one of the possible quantum states (the number of proteins being equal to the number of particles in the syste m, and the number of compartments being equal to the number of achievable q uantum states). Once the sought correspondence is established, we can utili ze a well-known formula from quantum statistical mechanics to calculate the overall occupation of a particular quantum state, associating the state wi th the corresponding subcellular compartment. In the present work we presen t the details of how we arrived at the formula for the compartment populati on, borrowing the tools from quantum statistical mechanics. (C) 2001 John W iley & Sons, Inc.