20S proteasome, what does the S stand for?
According to JLT's comment, the "S" has a specific meaning:
A Svedberg (symbol S, sometimes Sv, not to be confused with Sv for Sverdrup) is a non-SI physical unit used to characterize the behaviour of a particle type in ultracentrifugation. Bigger particles have higher svedberg values. It is a unit of time amounting to 10-13 s or 100 fs.
It is named after the Swedish physicist and chemist Theodor Svedberg (1884-1971), winner of the Nobel prize in chemistry in 1926 for his work in the chemistry of colloids and his invention of the ultracentrifuge.
The sedimentation rate or coefficient of a particle or macromolecule is computed through dividing the constant speed of sedimentation (in ms−1) by the acceleration applied (in ms−2). The speed is constant because the force applied by the ultracentrifuge (measuring typically in multiples of hundreds of thousands of gravities) is canceled by the viscous resistance of the medium (normally water) through which the particle is moving. The result has the dimensions of a unit of time and is expressed in svedbergs. One svedberg is defined as exactly 10−13s.
Bigger particles have higher svedberg values. The svedberg is not additive, since the sedimentation rate is associated with the size of the particle, when two particles bind together there is inevitably a loss of surface area. Thus when measured separately they will have svedberg values that do not add up to that of the particle formed when they bind together.
This is particularly the case with the ribosome. The most important measure used to distinguish ribosomes, which indicates their source organism, is the svedberg. A 70S ribosome comes from eubacteria, but is composed of a 50S subunit and a 30S subunit.
This is from Wikipedia, KS