INTRODUCTIONThe functions of most Arabidopsis genes cannot be understood simply by studying the expression patterns of their mRNAs. The level of a protein may not be closely correlated with the level of the corresponding mRNA, or a protein may need to be modified to be active. Furthermore, the function of proteins often depends on specific subcellular localization, so knowledge of this can exclude some hypotheses about possible function. The simplest way to analyze proteins is in unfractionated extracts. However, it is often desirable to fractionate proteins, e.g, by size. This procedure extracts total protein from Arabidopsis samples. Typical yields are ~2-3 mg/ml (using rosette leaves) or 6-8 mg/ml (using young seedlings).