Convergence of amino acid compositions of certain groups of proteins aids in their identification on two‐dimensional electrophoresis gels

The amino acid composition (AAC) versus the protein identity (PI) method was used for establishment of the identities of proteins from bovine brain and kidneys which were prefractionated on a CM52 cation exchanger and by preparative flat-bed isoelectric focusing. Established identities of proteins whose AACs converge with those of other members of their proper superfamily are reliable. Groups of convergent AACs can be extracted from protein databases using the standard root-mean-square rule (Rmsd) with measures the difference between the AAC of chosen protein versus those in the database. Convergence of AACs of proteins is dependent on several factors such as the upper limit of Rmsd, the limits of variations of molecular mass (m) and isoelectric point (pI), the number of proteins with similar AACs present in protein databases, and the domain structure of proteins. AACs of many proteins remain unique if the Rmsd is maintained with 1.5–1.0 with m ± 3kDA and pI ± 4. Certain groups of multidomain proteins have quasi-unique AACs only if the Rmsd is restrained to a value within 1.0 and 0.7. Convergence of AACs of certain groups of proteins may indicate that a common biological function exists for some members of each group. The AAC-PI method may become an additional search tool for protein functions.