Amino acid composition of the membrane and aqueous domains of integral membrane proteins.

To identify residues which might impart transport capability to the intramembranous regions of transport proteins, we surveyed available data for the 9991 amino acids contained in the aqueous and intramembranous regions of 24 integral membrane proteins: 10 transport (T) proteins and 14 nontransport (NT) proteins. Statistical comparison of percentage occurrence of each amino acid within T and NT samples provided a measure of "typical" composition of T and NT membrane-spanning regions, and showed that the residues partition into membrane and aqueous domains largely in accord with expectation from hydropathy indices. Comparison of aqueous and membrane domain composition between protein categories revealed a statistically similar distribution of residues in aqueous domains, but significant differences in membrane domains: seven residues (Asn, Asp, Gln, Glu, Phe, Pro, Tyr) were preferred in membrane regions of T proteins, and one (Val) was selectively excluded. Chemical and structural considerations suggested that three of these residues--Asn, Tyr, and Pro--are the most likely functional participants in transport processes.