title: Analysis of human red cell spectrin tetramer (head-to-head) assembly using complementary univalent peptides.
abstract: The mass-driven assembly of spectrin dimers to form tetramers involves two equal head-to-head alpha-beta associations and requires at least 30 degrees C for interconversion to occur readily. In this paper, the properties of tetramer formation were investigated using two complementary univalent peptides (the alpha I domain and beta monomers). Since the alpha I domain lacks an essential nucleation site required for side-to-side (lateral) heterodimer assembly [Speicher et al. (1992) J. Biol. Chem. 267, 14775-14782], these two peptides can only assemble head-to-head at a single site. This head-to-head assembly readily occurs at lower temperatures, indicating the temperature barrier for dimer-tetramer interconversion is caused by a conformational constraint of the dimer. This constraint, a closed hairpin loop, is released when the laterally associated partner is removed. The univalent alpha I-beta binding affinity at 37 degrees C (Ka = 1.4 x 10(5) M-1) is similar to the dimer-tetramer association constant at the same temperature. As the temperature is decreased from 37 to 0 degrees C, the alpha I-beta binding affinity increases about 32-fold. In contrast with head-to-head associations involving dimers, the second-order rate constants of two complementary univalent peptides (i.e., alpha I and beta) are dramatically higher, and the estimated activation energy (about 50 kJ mol-1) is about 5-fold lower. An open dimer conformation is an obligatory high-energy intermediate required for dimer-tetramer interconversion, and opening the dimer hairpin loop contributes about 190 kJ mol-1 to the activation energy for tetramer association.(ABSTRACT TRUNCATED AT 250 WORDS)
authors.affiliation: Wistar Institute of Anatomy and Biology, Philadelphia, Pennsylvania 19104.
authors.forename: T M
authors.initials: TM
authors.lastname: DeSilva
authors.forename: K C
authors.initials: KC
authors.lastname: Peng
authors.forename: K D
authors.initials: KD
authors.lastname: Speicher
authors.forename: D W
authors.initials: DW
authors.lastname: Speicher
mesh_terms: D002850:Chromatography, Gel; D002851:Chromatography, High Pressure Liquid; D006801:Humans; D007700:Kinetics; D046911:Macromolecular Substances; D011487:Protein Conformation; D013049:Spectrin / Q000737:chemistry*; D013696:Temperature; D013816:Thermodynamics
chemical_list: D046911:Macromolecular Substances; D013049:Spectrin