Structure of trimeric haemerythrin

Abstract

Several simplifying structural principles have been developed from the considerable data contained in the three-dimensional structures of proteins determined in the past two decades. One of these is based on the observation that particular folding motifs often occur in a variety of structural and functional settings¹. The compact bundle of four antiparallel α-helices², first seen in the structure of myohaemerythrin³, is an example. Several non-haemerythrin proteins have since been found to have the same folding pattern^4–7, and haemerythrins themselves exist in a wide variety of quaternary arrangements^8,9. The unusual ability of the haemerythrin fold to associate as dimers, trimers, tetramers, octamers or higher aggregates provides an opportunity for examining structural diversity in subunit association. We have used X-ray crystallography to study the subunit structure of trimeric haemerythrin from a Siphonosoma species. We report here that the pattern of intersubunit helix–helix interactions differs from the most common mode of association of other helix-bundle proteins. In a novel approach to structure analysis at low resolution, experimental phases for the structure determination were based on anomalous scattering from the iron atoms native to haemerythrin, using the new resolved-anomalous phasing procedure¹⁰.