Human annexin V (PP4), a member of the family of calcium, membrane binding proteins, has been crystallized in the presence of calcium and analysed by crystallography by multiple isomorphic replacement at 3 A and preliminarily refined at 2.5 A resolution. The molecule has dimensions of 64 x 40 x 30 A3 and is folded into four domains of similar structure. Each domain consists of five alpha‐helices wound into a right‐handed superhelix yielding a globular structure of approximately 18 A diameter. The domains have hydrophobic cores whose amino acid sequences are conserved between the domains and within the annexin family of proteins. The four domains are folded into an almost planar array by tight (hydrophobic) pair‐wise packing of domains II and III and I and IV to generate modules (II‐III) and (I‐IV), respectively. The assembly is symmetric with three parallel approximate diads relating II to III, I to IV and the module (II‐III) to (I‐IV), respectively. The latter diad marks a channel through the centre of the molecule coated with charged amino acid residues. The protein has structural features of channel forming membrane proteins and a polar surface characteristic of soluble proteins. It is a member of the third class of amphipathic proteins different from soluble and membrane proteins.

Crystal structure analysis and refinement at 2.0 A resolution of a rhombohedral crystal form of human annexin V at high calcium concentration revealed a domain motion compared to the previously analysed hexagonal crystal form. Five calcium ions were located on the convex face of the molecule. Three strongly bound calciums are liganded at protruding interhelical loops and Asp or Glu residues in homologous positions in repeats I, II and IV. Five proteinaceous oxygens and one solvent molecule form the coordination polyhedron in each case. The unoccupied seventh site is suggested as the phospholipid headgroup binding site. Two more weakly bound sites were identified by lanthanum labelling. The structural features suggest that annexin V attaches with its convex face to membranes by specified calcium mediated interactions with at least three phospholipids. The adjacent membrane bilayer may thus become locally disordered and permeable to allow calcium inflow through the central polar channel of the molecule.

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Human anaphylatoxin C3a, derived from complement component C3 has been crystallized and its crystal structure determined at 3.2 A resolution based on multiple isomorphous replacement. The electron density map was interpretable in terms of the known chemical sequence and a molecular model constructed. The molecule has dimensions of 42 22 16 A. It resembles a drumstick. It is constructed from two helical segments Tyr 15 to Met 27 and Gly 46 to Ser 71 connected by a loop. Residues 1 to 14 are flexible. The C-terminal residues are in irregular conformation. The crystal structure analysis establishes the disulphide linkages as Cys 22 --Cys 49 , Cys 23 -Cys 56 , Cys 36 -Cys 57 . Kristallstrukturanalyse und Molekülmodell des menschlichen C3a-Anaphylatoxins Zusammenfassung: Menschliches Anaphylatoxin C3a, gewonnen von der Komplement-Komponente C3, wurde kristallisiert und die Kristallstruktur bei 3.2 Ä Auflösung durch multiplen isomorphen Ersatz bestimmt. Die Elektronendichtekarte konnte nach der bekannten chemischen Aminosäuresequenz interpretiert werden. Ein Molekularmodell wurde aufgebaut. Das Molekül hat Dimensionen von 42 22 16 Ä. Es ähnelt einem Trommelstock. Es besteht aus zwei helikalen Segmenten Tyr 15 bis Met 27 und Gly 46 bis Ser 71 , die durch eine Peptidschleife verbunden sind. Die Reste l bis 14 sind flexibel. Die C-terminalen Reste sind unregelmäßig gefaltet. Die Kristallstrukturanalyse erlaubt die Festlegung der Disulfidbrücken: Cys 22 -Cys 49 , Cys 23 -Cys 56 , Cys 36 --Cys 57 .

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