MODELING OF A 5-STRANDED COILED-COIL STRUCTURE FOR THE ASSEMBLY DOMAIN OF THE CARTILAGE OLIGOMERIC MATRIX PROTEIN

The three-dimensional structure of the assembly domain of the cartilag e oligomeric matrix protein (COMP) has been modeled. The model demonst rates a parallel five-stranded coiled coil and fits well with a large amount of experimental data that describe the oligomerization state, t he alpha-helical conformation, the helix directionality and the proper ties of the (abcdefg)(n) repeat sequence containing apolar residues at (a) and (d) positions, Comparison of the pentamer model with the know n dimer, trimer, and tetramer coiled coils revealed interactions that could mediate the switch to the formation of the pentamer coiled coil, The most distinctive feature of the pentamer model involves ion pair interactions, Charged side chains of the pentamer can form (f-g), (f-b '), and (e-c') interhelical ion pairs, which are neither experimentall y observed nor modeled in the di-, tri-, and tetramers, A polar glutam ine residue could be adopted at an interior (d) position of the modele d structure due to the formation of a symmetrical network of buried hy drogen bonds between five such glutamines, The pentamer model contains an axial cavity that can accept water molecules, Conformational analy sis was carried out in an attempt to determine the three-dimensional s tructure of the disulfide bonded C-terminal region of the pentamer. Re cent data on crystallization of the COMP assembly domain (Efimov et al ., Proteins 24:259-262, 1996) indicate that the prediction can be test ed experimentally in the near future. (C) 1996 Wiley-Liss, Inc.