C1s is a multidomain serine protease that is responsible for the enzymatic activity of C1, the first component of the classical pathway of complement. Its catalytic region (gamma-B) comprises two contiguous complement control protein (CCP) modules, IV and V (about 60 residues each), a 15-residue intermediary segment, and the B chain (251 residues), which is the serine protease domain. With a view to identify domain-domain interactions within this region, the gamma-B fragment of C1s, obtained by limited proteolysis with plasmin, was chemically cross-linked with the water-soluble carbodiimide 1-ethyl-3-[3-(dimethylamino)propyl]carbodiimide; then cross-linked peptides were isolated after CNBr cleavage and thermolytic digestion. N-Terminal sequence and mass spectrometry analyses allowed us to identify two cross-links between Lys 405 of module V and Glu 672 of the B chain and between Glu 418 of the intermediary segment and Lys 608 of the B chain. Three-dimensional modeling of the CCP modules IV and V and of the catalytic B chain was also carried out on the basis of their respective homology with the 16th and 5th CCP modules of complement factor H and type I serine proteases. The information provided by both the chemical cross-linking studies and the homology modeling enabled us to construct a three-dimensional model for the assembly of the C-terminal part of the gamma-B region, comprising module V, the intermediary segment, and the B chain. This model shows that module V interacts with the serine protease B chain on the side opposite to both the activation site and the catalytic site. Functional implications of this interaction are discussed in terms of the possible role of module V in the specific recognition and positioning of C4, one of the two substrates of C1s.

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Absfract-Original mass spectra of uracil and thymine derivatives are presented with the corresponding fragmentation schemes.In the first series of spectra, the fragmentations of thymine derivatives, including 14C-2-thymine, dimers, and bromo thymines, confirm the basic retro Diels-Alder mechanism.The second series includes dihydro 5,6-derivatives of uracil (hydroxy and bromo substituents). The behaviour of these molecules is quite different; they are more sensitive to the substituents and a part of the fragmentation is often explained by protonated molecular ions.RBsum6-Nous presentom une serie de spectres de masse originaux, de derives de I'uracile et de la thymine, ainsi que les schemas de fragmentation correspondants.L'Ctude d'une premikre serie de mol&ules, & caractkre aromatique, dont une marquee au %, a permis de confirmer le mecanisme de base de la fragmentation comme &ant un 'retro Diels Alder'.Une deuxibme serie, saturee en position 5,6 du noyau uracile, montre le caractkre totalement different de ces mol6cules, plus fragiles, plus sensibles B la nature des substituants et donnant un ion protone [MH]+, responsable d'une partie des fragments observes.

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