The membrane attack complex (MAC: C5blC61C71C81C9n) is the terminal complex of
the mammalian complement cascade. Its principal function is to form a self-assembling,
potentially cytolytic pore spanning the membrane of a foreign cell. However,
inappropriate complement activation and subsequent aberrant MAC action is responsible
for tissue damage in several human pathologies.1 The C-terminal domains or modules
that are exclusive to the C6 and C7 components of the MAC, form a putative molecular
arm. These modules consist of two complement control protein modules (CCPs)
followed by a pair of Factor I-like Modules (FIMs). The C-terminal molecular arms of
C6 and C7 are important for linking the complement activation cascade with MAC
assembly and thereby ensuring MAC assembly occurs only when needed. They interact
with the C345C domain of C5 and C5b. Currently there are no high-resolution structural
data for any of the MAC components but efforts are underway to solve the structures of
the individual domains. For instance, the structure of the central MAC/perforin domain
of C8 has been determined, so has the structure of C5 and the FIM domains of C7.
However , the lack of structural information for the MAC is being balanced by an
increased understanding of mechanism, and eventually towards the rational design of
therapies designed to supress MAC formation.This report describes the preparation from bacterial and yeast cells of ¹³C,¹⁵N-labelled
samples of the C7 CCP-pair and of a triple module consisting of the second CCP
followed by the FIM-pair, as well as other constructs from the C6 and C7 C-terminal
arms. The NMR samples were used to solve 3D solutions of structures, thus allowing
for reconstruction of the four-module C-terminus of C7. Efforts were aimed at obtaining
additional structural information for C7. This included SAXS analysis as well as the
development of a novel approach using chemical cross-linking followed by tryptic
digestion and mass spectrometry-based identification of cross-linked peptides
