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membrane attack unit 487 membrane cofactor protein (MCP)
C8
C7 C9
C9
C9
C5b C6
C9
C9
C9
C9
MAC.
calcium and furnish domains that bind lipid, enabling
MAC to attach to membranes. Mechanisms proposed for
complement-mediated cytolysis include extrinsic protein
channel incorporation into the plasma membrane or
membrane deformation and destruction. Central regions of
C6, C7, C8α, C8β, and C9 have been postulated to contain
amphiphilic structures that may be membrane anchors.
A single C9 molecule per C5b678 leads to erythrocyte
lysis. Gram-negative bacteria that have both outer and
inner membranes resist complement action by lengthening
the O-antigen chain at the outer membrane or heightening
surface carbohydrate content, which interferes with
MAC binding. MAC assembly and insertion into the outer
membrane are requisite for lysis of bacteria. Nucleated
cells may rid their surfaces of MAC through endocytosis or
exocytosis. Platelets have provided much data concerning
sublytic actions of C5b–9 proteins. Control proteins acting
at different levels may inhibit killing of homologous cells
mediated by MAC. Besides C8-binding protein or homologous
restriction factor (HRF) found on human erythrocyte
membranes, the functionally similar but smaller
(a)
(b)
phosphatidyl inositol glycan (PIG)-tailed membrane protein
harnesses complement-induced cell lysis. Sublytic actions
of MAC may be of greater consequence for host cells than
its cytotoxic effects.
membrane attack unit
Lytic unit exclusive of the recognition and activation units.
C5b binds to cell membranes, followed by the successive
interaction of single molecules of C6, C7, and C8 with the
membrane-bound C5b. Finally, further interaction with
several C9 molecules finishes the formation of the lytic unit
through noncovalent interactions without enzymatic alteration.
Formation of a membrane attack unit or membrane
attack complex (MAC) leads to a cell membrane lesion that
permits loss of K + and ingress of Na + and water, leading to
hypotonic lysis of cells.
membrane-bound immunoglobulin (mIg)
An immunoglobulin molecule on the surface that possesses
a transmembrane region and extended C terminal region
and lacks a tail piece. It is the antigen-binding structure of
the B cell receptor.
membrane cofactor protein (MCP)
A type I transmembrane regulatory protein of the complement
system that is a cofactor for the factor-I-mediated
proteolytic cleavage and inactivation of C3b and C4b
deposited on self tissue. MCP protects the cell on which
it is located rather than neighboring cells. It also serves as
the measles virus receptor and participates in the adherence
of Streptococcus pyogenes (group A streptococcus) to
endothelial cells. It is a member of a group of structurally,
functionally, and genetically related proteins called regulators
of complement activation (RCAs). They include decayaccelerating
factor DAF (CD55), complement receptors 1
C9
C9
C9
C5bα
C5β
C8β
C6, C7
C8α–γ
C9
C9
C9
C9 C9
Perforations in cell membrane (a) and (b) induced by complement (poly C9 tubular complexes). (c) Model of membrane attack complex (MAC) subunit
arrangement.
(c)
M