Proteolytic activity of the different fragments of factor B on the third component of complement (C3). Involvement of the N-terminal domain of Bb in magnesium binding.
Kinetic experiments measuring the proteolytic activity of Bb and 33Kd fragment (the C-terminal domain of factor B) on C3 were performed in several conditions, in order to assess the role of factor B domains in the catalytic activity and magnesium binding. The experiments were carried out in fluid phase with 125I-C3 or C3(H2O) as substrates and in the presence of nonradioactive C3b as cofactor. The results indicate: (a) The C-terminal domain, 33Kd, possesses proteolytic activity on C3, which is Mg2(+)-independent, whereas proteolysis by Bb is enhanced in 5 mM Mg2+. (b) C3b behaves as cofactor of 33Kd proteolytic activity on C3 and factor H is able to inhibit this activity. (d) Kinetics of C3 proteolysis by 33Kd shows a lag phase which is also displayed by Bb in the absence but not in the presence of Mg2+. Taken together these data are consistent with the involvement of the N-terminal domain of Bb in Mg2+ binding, which results in an enhancement of the proteolytic activity on C3 of the adjacent C-terminal domain. A C3 convertase model accounting for these results is presented.
Sánchez-Corral
P
P
Department of Immunology, Fundación Jiménez Díaz, Madrid, Spain.
Antón
L C
LC
Alcolea
J M
JM
Marqués
G
G
Sánchez
A
A
Vivanco
F
F
D003176:Complement C3 / Q000378:metabolism*; D003179:Complement C3b / Q000502:physiology; D003180:Complement C3b Inactivator Proteins / Q000502:physiology; D011415:Complement Factor B / Q000737:chemistry / Q000378:metabolism*; D017242:Complement Factor H; D006801:Humans; D008274:Magnesium / Q000378:metabolism*; D010446:Peptide Fragments / Q000378:metabolism
C493495:CFH protein, human; D003176:Complement C3; D003180:Complement C3b Inactivator Proteins; D010446:Peptide Fragments; D003179:Complement C3b; D017242:Complement Factor H; D011415:Complement Factor B; D008274:Magnesium