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Mapping of the domains required for decay acceleration activity of the human factor H-like protein 1 and factor H
1Department of Molecular Biology, Bernhard Nocht Institute for Tropical Medicine, Hamburg, Germany.
Insights
Human factor H-like protein 1 (FHL-1) shares complement system regulatory functions with factor H. The N-terminal four short consensus repeats (SCR) are essential for this decay accelerating activity in both proteins.
Area of Science:
- Immunology
- Biochemistry
Background:
- The human complement system is regulated by proteins like factor H.
- Factor H-like protein 1 (FHL-1) shares structural similarities with factor H, possessing seven N-terminal short consensus repeats (SCR).
Purpose of the Study:
- To investigate the decay accelerating activity of FHL-1 on complement C3/C5-convertases.
- To compare the efficiency of FHL-1 and factor H in complement regulation.
- To identify the specific domain responsible for the decay accelerating activity in FHL-1 and factor H.
Main Methods:
- Assessing the ability of FHL-1 to dissociate C3/C5-convertases from sheep red blood cells.
- Utilizing recombinant FHL-1 and truncated mutants expressed via the baculovirus system.
- Mapping the functional domain by analyzing the activity of deletion mutants lacking specific SCRs.
Main Results:
- FHL-1 exhibits decay accelerating activity, similar to factor H, on C3/C5-convertases.
- Factor H is approximately 100-fold more efficient than FHL-1 in decay acceleration.
- The N-terminal four SCRs (SCR 1-4) of FHL-1 are essential and sufficient for its decay accelerating activity.
- Mutants lacking SCR 1 or SCR 4 were inactive, confirming the importance of these specific repeats.
Conclusions:
- FHL-1 and factor H possess identical and overlapping regulatory functions within the complement system.
- The essential domain for decay accelerating activity in both FHL-1 and factor H resides within the N-terminal four SCRs.
- These findings highlight the conserved functional role of the N-terminal SCRs in complement regulation.
Abstract:
The human factor H-like protein 1 (FHL-1) is composed of seven repetitive elements (short consensus repeats; SCR) that are identical in sequence to the seven N-terminal SCR of complement factor H. We show that the FHL-1 protein has decay acceleration activity in that it can dissociate C3/C5-convertases bound to the surface of sheep red blood cells. The same activity was also determined for factor H. However, compared to FHL-1, factor H was more efficient in decay acceleration, as about 100-fold less protein was required for a 50% inhibition of activity. The domain required for decay accelerating activity of FHL-1 and factor H was mapped by the use of recombinant fragments. FHL-1 and a series of truncated forms of the protein were expressed in the baculovirus system. Recombinant FHL-1 and all mutants which include SCR 1-4 were functionally active. These four N-terminal SCR are essential and sufficient for activity, as deletion mutants which lack SCR 1 or SCR 4 showed no activity. These results demonstrate that FHL-1 and factor H have identical and overlapping regulatory functions in the complement system and that the domain required for this activity is located in the overlapping region of both proteins within the N-terminal four SCR.