Description
General Description
Complement factor P (properdin) is purified from normal human serum. Factor P is a 53,000 Dalton cationic protein that circulates in blood in the form of dimers, trimers
and tetramers. It binds rapidly to C3b on surfaces where complement has begun to activate. Properdin binds most avidly to C3b,Bb the alternative pathway C3/C5
convertase, but also binds to C3b < C3b,B < C3b,Bb. Its primary function is to stabilize the C3b,Bb complex allowing increased alternative pathway activation (Pang burn, M.K.,
(1988); Nolan, K.F. and Reid, K.B.M. (1993)). Properdin enhances formation of the alternative pathway C3 convertase by increasing binding of factor B to P,C3b complexes.
Thus, properdin is an accelerator (positive regulator) of complement activation. Properdin has recently been proposed to be able to also initiate activation of the
alternative pathway by binding to the target surface and initiating C3/C5 convertase formation (Kemper C. and Fourcade D.E. (2008)).
Physical Characteristics & Structure
The basic subunit of this protein is a 53,000 Dalton single chain molecule which is composed of six thrombospondin-like repeat (TSR) domains. These basic units are
linked at the ends (C-terminal to N-terminal) to form circular dimers, trimers, tetramers and perhaps higher forms in blood (Pang burn, M.K. (1989)). Electron microscopic
images (Smith, C.A. et al. (1984)) have clearly demonstrated these structures. Higher oligomers of properdin are formed upon freeze thaw and perhaps during complement
activation and were originally called “activated” properdin due to their enhanced ability to bind and activate complement. Properdin has a rare post-translational modification in
that its tryptophan residues are highly mannosylated (Hartmann, S. and Hofsteenge, J. (2000)). There is also a single N-linked glycosylation site near the C-terminal. These
contribute to a carbohydrate content estimated at 10%. Properdin is highly positively charged at neutral pH and has a pI greater than 9.5. It is one of the most positively
charged proteins in blood (Morley, B.J. and Walport, M.J. (2000)).
EC Number: EC 3.4.21.45
Function
Properdin binds most avidly to C3b,Bb the alternative pathway C3/C5 convertase, but also binds to C3b < C3b,B < C3b,Bb. Its primary function is to stabilize the C3b,Bb
complex allowing increased alternative pathway activation. Properdin enhances formation of the alternative pathway C3 convertase by increasing binding of factor B to
P,C3b complexes. Thus, properdin is an accelerator (positive regulator) of complement activation.
Assays
Assays depend on the ability of properdin to bind to clusters of C3b or to accelerate the activation of the alternative pathway. ELISA assays for protein may be
performed by coating plates with excess C3b (CompTech Cat# A114), binding properdin and detection with goat anti-Properdin (CompTech Cat# A239). Functional assays are
best performed using P-depleted serum (CompTech Cat# A339) and then neutrophils. In neutrophils it is stored in granules that release the properdin in response to C5a, FMLP,
IL8 and TNF-alpha.measuring the accelerated lysis of rabbit erythrocytes (CompTech Cat# B300) in the presence of
properdin (CompTech Cat# A139) and MgEGTA (CompTech Cat# B106). Lysis does occur slowly in the absence of properdin so these assays must be timed assays to compare
the rates of lysis with and without properdin. Assays for “activated” properdin consist of incubating properdin in NHS for extended periods of time (30 to 60 min) and measuring
the residual C3 activity using C3-depleted serum (CompTech Cat# 313) and rabbit erythrocytes.
In vivo
Blood contains approximately 5 µg/mL properdin (Pang burn, M.K. (1989)). Properdin does not appear to be synthesized in the liver where most complement proteins
are synthesized. It is primarily made by monocytes, T-cells and
Regulation See In vivo above.