Complement factor P (CFP or properdin) was the first component of the alternative pathway to be identified, yet it eluded detailed characterization until recently. Its function is to bind to the C3 convertase, C3b, Bb, and to increase the stability of the complex. Native properdin does not bind to C3b bearing particles nor does it induce C3 convertase formation in serum. Interaction with C3 convertase activates properdin nonenzymatically, probably through a conformational rearrengement of the molecule. In mixtures of purified proteins, properdin is not an essential component for the activation of the pathway, but its presence does result in more rapid amplification of bound C3b. However, in certain cases, such as measles virus-infected cells, properdin needs to participate in order that activation of the pathway may progress to lysis of the cells. Once activated, properdin can bind directly to fluid phase or particle-bound C3b, stabilize the C3/C5 convertase, and induce complement consumption when incubated with serum.
Factor P (CFP or properdin) is a high molecular weight polymer composed of noncovalently bound subunits of 56000 daltons. Recently, high resolution electron microscopy has revealed that the activated properdin molecule is a polydisperse mixture of cyclic dimers, trimers, tetramers, and higher species, the tetramer being the prevalent form. Most other studies had described properdin as a tetramer. One study using sedimentation equilibrium ultracentrifugation found that properdin is a trimer and this form was the second most prevalent species observed by electron microscopy. Gel filtration of fresh EDTA plasma suggested that native properdin exhibits a polymeric distribution similar to that found for activated properdin. The properdin monomer appears to be a highly assymmetric flexible molecule 260A long and 30,~ wide. It associates head-to-tail to form cyclic polymers. The unusual amino acid composition of properdin suggested that collagen-like regions might exist and its elongated shape would be consistent with this suggestion. However, extensive amino acid sequence analysis indicated that there are no regions in the molecule where the repeating collagen unit is found.
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