C3c is a major proteolysis product of Complement C3. It is a processed fragment, distinct from the small inflammatory fragment C3a and the activated C3b from which later breakdown products can be generated. A fragment name identifies a particular piece of the original protein, rather than a separate gene product.
The 1969 fragment-isolation study separated C3c and a smaller fragment from enzyme-treated C3b and distinguished them antigenically. This is a biochemical way to establish that further processing yields different molecular pieces.[ref]
In 2005, X-ray crystallography provided crystal structures of native Complement C3 and C3c. Comparing their protein domain arrangements connected cleavage with changes in molecular architecture. The deposited structures are 2A73 for native Complement C3 and 2A74 for C3c.[ref]
Structures of complement component C3 provide insights into the function and evolution of immunity by Bert J. C. Janssen and others. Abstract; Figures1–2; coordinate-deposition statement
Consider an original interpretation exercise: an antibody signal from a region retained in C3c could persist after the intact precursor has disappeared. Such a signal would not by itself measure intact Complement C3, newly generated C3a or surface-attached C3b. Knowing the recognized region and the sample's processing state is essential when choosing what an assay measures.
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