Two variants of the complement protein C3, each linked to severe depletion of the protein in blood plasma, have revealed an unexpected split in how C3 behaves inside and outside cells. Researchers studying the variants c.168_169del and c.1682G>A, which changes the amino acid glycine at position 561 to aspartic acid, found that both remained detectable inside cells even though extracellular C3 levels in the affected patients were markedly reduced. The work, published as an open access original article in Cellular and Molecular Life Sciences, provides evidence that intracellular and extracellular C3 represent functionally independent compartments.
Complement component C3 is the central effector of extracellular complement activation. Recent studies have also identified intracellular forms of C3 with distinct non-canonical functions, but how pathogenic variants of the C3 gene affect this intracellular biology remained unknown. The two variants examined in the study were previously described in association with C3 deficiency and severe plasma C3 depletion.
The c.168_169del variant is a frameshift mutation predicted to abolish protein expression. Instead of eliminating the protein, the mutation produced a smaller non-secreted pro-C3 species that lacked glycosylation and localized exclusively to the cytosol. The researchers used cellular fractionation, antigenic characterization, and split-GFP localization assays in cell models to characterize this unexpected product. Together, these experiments showed that the truncated protein resembles a non-canonical cytosolic C3 isoform generated through alternative translation initiation downstream of the mutation site.
This finding matters because it provides clinical support for the existence of cytosol-restricted C3 generated independently of the canonical secretory pathway.
The p.G561D variant displayed normal intracellular expression, antigenicity, glycosylation, and secretion. It showed impaired alternative pathway convertase activity, increased susceptibility to degradation mediated by the complement regulators factor H and factor I, and enhanced spontaneous turnover in serum. These functional failures explain the severe plasma C3 depletion observed in the patient carrying this variant.
Taken together, the two cases demonstrate that pathogenic C3 variants can exert fundamentally different effects on the extracellular and intracellular pools of the protein.
The authors argue that these findings support integrating intracellular complement biology into the interpretation of complement-associated diseases.
Subject of Research: How two pathogenic C3 gene variants affect extracellular and intracellular pools of complement C3 differently
Article Title: Genetic C3 variants reveal functional independence between extracellular and intracellular forms of complement C3
Article References: Chacon-Arguedas, D., King, B. C., Marin, A. V., Tortajada, A., Andersen, G. R., Regueiro, J. R., & Blom, A. M. (2026). Genetic C3 variants reveal functional independence between extracellular and intracellular forms of complement C3. Cellular and Molecular Life Sciences, 83(1), Article 409. https://doi.org/10.1007/s00018-026-06517-2
Image Credits: AI Generated
DOI: 10.1007/s00018-026-06517-2
Keywords: complement C3, C3 deficiency, intracellular complement, cytosolic C3 isoform, alternative translation initiation, frameshift variant, p.G561D, alternative pathway convertase, factor H, factor I, glycosylation, plasma C3 depletion
Tags: alternative pathway convertasealternative translation initiationC3 deficiencycomplement C3cytosolic C3 isoformFactor Hfactor Iframeshift variantGlycosylationintracellular complementp.G561Dplasma C3 depletion
