Intracellular distribution and degradation of immunoglobulin G and immunoglobulin G fragments injected into HeLa cells.

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Abstract
Intact rabbit IgG molecules and their papain or pepsin fragments were radioiodinated and injected into HeLa [human cervical carcinoma] cells. Whole IgC, Fab2 and Fc fragments were degraded with half-lives of 60-90 h; half-lives of Fab fragments were 110 h. Proteolytic cleavage in the hinge region of the IgG molecule apparently is not the rate-limiting step in its intracellular degradation. The hingeless human myeloma protein, Mcg, was degraded at the same rate as bulk human IgG, providing further evidence that the proteolytically susceptible hinge region is not important for intracellular degradation of IgG molecules. SDS [sodium dodecyl sulfate] acrylamide gel analyis of injected rabbit IgG molecules revealed that H and L chains were degraded at the same rate. Injected rabbit IgG and rabbit IgG fragments were also examined on isoelectric focusing gels. Fab, Fab2 and Fc fragments were degraded without any correlations with respect to isoelectric point. Positively charged rabbit IgG disappeared more rapidly than their negative counterparts, contrary to the trend reported for normal intracellular proteins. The isoelectric points of 2 mouse monoclonal antibodies were essentially unchanged after injection into HeLa cells, suggesting that the altered isoelectric profile observed for intact rabbit IgG resulted from degradation and not protein modification. The intracellular distributions of IgG fragments and intact rabbit IgG molecules were determined by autoradiography of thin sections through injected cells. Intact IgG molecules were excluded from HeLa nuclei whereas both Fab and Fc fragments readily entered them. For some proteins, entry into the nuclear compartment is determined primarily by size.