Degradation of structurally characterized proteins injected into HeLa cells. Tests of hypotheses

We have compared sequence and structural features of 35 proteins to their metabolic stabilities in HeLa cells. No relationship was observed between the half-life of an injected protein and its subunit molecular weight, isoelectric point, hydrophobicity, thermostability, surface charge density, or N-terminal residue. Other properties, including susceptibility to oxidation, specific combinations of amino acids, secondary structure composition, and solvent exposed residues, also failed to correlate with protein stability. Although a weak inverse correlation was obtained when stability was compared to asparagine and glutamine content, we conclude that the degradation of an injected protein is unlikely to be related to any single structural parameter. Rather, we hypothesize that it results from an interplay between subcellular location and still poorly defined surface features of the injected proteins.