A comparison of lysosomal involvements in the degradation of normal and abnormal endogenous proteins of differing half-lives in MRC5 cells.

Protein degradation by diploid human-embryo lung fibroblasts (MRC5 cells) in monolayer culture was studied. 1. Varying the labelling period of proteins was found to alter the half-lives of labelled abnormal canavanine-containing proteins to an extent very similar to that obtained with normal proteins. 2. By manipulating the times of labelling it was possible to generate a species of abnormal protein with a greater half-life than that of a species of normal protein. A comparison of the lysosomal involvement in their degradation as determined both by inhibition by methylamine, a lysosomotropic agent, and by the degree of increase in protein degradation in step-down conditions, indicated that the degree of lysosomal involvement was not entirely dependent upon the half-life of the protein, but that abnormal proteins are preferentially degraded non-lysosomally. 3. The microtubule inhibitors colchicine and vinblastine were found to stimulate statistically basal protein degradation of normal long-labelled protein, whereas they had less effect upon the basal degradation of the other species of proteins studied and very little effect upon step-down degradation of all proteins studied. The stimulation in protein degradation found did not seem to involve the acid proteinases of lysosomes.