Megalomicin disrupts lysosomal functions.

Megalomicin (MGM) has been shown to cause a dilation of the most distal cisternae of the Golgi complex. The effects of MGM on Golgi morphology correlated with an inhibition of protein transport to the trans-Golgi resulting in an accumulation of poorly sialylated glycoproteins. Here we show that the addition of 50 microM MGM caused a rapid swelling of lysosomes in cultured cells and inhibited the degradation of the newly synthesized T cell antigen receptor CD36 subunit. Although MGM did not affect the uptake of fluid phase markers, it prevented their degradation. Interestingly, endocytosed ovalbumin did not colocalize with lysosomes in MGM-treated cells, suggesting an MGM-induced impairment in the delivery to lysosomes. This was confirmed by Percoll density gradients, where the fluid phase marker remained in endosomal fractions, even after long chase times, whereas in control cells the endocytosed marker was located in lysosomes. The effect of MGM was not confined to soluble proteins since it did also inhibit the delivery of the membrane-bound epidermal growth factor receptor to lysosomes. Finally, MGM strongly inhibited the ATP-dependent acidification of lysosomes in vitro, suggesting a possible mechanism for its in vivo activity.