Accelerated secretion of human lysozyme with a disulfide bond mutation

The mutant human lysozyme, [Ala77, Ala95]lysozyme, in which the disulfide bond Cys77-Cys95 is eliminated, is known to exhibit increased secretion in yeast, compared to wild-type human lysozyme [Taniyama, Y., Yamamoto, Y., Nakao, M., Kikuchi, M. & Ikehara, M. (1988) Biochem. Biophys. Res. Commun. 152, 962-967]. To investigate this phenomenon, mammalian cells were used to analyze the secretion kinetics of [Ala77, Ala95]lysozyme and wild-type human lysozyme. The secretion rate of [Ala77, Ala95]lysozyme during the 150-min chase period was significantly accelerated [half-life (t1/2) = 29 min] compared to that of wild-type human lysozyme (t1/2 = 83 min), when expressed at the same levels within the cells. In contrast, after the 150-min chase, the rates of disappearance of both wild-type and mutant human lysozymes within the cells were similar, and considerably slower (t1/2 = 220 min), respectively. The remaining intracellular wild-type human lysozyme was localized mainly in the endoplasmic reticulum, whereas accelerated transport of the [Ala77, Ala95]lysozyme mutant protein from the endoplasmic reticulum to the Golgi apparatus was observed. Also in yeast cells, similar secretion kinetics and the differences in t1/2 for wild-type and mutant human lysozymes during the early chase period were observed. The two-phase kinetics of disappearance of intracellular human lysozymes suggest that only a proportion of the proteins becomes secretion competent soon after synthesis and is completely secreted during the early chase period, whereas others enter the distinct, slow pathways of intracellular transport and/or degradation. Increased secretion of [Ala77, Ala95]lysozyme is possibly due to enhanced competence for secretion acquired in the endoplasmic reticulum at the early stage of transport events, which is closely connected with the removal of a disulfide bond.