Cysteine proteinase plays a key role for the initiation of yolk digestion during development of Xenopus laevis.

In electrophoretic analyses, extracts of Xenopus laevis neurulae exhibited activities digesting yolk proteins maximally at pH 4.8. These activities were completely inhibited by a mixture of pepstatin A and Z-Phe-Phe-CHN2, thus being identifiable as cathepsin D and cysteine proteinase. The electrophoretic profiles of yolk proteins cleaved by embryonic extracts changed at gastrula stages; the profile before stage 13 was the same as that given by cathepsin D treatment and the profile at stage 13 was a combination of the profile given by cathepsin D treatment and that given by cysteine proteinase treatment. Quantitative measurement of enzyme activities showed that the cathepsin D activity that was preserved from the beginning of development increased from stages 13 to 25 and decreased thereafter, whereas the cysteine proteinase activity appeared at stage 13, gradually increased until stage 35 and strongly increased thereafter. Immunoblot analyses showed that the 43 kDa form of cathepsin D was processed to its 36 kDa form, presumably by cysteine proteinase. This change can explain the increase of cathepsin D activity at stage 13 and thereafter. Immunofluorescent staining with the antibody against cysteine proteinase occurred in mesodermal and ectodermal cells other than neural ones at stages 13-24, and in the endodermal cells at stages 24-36. Faint staining in the neural ectoderm persisted from stages 18 to 36. Immunoelectron microscope observation showed that what stained was the superficial layer of yolk platelets. All these results indicate that cysteine proteinase plays a key role in the initiation of yolk digestion during embryonic development.