18 kDa protein tyrosine phosphatase in the ocular lens

Protein tyrosyl phosphorylation and dephosphorylation play essential roles in regulating cellular events such as proliferation and differentiation, an d their involvement in the lens development and transparency is also sugges ted. The level of tyrosine phosphorylation in a given protein is regulated by the opposing actions of protein-tyrosine kinases (Tyr kinases) and prote in-tyrosine phosphatases (TyrPases). Recent studies have revealed that some Tyr kinases, such as platelet-derived growth factor receptor and fibroblas t growth factor receptor, are present in the lens, however, little is known about TyrPases in the lens. In this study, we found a 18 kDa protein tyros ine phosphatase (18 kDa TyrPase) predominantly present in the ocular lens o f various animals. We purified the phosphatase from the lens of chick embry o and characterized its activity. Phosphatase activity was determined in chick embryo, mouse, rabbit and bovi ne lenses using p-nitrophenyl phosphate (pNPP) as substrate. All lenses exa mined dephosphorylated pNPP under acidic conditions, and a large portion of the activity resided in a low molecular weight protein, ca, 18 kDa. follow ing high-resolution gel permeation column chromatography. The brain and liv er showed high dephosphorylation activities. but most of their activity was present in high molecular weight fractions, unlike that in the lens. The 1 8 kDa phosphatase was purified from the lens of 17 day old chick embryos to near-homogeneity with two-step rapid chromatography. This phosphatase show ed strict substrate specificity for phosphotyrosine and phosphotyrosyl pept ides, suggesting that it was a kind of protein tyrosine phosphatases (TyrPa ses). Several known inhibitors of TyrPases, such as SH blockers, vanadate a nd phenylarsine oxide, strongly inhibited the enzyme activity. The molecula r weight, substrate specificity, and responses to various inhibitors and ac tivators coincide well with those reported for the low molecular weight pro tein tyrosine phosphatase (LMW-TyrPase), belonging to the TyrPase superfami ly, These results suggest that the 18 kDa phosphatase found in the lens is a LMW-TyrPase, The 18 kDa TyrPase is the predominant phosphatase in the ocu lar lens. It may be involved in regulation of lens cell proliferation, diff erentiation and/or lens transparency.(C) 2001 Academic Press.