Analysis of expression patterns of protein phosphatase-1 and Phosphatase-2A in rat and bovine lenses

PURPOSE. The reversible phosphorylation and dephosphorylation at the serine and threonine residues on proteins play distinct roles in regulating multi ple cellular activities. Whereas the protein serine-threonine kinases have been well studied in the lens system, very little is known about the expres sion and function of the serine-threonine phosphatases. The present article reports the expression patterns of protein phosphatase (PP)-1 and -2A in a dult rat and bovine lenses. METHODS. Total RNAs and proteins were extracted from the epithelial and fib er cells of rat and bovine lenses. RT-PCR and Northern blot analysis were u sed to detect the mRNA expression levels in the epithelial cells and differ ent fractions of fiber cells of these two types of lenses. Western blot was used to examine the protein expression levels in these different samples. An enzymatic assay was used to detect the activity distribution of PP-1 and -2A in these samples. RESULTs. The mRNAs for the PP-1 catalytic subunit (PP-1cs) and PP-2A cataly tic subunit (PP-2Acs) were expressed in both epithelial and fiber cells of rat and bovine lenses. A detailed examination of the expression patterns of the two mRNAs in different fractions of fiber cells revealed that the cort ical fiber cells (Fl) contain the highest level of PP-1cs and -2Acs mRNAs ( similar to those in the epithelial cells) among different fractions of fibe r cells. The levels of the two mRNAs were sequentially decreased in the nex t layers of fiber cells (F2 and F3) and became barely detectable in the inn er layers of fiber cells (F4 and N), In contrast to the mRNA expression pat terns, the PP-1 cs protein was mainly found in the epithelial cells. Among different layers of fiber cells, only cortical (Fl) fiber cells contained d etectable level of PP-Ics protein (bovine lenses contained a relatively hig her level of PP-1cs than rat lenses in this region). In the remaining fiber cells, the PP-1cs protein was hardly detectable in rat lenses and slightly detectable in bovine lenses. The PP-2Acs protein was detectable only in th e lens epithelial cells. Enzymatic assays revealed that the distribution pa tterns of PP-1 and -2A activities were similar to those of PP-1cs and -2Acs proteins. Furthermore, PP-1 activity was approximately four to five times higher than PP-2A activity in the lens epithelial cells.