Identification and characterization of alternative splice products encodedby the human phosphotyrosyl phosphatase activator gene

The phosphotyrosyl phosphatase activator (PTPA), a protein phosphatase 2A ( PP2A) regulatory protein, specifically stimulates the phosphotyrosyl phosph atase activity of PP2A in vitro. Human PTPA is encoded by a single gene, th e structure and chromosomal localization of which have been determined in o ur previous work. In this paper, we report the identification and character ization of six additional splice variants, termed PTPA beta to PTPA eta, in addition to the originally identified PTPA alpha form. Interestingly, PTPA beta and PTPA gamma contain a novel exon that had been overlooked in the f ormerly identified gene structure. As revealed by nested PCR, all these PTP A transcripts are expressed in various human cDNA libraries and cell lines. However, a quantitative approach, using a single PCR reaction followed by detection of the reaction products with a radioactively labeled probe, reve aled only PTPA alpha, beta and delta, suggesting that the other transcripts are expressed very poorly. In vitro transcription-translation revealed tha t only PTPA alpha, beta, delta and epsilon are translated into functional p roteins, whereas translation of PTPA gamma, zeta and eta is stopped prematu rely due to a frameshift resulting from skipping exon 2, suggesting that th e latter isoforms may result from splicing errors. By western analysis of H epG2 and Saos-2 cell extracts, only PTPA alpha and beta were detected. PTPA alpha and beta were expressed as GST fusion proteins in bacteria, and were found to contain the same specific phosphotyrosyl phosphatase stimulatory activity towards PP2A. The identification of this family of PTPA variants a dds another level of complexity to the in vivo function(s) of PTPA, opening up the possibility that different isoforms may perform different functions .