Structure and transcriptional regulation of protein phosphatase 2A catalytic subunit genes

Abstract

The alpha and beta isoforms of the human protein phosphatase 2A catalytic subunit are encoded by distinct genes whose expression appears to be differentially regulated. To obtain a better understanding of the mechanism(s) that regulate(s) the expression of these two transcripts, we have cloned the genes encoding both isoforms. Both genes (each approximately 30 kbp) are composed of seven exons and six introns which intervene at identical locations, suggesting that they were derived from a common ancestral gene. However, the 5' upstream regions as well as the regions encoding the 5' and 3' untranslated sequences of each mRNA are different. The promoters of both genes are very G+C rich and lack the TATA and CCAAT sequences typical of many housekeeping genes. The C alpha gene contains several potential Sp1 binding sites and a potential cAMP-responsive element. Northern analysis using RNAs isolated from several different human cell lines showed that the steady-state C alpha mRNA was, in general, more abundant than the C beta mRNA. To determine whether the promoters regulate the differential C alpha and C beta RNA expression, they were fused to the reporter gene chloramphenicol acetyltransferase and transiently expressed in HeLa cells. Expression from the C alpha promoter was 7-10 times stronger than that from the C beta promoter, which paralleled the endogenous C alpha and C beta mRNA levels in HeLa cells. These data suggest that the steady-state levels of the C alpha and C beta mRNAs, are due, at least in part, to different promoter activities.

The Formation and Activity of PP2A Holoenzymes Do Not Depend on the Isoform of the Catalytic Subunit

“…Because both enzymes contained predominantly A␣ and C␣, it was concluded that the isoforms of A or C do not determine which B subunit associates with the core enzyme (21). However, the fact that both enzymes contained predominantly A␣ and C␣ is not surprising, because most cells and tissues, including rabbit skeletal muscle, express at least 10-fold more A␣ and C␣ than A␤ and C␤ (6,22). Therefore, no direct and quantitative comparison of A␣C␣ with A␤C␣ or of A␣C␤ with A␤C␤ in binding B subunits was made.…”

“…They differ by only eight amino acids, seven of which are localized within a sequence of 30 amino acids at the N terminus (1,5). C␣ and C␤ are both encoded by genes composed of seven exons and six introns (6). It has been suggested that exons I and VII encode sequences involved in regulation, whereas exons II-VI are important for substrate binding and catalysis.…”

“…It has been suggested that exons I and VII encode sequences involved in regulation, whereas exons II-VI are important for substrate binding and catalysis. C␣ is expressed at 10-fold higher levels than C␤ because of differences in transcriptional regulation (6). Interestingly, all catalytically relevant amino acids in protein phosphatase 1 (PP1), whose three-dimensional structure has been elucidated, are completely conserved in PP2A and PP2B (7,8), suggesting that the three-dimensional structures of PP2A and PP2B are similar to that of PP1.…”

Subunit composition and developmental regulation of hepatic protein phosphatase 2A (PP2A)

“…Some results derived from immunoprecipitation using different antibodies suggest that the dimeric (AC) form of PP2A may constitute up to a third of the total PP2A pool [22]. Previous studies on the expression of hepatic PP2A components in the adult rat [23][24][25] have identified both dimeric and heterotrimeric PP2A complexes by using affinity purified antibodies specific for PP2A subunits [26]. Limited data are available on the forms of PP2A in fetal liver [27].…”

Light-Driven Translocation of the Protein Phosphatase 2A Complex Regulates Light/Dark Dephosphorylation of Phosducin and Rhodopsin

“…The PP2A enzyme exists either as a dimer (AC) or as a trimer (ABC) complex of subunits: a 36-38 kDa catalytic (C), a 60-65 kDa structural (A), and multiple nonhomologous 54-130 kDa regulatory (B) subunits. The gene structure and transcriptional regulation are known for the two catalytic isoforms, CR and C , which are 97% identical and were potentially derived through gene duplication (1). Similarly, the A subunit has two isoforms and is firmly complexed with the C subunit (2).…”