Partial purification and characterization of heat stable protein phosphatase inhibitor-2 from rabbit reticulocytes

Identification and characterization of two distinct PPP1R2 isoforms in human spermatozoa

Background

Protein Ser/Thr Phosphatase PPP1CC2 is an alternatively spliced isoform of PPP1C that is highly enriched in testis and selectively expressed in sperm. Addition of the phosphatase inhibitor toxins okadaic acid or calyculin A to caput and caudal sperm triggers and stimulates motility, respectively. Thus, the endogenous mechanisms of phosphatase inhibition are fundamental for controlling sperm function and should be characterized. Preliminary results have shown a protein phosphatase inhibitor activity resembling PPP1R2 in bovine and primate spermatozoa.

Results

Here we show conclusively, for the first time, that PPP1R2 is present in sperm. In addition, we have also identified a novel protein, PPP1R2P3. The latter was previously thought to be an intron-less pseudogene. We show that the protein corresponding to the pseudogene is expressed. It has PPP1 inhibitory potency similar to PPP1R2. The potential phosphosites in PPP1R2 are substituted by non-phosphorylable residues, T73P and S87R, in PPP1R2P3. We also confirm that PPP1R2/PPP1R2P3 are phosphorylated at Ser121 and Ser122, and report a novel phosphorylation site, Ser127. Subfractionation of sperm structures show that PPP1CC2, PPP1R2/PPP1R2P3 are located in the head and tail structures.

Conclusions

The conclusive identification and localization of sperm PPP1R2 and PPP1R2P3 lays the basis for future studies on their roles in acrosome reaction, sperm motility and hyperactivation. An intriguing possibility is that a switch in PPP1CC2 inhibitory subunits could be the trigger for sperm motility in the epididymis and/or sperm hyperactivation in the female reproductive tract.

The structure, role, and regulation of type 1 protein phosphatases

Type 1 protein phosphatases (PP-1) comprise a group of widely distributed enzymes that specifically dephosphorylate serine and threonine residues of certain phosphoproteins. They all contain an isoform of the same catalytic subunit, which has an extremely conserved primary structure. One of the properties of PP-1 that allows one to distinguish them from other serine/threonine protein phosphatases is their sensitivity to inhibition by two proteins, termed inhibitor 1 and inhibitor 2, or modulator. The latter protein can also form a 1:1 complex with the catalytic subunit that slowly inactivates upon incubation. This complex is reactivated in vitro by incubation with MgATP and protein kinase FA/GSK-3. In the cell the type 1 catalytic subunit is associated with noncatalytic subunits that determine the activity, the substrate specificity, and the subcellular location of the phosphatase. PP-1 plays an essential role in glycogen metabolism, calcium transport, muscle contraction, intracellular transport, protein synthesis, and cell division. The activity of PP-1 is regulated by hormones like insulin, glucagon, alpha- and beta-adrenergic agonists, glucocorticoids, and thyroid hormones.