New monoclonal antibodies recognizing phosphorylated proteins in mitotic cells

Multisite phosphorylation of Pin1-associated mitotic phosphoproteins revealed by monoclonal antibodies MPM-2 and CC-3

Background

The peptidyl-prolyl isomerase Pin1 recently revealed itself as a new player in the regulation of protein function by phosphorylation. Pin1 isomerizes the peptide bond of specific phosphorylated serine or threonine residues preceding proline in several proteins involved in various cellular events including mitosis, transcription, differentiation and DNA damage response. Many Pin1 substrates are antigens of the phosphodependent monoclonal antibody MPM-2, which reacts with a subset of proteins phosphorylated at the G2/M transition.

Results

As MPM-2 is not a general marker of mitotic phosphoproteins, and as most mitotic substrates are phosphorylated more than once, we used a different phosphodependent antibody, mAb CC-3, to identify additional mitotic phosphoproteins and eventual Pin1 substrates by combining affinity purification, MALDI-TOF mass spectrometry and immunoblotting. Most CC-3-reactive phosphoproteins appeared to be known or novel MPM-2 antigens and included the RNA-binding protein p54^nrb/nmt55, the spliceosomal protein SAP155, the Ki-67 antigen, MAP-1B, DNA topoisomerases II α and β, the elongation factor hSpt5 and the largest subunit of RNA polymerase II. The CC-3 mitotic antigens were also shown to be Pin1 targets. The fine CC-3- and MPM-2-epitope mapping of the RNA polymerase II carboxy-terminal domain confirmed that the epitopes were different and could be generated in vitro by distinct kinases. Finally, the post-mitotic dephosphorylation of both CC-3 and MPM-2 antigens was prevented when cellular Pin1 activity was blocked by the selective inhibitor juglone.

Conclusion

These observations indicate that the mitotic phosphoproteins associated with Pin1 are phosphorylated on multiple sites, suggesting combinatorial regulation of substrate recognition and isomerization.

A hyperphosphorylated form of RNA polymerase II is the major interphase antigen of the phosphoprotein antibody MPM-2 and interacts with the peptidyl-prolyl isomerase Pin1

The monoclonal antibody MPM-2 recognizes a subset of M phase phosphoproteins in a phosphorylation-dependent manner. It is believed that phosphorylation at MPM-2 antigenic sites could regulate mitotic events since most of the MPM-2 antigens identified to date have M phase functions. In addition, many of these proteins are substrates of the mitotic regulator Pin1, a peptidyl-prolyl isomerase which is present throughout the cell cycle and which is thought to alter its mitotic targets by changing their conformation. In interphase cells, most MPM-2 reactivity is confined to nuclear speckles. We report here that a hyperphosphorylated form of the RNA polymerase II largest subunit is the major MPM-2 interphase antigen. These findings were made possible by the availability of another monoclonal antibody, CC-3, that was previously used to identify a 255 kDa nuclear matrix protein associated with spliceosomal components as a hyperphosphorylated form of the RNA polymerase II largest subunit. MPM-2 recognizes a phosphoepitope of the large subunit that becomes hyperphosphorylated upon heat shock in contrast to the phosphoepitope defined by CC-3, whose reactivity is diminished by the heat treatment. Therefore, these two antibodies may discriminate between distinct functional forms of RNA polymerase II. We also show that RNA polymerase II large subunit interacts with Pin1 in HeLa cells. Pin1 may thus regulate transcriptional and post-transcriptional events by catalyzing phosphorylation-dependent conformational changes of the large RNA polymerase II subunit.

Gene rearrangements in the molecular pathogenesis of acute promyelocytic leukemia

Acute Promyelocytic Leukemia (APL) is a distinct subtype of myeloid leukemia that in the USA alone affects more than 3,000 individuals every year. APL is characterized by three distinct and unique features: i) the accumulation in the bone marrow of tumor cells with promyelocytic features; ii) the invariable association with specific translocations which always involve chromosome 17 and the Retinoic Acid Receptor alpha (RAR alpha) locus; iii) the exquisite sensitivity of APL blasts to the differentiating action of Retinoic Acid (RA). These features have led APL to become the paradigm for therapeutic approaches utilizing differentiating agents. The last 5 years have provided crucial insights into the molecular basis of APL. RAR alpha translocates in 99% of cases to a gene located on chromosome 15 that we initially named myl and subsequently has been called PML. In a few cases, RAR alpha variably translocates to chromosome 11 where it fuses to the PLZF gene or to a newly described partner, NuMA. In addition, RAR alpha is also found translocated to chromosome 5 where it fuses to the NPM gene. The cloning of variant translocations in APL and the comparative analysis of their associated products is crucial for the understanding of the molecular etiopathogenesis of the disease. The generation of animal models, i.e., transgenic mice expressing the fusion genes, will be instrumental in determining the precise contribution of these fusion genes to leukemogenesis. In fact, mice harboring a PML/RAR alpha transgene whose expression is specifically targeted to the myeloid-promyelocytic lineage develop acute myeloid leukemia with promyelocytic features. Moreover, the functional analysis of the various fusion proteins, as well as RAR alpha partners, is revealing striking common features beneath a misleading structural heterogeneity which unravels a possible unifying molecular mechanism towards APL leukemogenesis.

Cell cycle-dependent phosphorylation of mammalian protein phosphatase 1 by cdc2 kinase

Protein phosphatase 1 (PP-1) is known to be a critical component of eukaryotic cell cycle progression. In vitro, our previous studies showed that cdc2 kinase phosphorylates Thr-320 (T320) in PP-1, and that this leads to inhibition of enzyme activity. To examine directly the phosphorylation of PP-1 in intact mammalian cells, an antibody has been prepared that specifically recognizes PP-1C alpha phosphorylated at T320. Cell synchronization studies revealed in a variety of cell types that T320 of PP-1 was phosphorylated to high levels only during early to mid-mitosis. The phosphorylation of T320 of PP-1 was reduced by the cyclin-dependent protein kinase inhibitor, olomoucine, and increased by the PP-1/PP-2A inhibitor, calyculin A. Immunofluorescence microscopy using phospho-T320 antibody indicated that in NIH 3T3 cells the phosphorylation of PP-1 began to increase from basal levels in prophase and to peak at metaphase. Immunostaining indicated that phospho-PP-1 was localized exclusively to nonchromosomal regions. Furthermore, in cell fractionation studies of mitotic cells, phospho-PP-1 was detectable only in the soluble fraction. These observations suggest that phosphorylation by cdc2 kinase in early to mid-mitosis and inhibition of PP-1 activity is likely to contribute to the increased state of phosphorylation of proteins that is critical to the initiation of normal cell division.