Alternative splice variants of phospholipase C-beta2 are expressed in platelets: effect on Galphaq-dependent activation and localization.

Phospholipase C (PLC) beta2 plays a pivotal role in G-protein dependent signal transduction in platelets. We have previously demonstrated in platelets, leukocytes and human erythroleukemia cells the presence of transcripts of two forms of PLC-beta2 generated by alternative splicing. They differ by 45 nucleotides in the carboxyl-terminal region and are designated as PLC-beta2a and PLC-beta2b, with and without by 15 amino acid residues (corresponding to 864-878). The presence of the two variants has not been shown at the protein level in cells. Moreover, the carboxy-terminal region of PLC-beta has been implicated in Galphaq activation, particulate association, and nuclear localization, suggesting that the PLC-beta2 splice variants may be regulated differentially. We demonstrate for the first time that both PLC-beta2 isoforms are expressed in platelets at the protein level. Studies in CV-1 cells transfected with PLC-beta2a or beta2b cDNAs, along with constitutively activated Galphaq (Q209L), showed that inositolphosphate formation was comparable between the two variants. However, the nuclear localization of the two isoforms was different with a higher cytoplasmic to nuclear ratio for PLC-beta2b compared to PLC-beta2a, suggesting that a great proportion of the total PLC-beta2a was in the nucleus relative to PLC-beta2b. There was no difference in the relative distribution of the two variants between the cytosol and particulate fractions. Both PLC-beta2 alternative splice variants are expressed at the protein level in platelets. In transfected CV-1 cells, PLC-beta2a is relatively more enriched in the nuclei than PLC-beta2b suggesting that the two variants may have different effects in cell proliferation and differentiation.