The localization of the cAMP-dependent protein kinase phosphorylation site in the platelet rat protein, rap 1B

Phosphorylation of Rap1 by cAMP-dependent Protein Kinase (PKA) Creates a Binding Site for KSR to Sustain ERK Activation by cAMP

Cyclic adenosine monophosphate (cAMP) is an important mediator of hormonal stimulation of cell growth and differentiation through its activation of the extracellular signal-regulated kinase (ERK) cascade. Two small G proteins, Ras and Rap1 have been proposed to mediate this activation. Using HEK293 cells as a model system, we have recently shown that both Ras and Rap1 are required for cAMP signaling to ERKs. However, cAMP-dependent Ras signaling to ERKs is transient and rapidly terminated by PKA phosphorylation of the Raf isoforms C-Raf and B-Raf. In contrast, cAMP-dependent Rap1 signaling to ERKs and Rap1 is potentiated by PKA. We show that this is due to sustained binding of B-Raf to Rap1. One of the targets of PKA is Rap1 itself, directly phosphorylating Rap1a on serine 180 and Rap1b on serine 179. We show that these phosphorylations create potential binding sites for the adaptor protein 14-3-3 that links Rap1 to the scaffold protein KSR. These results suggest that Rap1 activation of ERKs requires PKA phosphorylation and KSR binding. Because KSR and B-Raf exist as heterodimers within the cell, this binding also brings B-Raf to Rap1, allowing Rap1 to couple to ERKs through B-Raf binding to Rap1 independently of its Ras-binding domain.

Protein kinase A-dependent phosphorylation of Rap1 regulates its membrane localization and cell migration

The small G protein Rap1 can mediate "inside-out signaling" by recruiting effectors to the plasma membrane that signal to pathways involved in cell adhesion and cell migration. This action relies on the membrane association of Rap1, which is dictated by post-translational prenylation as well as by a stretch of basic residues within its carboxyl terminus. One feature of this stretch of acidic residues is that it lies adjacent to a functional phosphorylation site for the cAMP-dependent protein kinase PKA. This phosphorylation has two effects on Rap1 action. One, it decreases the level of Rap1 activity as measured by GTP loading and the coupling of Rap1 to RapL, a Rap1 effector that couples Rap1 GTP loading to integrin activation. Two, it destabilizes the membrane localization of Rap1, promoting its translocation into the cytoplasm. These two actions, decreased GTP loading and decreased membrane localization, are related, as the translocation of Rap1-GTP into the cytoplasm is associated with its increased GTP hydrolysis and inactivation. The consequences of this phosphorylation in Rap1-dependent cell adhesion and cell migration were also examined. Active Rap1 mutants that lack this phosphorylation site had a minimal effect on cell adhesion but strongly reduced cell migration, when compared with an active Rap1 mutant that retained the phosphorylation site. This suggests that optimal cell migration is associated with cycles of Rap1 activation, membrane egress, and inactivation, and requires the regulated phosphorylation of Rap1 by PKA.

Phosphorylation of H-ras proteins by protein kinase A

Protein phosphorylation is one of several representative post-translational modifications. Cyclic AMP-dependent protein kinase (PKA) plays the crucial and varying role of signal transduction. On the other hand, ras proteins plays an important role in cell proliferation and growth. Although a previous report showed that H-ras protein was phosphorylated by PKA, the stoichiometry was not determined, so we investigated the stoichiometry of phosphorylation of the protein by PKA. H-ras cDNA inserted into a pGEX-2T expressing vector produced high levels of recombinant H-ras (rH-ras) in a fusion protein with glutathione S-transferase. rH-ras was obtained after cleavage by thrombin. Phosphorylation of ras protein by the catalytic subunit of PKA was performed, and the radioactivity was counted after SDS-PAGE and autoradiography. The results indicate that less than 0.1 mol of phosphate was incorporated per mol of H-ras protein, and suggest that H-ras protein could not be a physiologically meaningful substrate for PKA.

Evidence for Rap1 in vascular smooth muscle cells. Regulation of their expression by platelet-derived growth factor BB

The effect of platelet-derived growth factor (PDGF) on Rap1 expression was investigated in rat vascular smooth muscle cells (SMC). First, evidence for Rap1 proteins was shown by their: (i) detection in membranes using a specific anti-Rap1 antibody, (ii) typical shift in electrophoretic mobility as a consequence of reduction, and (iii) cAMP-induced phosphorylation and immunoprecipitation. Then, the mitogenic activity of 10 ng/ml PDGF AA and BB for 48 h, resulting in a 2- and 5-fold increase in [3H]thymidine incorporation, was correlated with that of total Rap1 protein expression which was found to be 99% +/- 36% and 260% +/- 70%, respectively. Further time-course studies established that this up-regulation of Rap1 proteins was only observed after 48 h of PDGF BB treatment. Lastly, comparative RT-PCR of both rap1a and rap1b mRNAs showed that PDGF BB also up-regulated the rap1a mRNA species, which was 1.5-fold increased in contrast with the rap1b mRNA species. It is concluded that the PDGF BB-induced SMC proliferation is associated with an up-regulation of Rap1a protein.

Phosphorylation of rap1B by protein kinase A is not involved in platelet inhibition by cyclic AMP

The functional consequence of cyclic AMP-dependent phosphorylation of rap1B for stimulus-induced platelet activation is not known. Platelets were pretreated with the stable prostacyclin-analogue iloprost and resuspended in plasma without iloprost. Western blot analysis showed that rap1B was completely converted into its phosphorylated form in the iloprost-pretreated platelets. Surprisingly, the platelets that contained phosphorylated rap1B were found to respond fully to activation by a wide variety of stimuli: aggregation upon stimulation by collagen, phorbol ester, vasopressin, ADP, epinephrine, and ATP-secretion from dense granules induced by collagen, thrombin-receptor activating peptide, vasopressin and phorbol ester were unchanged as compared to control. The results indicate that cyclic AMP-dependent phosphorylation of rap1B does not play a role in the inhibition of the various signal transduction pathways that lead to platelet aggregation and dense granule secretion.

Cytoskeletal interactions of Rap1b in platelets

We have presented evidence that rap1b, a 22 kDa low molecular weight GTP binding protein, becomes associated with the cytoskeleton in thrombin-activated platelets. The initial incorporation is very rapid and occurs as fast as we can measure it. Thus, some rap1b is associated with the cytoskeleton as fast as it is formed. The remainder of the rap1b is incorporated more slowly. This biphasic incorporation of rap1b is similar to the incorporation of GPIIb/IIIa into the cytoskeleton, but no interaction between GPIIb/IIIa and rap1b could be demonstrated. Phosphorylation of rap1b by cAMP-dependent protein kinase did not inhibit its association with the cytoskeleton. We conclude that rap1b is one of an increasing number of proteins that associate with the cytoskeleton during cell activation. The function of rap1b in the cytoskeleton is unclear at this time. However, it is possible to speculate on potential roles. There is growing evidence that low molecular weight G proteins participate in the formation of multi-molecular aggregates. For example, p21rac promotes the assembly of a membrane-associated complex composed of NADPH oxidase, p47, and p67 and this complex is important for activation of NADPH oxidase in neutrophils. Similarly, in yeast, BUD1, a homolog of rap1, forms a complex with BUD5 (a homolog of GDI), BEMI, CDC24, and CDC42 (a homolog of G25K). This multi-protein aggregate may be important in cytoskeletal structure in yeast. In platelets, rad1b, which is membrane associated, may promote the assembly of a complex of proteins during cell activation and may localize this complex to the plasma membrane.(ABSTRACT TRUNCATED AT 250 WORDS)