Effect of protein kinase A on accumulation of brefeldin A-inhibited guanine nucleotide-exchange protein 1 (BIG1) in HepG2 cell nuclei

Brefeldin A-inhibited guanine nucleotide-exchange proteins, BIG1 and BIG2, are activators of ADP-ribosylation factor GTPases that are essential for regulating vesicular traffic among intracellular organelles. Biochemical analyses and immunofluorescence microscopy demonstrated BIG1 in nuclei as well as membranes and cytosol of serum-starved HepG2 cells. Within 20 min after addition of 8-Br-cAMP, BIG1 accumulated in nuclei, and this effect was blocked by protein kinase A (PKA) inhibitors H-89 and PKI, suggesting a dependence on PKA-catalyzed phosphorylation. BIG2 localization was notaltered by CAMP, nor did BIG2 small interfering RNA influence nuclear accumulation of BIG1 induced by cAMP. Mutant BIG1 (S883A) in which Ala replaced Ser-883, a putative PKA phosphorylation site, did not move to the nucleus with cAMP addition, whereas replacement with Asp (S883D) resulted in nuclear accumulation of BIG1 without or with CAMP exposure, consistent with the mechanistic importance of a negative charge at that site. Mutation (712KPK714) of the nuclear localization signal inhibited BIG1 accumulation in nuclei, and PKA-catalyzed phosphorylation of S883, although necessary, was not sufficient for nuclear accumulation, as shown by the double mutation S883D/ nuclear localization signal. A role for microtubules in cAMP-induced translocation of BIG1 is inferred from its inhibition by nocodazole. Thus, two more critical elements of BIG1 molecular structure were identified, as well as the potential function of microtubules in a novel PKA effect on BIG1 translocation.