Pulmonary-Critical Care Medicine Branch, National Heart, Lung, and Blood
Institute, National Institutes of Health, Building 10, Room 5N307, MSC 1434,
Bethesda, MD 20892-1434.
BIG1 and BIG2 are brefeldin A-inhibited guanine nucleotide-exchange proteins
that activate ADP-ribosylation factors (ARFs), critical components of vesicular
trafficking pathways. These proteins can exist in macromolecular complexes and
move between Golgi membranes and cytosol. In the BIG1 molecule, a centrally
located Sec7 domain is responsible for ARF activation, but functions of other
regions are largely unknown. Yeast two-hybrid screens of a human placenta cDNA
library with BIG1 cDNA constructs revealed specific interaction of the
N-terminal region (amino acids 1-331) with FK506-binding protein 13 (FKBP13).
The association was confirmed by immunoprecipitation of both endogenous BIG1 and
FKBP13 from Jurkat T cells with antibodies against either one. Binding of BIG1,
BIG2, and ARF to cell membranes in vitro was increased by guanosine
5'-[gamma-thio]triphosphate, and further increases were induced by FK506.
Incubation of Jurkat T cells with FK506 increased binding of BIG1, BIG2, and ARF
to Golgi and other membranes in a time- and concentration-dependent manner,
without effects on clathrin or gamma-adaptin binding. Binding of BIG1, BIG2, and
ARF to membranes was also increased by L-732,531, an agonist structurally
related to FK506, but was not increased by a related antagonist, L-685,818, nor
by cyclosporin A or rapamycin. These findings are consistent with a role for
FKBP13 and FK506 in vesicular trafficking, influencing ARF activity through
their guanine nucleotide-exchange proteins.