A conserved domain of the arabidopsis GNOM protein mediates subunit interaction and cyclophilin 5 binding

The Arabidopsis GNOM protein, a guanine nucleotide exchange factor (GEF) th at acts on ADP ribosylation factor (ARF-type G proteins, is required for co ordination of cell polarity along the apical-basal embryo axis. Interalleli c complementation of gnom mutants suggested that dimerization is involved i n GNOM function. Here, direct interaction between GNOM molecules is demonst rated in vitro and by using a yeast two-hybrid system, Interaction was conf ined to an N-terminal domain conserved within a subgroup of large ARF GEFs. The same domain mediated in vitro binding to cyclophilin 5 (Cyp5), which w as identified as a GNOM interactor in two-hybrid screening. Cyp5 displayed peptidylprolyl cis/trans-isomerase and protein refolding activities that we re sensitive to cyclosporin A. Cyp5 protein accumulated in several plant or gans and, like GNOM, was partitioned between cytosolic and membrane fractio ns. Cyp5 protein was also expressed in the developing embryo. Our results s uggest that Cyp5 may regulate the ARF GEF function of the GNOM protein duri ng embryogenesis.