Submitted to: Journal of Biological Chemistry
Publication Type: Peer reviewed journal
Publication Acceptance Date: 12/9/2002
Publication Date: 2/21/2003
Citation: CHENG,N., HIRSCHI,K. ., CLONING AND CHARACTERIZATION OF CXIP1. A NOVEL PICOT DOMAIN-CONTAINING ARABIDOPSIS PROTEIN THAT ASSOCIATES WITH CAX1, JOURNAL OF BIOLOGICAL CHEMISTRY, 2003. v. 278. p. 6503-6509. Interpretive Summary: Calcium is an important nutrient for both humans and plants. Both plants and animals regulate calcium levels by modifying the movement of this substance using various transporters. In this paper, we have identified and characterized a novel regulatory of calcium transporters. In other words, this molecular "opens and closes" calcium transporters. Potentially, this regulator can be highly expressed in plants as a means of "opening" the transporters to boost calcium levels in plants- thus making them more nutritious.
Technical Abstract: Regulation of Ca2+ transporters is a vital component of signaling. The Arabidopsis H+/Ca2+ exchanger, CAX1 contains an N-terminal autoinhibitory domain that prevents Ca2+ transport when CAX1 is heterologously expressed in yeast. Using a yeast screen, we have identified three different proteins that activate CAX1. One of these, CAX interacting protein 1 (CXIP1; 19.3 kDa) has amino acid similarity with the C-terminus of PICOT (protein kinase C-interacting cousin of thioredoxin) proteins. Although PICOT proteins are found in a variety of organisms, a function has not been previously ascribed to a plant PICOT protein. We demonstrate that CXIP1 activated the CAX1 homolog CAX4 but not CAX3. An Arabidopsis homolog of CXIP1, CXIP2, weakly activated CAX4 but not CAX1. Using a yeast two-hybrid assay, CXIP1 interacted with the N-terminus of CAX1. Through competition analysis, CXIP1 and a CAX1 N-terminal peptide appeared to bind to similar N-terminal domains of CAX1. Chimeric CAX3 constructs that contained the N-terminus of CAX1 were activated by CXIP1. Mutations in the CXIP1 PICOT domain abolished CAX1 activation. In Arabidopsis, CXIP1 transcripts, like CAX1, accumulated in response to different metal conditions. This report thus characterizes a new class of signaling molecules in plants, which may regulate CAX transporters in vivo.