Minisymposium 7: Photomorphogenesis

Abs # 17003: A cyclic nucleotide-gated, calmodulin-binding potassium channel involved in photomorphogenesis in Arabidopsis thaliana

Presenter:	Borsics, Tamas
Authors	Borsics, Tamas  (A)   Li, Xinli  (A)   Harrington, Michael  (A)   Christopher, David A (A)
Affiliations:	(A): University of Hawaii, Dept. of Molecular Biosciences & Bioengineering

Cyclic nucleotide-gated and calmodulin (CaM)-regulated K+ channels play essential roles in animal and plant cell signal transduction, metabolic regulation, growth and adaptation to the environment. We are interested in the signaling pathways that coordinate leaf and chloroplast development in response to light. Previous research from other laboratories has shown that CaM and cGMP are required for mediating normal chloroplast development during de-etiolation and in phytochrome mutants. The mechanisms and pathways by which these molecules operate in photomorphogenesis is not known. We have isolated and characterized a new member of the family of inward rectifying K+ channels, designated ACBK1, from Arabidopsis thaliana. ACBK1 is homologous to the Shaker-type superfamily and the cyclic nucleotide-gated and CaM-regulated K+ channels. ACBK1 has six membrane spanning domains, a pore domain, and a putative cNMP-binding domain that overlaps a CaM-binding domain located at the C-terminus. CaM-gel overlay analysis revealed that ACBK1 is a CaM-binding protein and forms a typical basic amphiphilic structure. Although ACBK1 lacks a typical K+-signature motif, complementation tests in K+ channel mutants of E. coli, yeast and Arabidopsis confirm ACBK1 is a K+ channel. ACBK1 is not a highly selective ion channel in bacteria, but it is highly selective in yeast. Biochemical analysis revealed Ca2+/CaM inhibited K+ uptake, while cGMP reversed this inhibition in E. coli. Reverse genetics experiments using antisense ACBK1 lines in the Col background were conducted. Relative to wild type Col, the antisense ACBK1 plants flowered 12 and 5 days earlier on short day (11 hr light/13 hr dark cycle) and long-day (16 hr light/8 hr dark cycle) conditions, respectively. These flowering times were comparable to the phyB mutant, which flowered 10 days (short day cycle) and 3 days (long day cycle) earlier relative to Landsberg. The antisense lines also had a 30% reduction in leaf thickness, 2.2-fold longer hypocotyls and altered chloroplast mRNA levels in response to light, and chloroplasts with aberrant starch accumulation at higher light fluences. The results suggest a role for ACBK1 and its interacting molecules (CaM, cGMP) in K+ balance involved in various light-regulated processes, including photomorphogenesis.