Arabidopsis thaliana sequence analysis confirms the presence of cyt b-561 in plants: Evidence for a novel protein family

Recent advances in the Arabidopsis sequencing project has elucidated the pr esence of two genes Arb561-A and Atb561-B that show limited homology to the DNA sequence encoding for the mammalian chromaffin granule cytochrome b-56 1 (cyt b-561). Detailed analysis of the structural features and conserved r esidues reveals, however, that the structural homology between the presumpt ive Arabidopsis proteins and the animal proteins is very high. All proteins are hydrophobic and show highly conserved transmembrane helices. The presu mably heme-binding histidine residues in the plant and animal sequences as well as the suggested binding site for the electron acceptor, monodehydroas corbate, an strictly conserved. In contrast, the suggested electron donor ( ascorbate) binding site is not very well conserved between the plant and an imal sequences questioning the function of this motif. Sequence analysis of the Atb561-B gene demonstrates a different splicing than that initially pr edicted in silico resulting in a protein with nine extra amino acids and a significantly higher homology to the other cyt b-561 sequences. The homolog y between the plant and animal sequences is further supported by the strong similarity between a number of biochemical properties of the chromaffin cy t b-561 and the cyt b-561 isolated from bean hook plasma membranes. Since t he mammalian cyt b-561 is considered specific to neuroadrenergic tissues, t he identification of a closely related homologue in an aneural organism dem onstrates that these proteins constitute a new class of widely occurring me mbrane proteins. Both the plant and animal cyt b-561 are involved in transm embrane electron transport using ascorbate as an electron donor. The simila rity between these proteins therefore suggests, for the first time, that th is transport supports a number of different cell physiological processes. A n evolutionary relationship between the plant and animal proteins is presen ted. (C) 2000 Editions scientifiques et medicales Elsevier SAS.