Analysis of an Arabidopsis thaliana protein family, structurally related to cytochromes b561 and potentially involved in catecholamine biochemistry in plants

Cytochromes b561 (cyts b561) constitute a family of eukaryotic membrane proteins, catalysing ascorbate (Asc)-mediated trans-membrane electron transport, and hence likely involved in Asc regeneration. A class of proteins (DoH-CB) has been identified in plants and animals, containing the cyt b561 electron-transport domain (CB), combined with the catecholamine-binding regulatory domain of dopamine-beta-hydroxylase (DoH). A mammalian DoH-CB protein was previously reported to function as a cell-derived growth factor receptor (SDR2). We have performed an in silico analysis on DoH-CB proteins from Arabidopsis thaliana and demonstrate that structural features of both CB and DoH domains are well conserved. The combination of both domains may have evolved from a functional interaction between a cyt b561 and a DoH-containing protein, illustrating the so-called "Rosetta Stone" evolutionary principle, and this hypothesis is supported by sequence comparisons. DoH-CB proteins form a newly identified group of proteins, likely to play a key role in catecholamine action in plants. It is suggested that these proteins may function as trans-membrane electron shuttles, possibly regulated by catecholamines. The role and action of catecholamines in plants is poorly documented, but it is clear that they are involved in many aspects of growth and development. Whether the DoH-CB proteins functionally interact with Asc, as is the case for cyts b561, remains to be determined.