The recently reported NIβ domain is already known as the Calx-β motif

Abstract

May and Ponting recently described homologies between integrin-α-subunit and -β-subunit domains and cyanobacterial proteins. However, their analysis of integrin-β homologies reports findings that we published 16 months earlier. Before discussing this, we wish to make clear that this recapitulation of our work was not the sole focus of their article: their demonstration that integrin-α and -β domains coexist in a single bacterial polypeptide remains novel and interesting. We discovered a motif, the Calx-β motif, that is present as a tandem repeat in the cytoplasmic domains of mammalian 3Na+/1Ca2+ exchangers (NCX1, etc.) and of their invertebrate orthologs (CALX, C10G8.5, etc.). The Calx-β motif is also present in the cytoplasmic tail of mammalian integrin β4 and in three functionally uncharacterized Synechocystis sp. PCC6803 proteins (slr0408, slr1028 and slr1403). The Calx-β motif overlaps domains used by NCX1 for calcium binding and regulation. Our analysis of the Calx-β motif, using the PHD neural network, indicated that it contains a series of β-strands and turns, which is consistent with the Calx-β motif forming a self-contained β-sheet.