Potassium Channel Modulation by a Toxin Domain in Matrix Metalloprotease 23

Abstract

Mechanisms that fine tune the activity of potassium channels are crucial to a cell's ability to integrate and respond to a plethora of internal and external signals. Peptide toxins from venomous creatures have served as vital tools to define the molecular mechanisms underlying K ϩ channel function (1, 2). It has been suggested that toxins evolved from endogenous genes that function in normal cellular pathways (3, 4). Indeed, venomous creatures possess toxins with homology to several proteins, including acetylcholinesterases (5), phospholipases (6, 7), nerve growth factor (8), endothelins (9), Lynx-1 (10, 11), Kunitz-type serine protease inhibitors (12), and the ion channel regulatory (ICR) 5 domains of cysteinerich secretory proteins (CRISPs) (3,13,14). Mammalian proteins containing toxin-like domains (TxDs) that block K ϩ channels have not been characterized previously.BgK, a 37-residue peptide toxin from the sea anemone Bunodosoma granulifera (15,16), and ShK, a 35-residue peptide toxin from the sea anemone Stichodactyla helianthus (17,18) are potent inhibitors of K ϩ channels. The Simple Modular Architecture Research Tool (SMART) (available on the World Wide Web) predicts the existence of a large superfamily of proteins that contain domains (referred to as ShKT domains in the SMART data base) resembling these two toxins (Fig. 1A). Many of these proteins (ϳ70 proteins) are metallopeptidases, whereas others are prolyl-4-hydroxylases, tyrosinases, peroxidases, oxidoreductases, or proteins containing epidermal growth factor-like domains, thrombospondin-type repeats, or trypsin-like serine protease domains (Fig. 1B). The only human protein containing a ShKT domain in the SMART data base is MMP23 (matrix metalloprotease 23). Matrix metalloproteases belong to the metzincin superfamily and play important roles in tissue remodeling, development, and the immune response (19).MMP23 is expressed in many tissues and exists either as a type II transmembrane protein in ER/nuclear membranes or as a secreted form following cleavage of the RRRRY motif just N-terminal to the Zn 2ϩ -dependent metalloprotease domain (20 -23). The ShKT domain of MMP23 (MMP23 TxD ) lies between the metalloprotease domain and an immunoglobulinlike cell adhesion molecule (IgCAM) domain ( Fig. 2A). MMP23 has been implicated in prostate, brain, and breast cancer (24 -26). In humans, two related sequences, MMP23A (a pseudogene) and MMP23B, are co-located on chromosome 1p36 (20). We have investigated MMP23 to gain insight into the structure and physiological functions of ShKT toxin domains and describe the solution structure of the MMP23 TxD domain, its * This work was supported, in whole or in part, by National Institutes of Health

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“…Some tumor cells produce MMP-23, a large protein that contains a domain analogous to the Kv1.3 blocker ShK [49]. This may represent a mechanism for tumor cells to reduce their destruction by immune cells.…”

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