Abstract

OSR1 (oxidative stress-responsive-1) and SPAK (Ste20/Sps1-related proline/alanine-rich kinase) are related kinases belonging to the GCK-VI subfamily of Ste20 group kinases (Lee et al., 2009). They phosphorylate clusters of threonine residues in their best known substrates the Na+ Cl- cotransporters NCC and NKCC2. Both SPAK and OSR1 possess similar architectures: the kinase catalytic domain, followed by a flexible region and then a conserved C-terminal (CCT) domain (Vitari et al., 2006). SPAK has a flexible N-terminal region with an Ala-Pro stretch thought to be a positional targeting module. The CCT domains of OSR1 and SPAK are 79% identical at their sequence level and form a unique fold called SPOC (SPak/Osr1 C-terminus) fold (van Aalten DM et al.,2007).
The CCT domain is involved in the interactions of these kinases - not just with their substrates but also their activator kinases - by recognizing the docking motif RFxV (Piechotta et al., 2002) which these all possess. Both SPAK and OSR1 are involved in many cellular process including cell differentiation, cytoskeletal rearrangement, cell proliferation, transformation and most notably are involved in the regulation of ion homeostasis and volume control in mammalian cells. Since mutations in their WNK1 and WNK4 upstream-activating kinases can cause Gordon's syndrome, an inherited disregulation of Na-Cl co-transporters, SPAK/OSR1 are also considered to be a potential therapeutic target for hypertension. Inhibition of these enzymes reduces the activity of the substrates NCC and NKCC2 and thereby reduces renal salt re-absorption and consequent lowering of blood pressure (Richardson and Alessi, 2008).
The structure of the CCT domain consists of four antiparallel beta-strands packed against two large and one smaller alpha helices (van Aalten DM et al.,2007). The interface between alpha1 and beta2 creates an elongated, negatively charged groove called the primary pocket while a secondary hydrophobic pocket is formed by the large loop connecting alpha3 and beta4. The CCT domain interacts with its docking motif via the primary pocket with the residues involved in the interaction being identical in both SPAK and OSR1. RFxV motifs with a following Thr/Ser residue are sensitive to inhibition of binding by phosphorylation which causes a steric clash between the phosphate group and the backbone of the CCT domain. It is not yet clear if SPAK and OSR1 kinase activities can themselves autoregulate RFxV interactions.