Sprouty proteins: modified modulators, matchmakers or missing links?

Abstract

Sprouty proteins are involved in organogenesis, particularly during the branching of endothelial tubes, and existing evidence suggests that Sprouty's point of action lies downstream of receptor signaling to inhibit the activation of the central Ras/Erk pathway. How Sprouty proteins accomplish their inhibitory action and whether they interact with other signaling pathways are significant questions. Sprouty proteins are devoid of any recognizable protein interaction domain, and clues as to how they function have been mainly derived from screening for interacting partners. Conserved across all the Sprouty proteins are three sequences: a Cbl-tyrosine kinase-binding (TKB) binding motif centered on an obligatorily phosphorylated tyrosine (Y55 in Sprouty2), a serine-rich motif (SRM) and a cysteine-rich domain (CRD). With the exception of a handful of proteins that bind to the N-terminus, most of the binding to Sprouty occurs via the CRD, predominantly by serine/threonine kinases that target sites within the SRM on Sprouty. Some of the resultant increase in phosphorylation is opposed by activated protein phosphatase 2A that binds to the N-terminal Cbl-TKB binding motif. Significantly, two ubiquitin E3 ligases also bind to the N-terminus of Sprouty: c-Cbl binds with high affinity to the TKB binding motif and SIAH2 binds constitutively to a different site; both proteins are able to direct the ubiquitination of Sprouty proteins and its destruction. The collective evidence points to Sprouty proteins as being substantially covalently-modified to control its location, stability, association, and destruction. With such stringent control of the Sproutys, the main question is what key proteins does this facilitator bring together?