YXXL motifs in SH2-Bβ are phosphorylated by JAK2, JAK1, and platelet-derived growth factor receptor and are required for membrane ruffling

Abstract

SH2-Bβ binds to the activated form of JAK2 and various receptor tyrosine kinases. It is a potent stimulator of JAK2, is required for growth hormone (GH)-induced membrane ruffling, and increases mitogenesis stimulated by platelet-derived growth factor (PDGF) and insulin-like growth factor I. Its domain structure suggests that SH2-Bβ may act as an adapter protein to recruit down-stream signaling proteins to kinase SH2-Bβ complexes. SH2-Bβ is tyrosyl-phosphorylated in response to GH and interferon-γ, stimulators of JAK2, as well as in response to PDGF and nerve growth factor. To begin to elucidate the role of tyrosyl phosphorylation in the function of SH2-Bβ, we used phosphopeptide mapping, mutagenesis, and a phosphotyrosine-specific antibody to identify Tyr-439 and Tyr-494 in SH2-Bβ as targets of JAK2 both in vitro and in intact cells. SH2-Bβ lacking Tyr-439 and Tyr-494 inhibits GH-induced membrane ruffling but still activates JAK2. We provide evidence that JAK1, like JAK2, phosphorylates Tyr-439 and Tyr-494 in SH2-Bβ and that PDGF receptor phosphorylates SH2-Bβ on Tyr-439. Therefore, phosphorylated Tyr-439 and/or Tyr-494 in SH2-Bβ may provide a binding site for one or more proteins linking cytokine receptor-JAK2 complexes and/or receptor tyrosine kinases to the actin cytoskeleton.