A Novel Src Homology 3 Domain-containing Adaptor Protein, HIP-55, That Interacts with Hematopoietic Progenitor Kinase 1

Abstract Hematopoietic progenitor kinase 1 (HPK1) is a member of the mitogen-activated protein kinase kinase kinase kinase (MAP4K) family and an upstream activator of the c-Jun N-terminal kinase (JNK) signaling cascade. HPK1 interacts, through its proline-rich domains, with growth factor receptor-bound 2 (Grb2), CT10-regulated kinase (Crk), and Crk-like (CrkL) adaptor proteins. We identified a novel HPK1-interacting protein of 55 kDa (HIP-55), similar to the mouse SH3P7 protein, containing an N-terminal actin-binding domain and a C-terminal Src homology 3 domain. We found that HPK1 bound to HIP-55 both in vitro and in vivo. When co-transfected, HIP-55 increased HPK1's kinase activity as well as JNK1's kinase activity. A dominant-negative HPK1 mutant blocked activation of JNK1 by HIP-55 showing that HIP-55 activates the JNK1 signaling pathway via HPK1. Our results identify a novel protein, HIP-55, that binds to HPK1 and regulates the JNK1 signaling cascade.