ALK4

ALK4, also known as activin receptor type-1B (ACVR1B), is a type I serine/threonine kinase receptor of the transforming growth factor-β (TGF-β) superfamily that functions as a key transducer of activin signaling^[1][2]. Upon ligand engagement, activin first binds type II receptors, including ACVR2A or ACVR2B, which subsequently recruit and activate ALK4, leading to downstream phosphorylation of SMAD2 and SMAD3 and regulation of transcriptional programs involved in cell growth, differentiation, and homeostasis^[1][2]. Mechanistically, ALK4 also mediates signaling induced by activin-like ligands and participates in canonical activin-SMAD signaling pathways that govern diverse physiological processes^[1][2]. In experimental and disease-related contexts, genetic and functional studies have linked ACVR1B to tumor biology, including the identification of somatic ACVR1B mutations in pancreatic carcinoma, supporting a role for disrupted ALK4 signaling in cancer-associated pathways^[3]. Compared with related type I receptors such as ALK5 (TGFBR1) and ALK7 (ACVR1C), ALK4 primarily functions as a receptor for activin-family ligands and therefore represents a distinct entry point into activin-dependent signaling networks^[1][2]. For experimental applications, pharmacological blockade of ALK4 signaling is frequently achieved using SB-431542, a small-molecule inhibitor that targets ALK4 together with ALK5 and ALK7 and is widely used to interrogate activin/TGF-β pathway function in cellular models^[4].