Golgi serine/threonine protein kinase required for intermediate growth in the proximal-distal axis. Phosphorylates specific residues within extracellular cadherin domains of Fat (ft) and Dachsous (ds) as they transit through the Golgi (PubMed:18635802). Acts in ommatidial polarity determination as a secondary signal downstream of Notch, JAK/STAT and wingless. Also necessary for the initiation, up-regulation or maintenance of Notch ligand, Serrate (Ser) expression in legs, thereby participating in a feedback loop with N signaling. Sufficient for joint formation and growth in the leg (PubMed:10607560, PubMed:11566858, PubMed:14757640, PubMed:7555705, PubMed:8606003). ATP + L-seryl-[protein] = ADP + H(+) + O-phospho-L-seryl-[protein] ATP + L-threonyl-[protein] = ADP + H(+) + O-phospho-L-threonyl-[protein] Golgi apparatus, in the wing pouch. In the eye disk, expressed in a gradient ahead of the morphogenetic furrow, high at the equator and low at the poles of the eye. In the leg disk, expressed in concentric rings, possibly corresponding to segmental boundaries. In the wing disk, expression is localized in the wing pouch; low in peripheral regions and high towards the center. Proteolytically cleaved to yield a secreted protein. Mutants show reduced growth and altered differentiation only within restricted sectors of the proximal-distal (PD) axis in the leg and wing. Belongs to the FJX1/FJ family.